## Peer-reviewed publications

### 2022

- Markus Grassl, Felix Huber, and Andres Winter. Entropic proofs of singleton bounds for quantum error-correcting codes.
*Ieee transactions on information theory*, 68(6):3942-3950, 2022. doi:10.1109/TIT.2022.3149291

[BibTeX] [Download PDF]`@Article{grassl_entropic_2022, author = {Grassl, Markus and Huber, Felix and Winter, Andres}, journal = {IEEE Transactions on Information Theory}, title = {Entropic Proofs of Singleton Bounds for Quantum Error-Correcting Codes}, year = {2022}, issn = {0018-9448, 1557-9654}, month = jun, volume = {68}, number = {6}, pages = {3942-3950}, doi = {10.1109/TIT.2022.3149291}, language = {en}, shorttitle = {Algebraic quantum codes}, url = {https://doi.org/10.1109/TIT.2022.3149291}, urldate = {2021-05-19} }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. Remarks about bell-nonclassicality of a single photon.
*Physics letters a*, 435:128031, 2022. doi:https://doi.org/10.1016/j.physleta.2022.128031

[BibTeX] [Abstract] [Download PDF]

We discuss the interferometric scheme put forward by Tan, Walls, and Collett (TWC) that claims to reveal Bell non-classicality of a single photon. We show that the violation of local realism they report hinges on an additional assumption about hidden variables. Without it, one can construct a classical model that correctly predicts all the probabilities of the TWC scheme. To illustrate this, we construct a proper Bell inequality, devoid of additional assumptions, that is not violated by the TWC setup. We then show a modification of the interferometric scheme, consisting in making the amplitude of auxiliary fields tunable, that enables the Bell violation. Finally, we present an alternative point of view on the shortcomings of the original TWC analysis, arguing that it effectively treats the weak coherent states classically.

`@article{DAS2022128031, title = {Remarks about Bell-nonclassicality of a single photon}, journal = {Physics Letters A}, volume = {435}, pages = {128031}, year = {2022}, issn = {0375-9601}, doi = {https://doi.org/10.1016/j.physleta.2022.128031}, url = {https://www.sciencedirect.com/science/article/pii/S037596012200113X}, author = {Tamoghna Das and Marcin Karczewski and Antonio Mandarino and Marcin Markiewicz and Bianka Woloncewicz and Marek Żukowski}, keywords = {Bell's theorem, Single photon, Local hidden variable model}, abstract = {We discuss the interferometric scheme put forward by Tan, Walls, and Collett (TWC) that claims to reveal Bell non-classicality of a single photon. We show that the violation of local realism they report hinges on an additional assumption about hidden variables. Without it, one can construct a classical model that correctly predicts all the probabilities of the TWC scheme. To illustrate this, we construct a proper Bell inequality, devoid of additional assumptions, that is not violated by the TWC setup. We then show a modification of the interferometric scheme, consisting in making the amplitude of auxiliary fields tunable, that enables the Bell violation. Finally, we present an alternative point of view on the shortcomings of the original TWC analysis, arguing that it effectively treats the weak coherent states classically.}, urldate={2022-03-31} }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. Comment on `single particle nonlocality with completely independent reference states’.
*New journal of physics*, 24(3):38001, mar 2022. doi:10.1088/1367-2630/ac55b1

[BibTeX] [Abstract] [Download PDF]

We comment on the purported violation of local realism, by a single photon induced correlations between homodyne detectors, which one can find in: (2008) New J. Phys. 10 113024 [arXiv:0807.0109]. The claim is erroneous, due to a calculational mistake. As the result is the basis of other claims in the paper, they are unsubstantiated.

`@article{Das_2022, doi = {10.1088/1367-2630/ac55b1}, url = {https://doi.org/10.1088/1367-2630/ac55b1}, year = 2022, month = {mar}, publisher = {{IOP} Publishing}, volume = {24}, number = {3}, pages = {038001}, author = {Tamoghna Das and Marcin Karczewski and Antonio Mandarino and Marcin Markiewicz and Bianka Woloncewicz and Marek {\.{Z}}ukowski}, title = {Comment on `Single particle nonlocality with completely independent reference states'}, journal = {New Journal of Physics}, abstract = {We comment on the purported violation of local realism, by a single photon induced correlations between homodyne detectors, which one can find in: (2008) New J. Phys. 10 113024 [arXiv:0807.0109]. The claim is erroneous, due to a calculational mistake. As the result is the basis of other claims in the paper, they are unsubstantiated.}, urldate={2022-03-31} }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. Wave–particle complementarity: detecting violation of local realism with photon-number resolving weak-field homodyne measurements.
*New journal of physics*, 24(3):33017, 2022. doi:10.1088/1367-2630/ac54c8

[BibTeX] [Download PDF]`@article{das2022wave, title={Wave--particle complementarity: detecting violation of local realism with photon-number resolving weak-field homodyne measurements}, author={Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Woloncewicz, Bianka and {\.Z}ukowski, Marek}, journal={New Journal of Physics}, volume={24}, number={3}, pages={033017}, year={2022}, publisher={IOP Publishing}, url={https://doi.org/10.1088/1367-2630/ac54c8}, doi={10.1088/1367-2630/ac54c8}, urldate={2022-03-31} }`

### 2021

- Markus Grassl. Algebraic quantum codes: linking quantum mechanics and discrete mathematics.
*International Journal of Computer Mathematics: Computer Systems Theory*, 6(4):243-250, 2021. doi:10.1080/23799927.2020.1850530

[BibTeX] [Download PDF]`@Article{grassl_algebraic_2021, author = {Grassl, Markus}, journal = {International {J}ournal of {C}omputer {M}athematics: {C}omputer {S}ystems {T}heory}, title = {Algebraic quantum codes: linking quantum mechanics and discrete mathematics}, year = {2021}, issn = {2379-9927, 2379-9935}, month = dec, volume = {6}, number = {4}, pages = {243-250}, doi = {10.1080/23799927.2020.1850530}, language = {en}, shorttitle = {Algebraic quantum codes}, url = {https://www.tandfonline.com/doi/full/10.1080/23799927.2020.1850530}, urldate = {2021-05-10} }`

- Victoria J. Wright and Stefan Weigert. General Probabilistic Theories with a Gleason-type Theorem.
*Quantum*, 5:588, 2021. doi:10.22331/q-2021-11-25-588

[BibTeX] [Download PDF]`@article{Wright2021general, doi = {10.22331/q-2021-11-25-588}, url = {https://doi.org/10.22331/q-2021-11-25-588}, title = {General {P}robabilistic {T}heories with a {G}leason-type {T}heorem}, author = {Wright, Victoria J and Weigert, Stefan}, journal = {{Quantum}}, issn = {2521-327X}, publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}}, volume = {5}, pages = {588}, month = nov, year = {2021} }`

- Tomasz Miller, Michał Eckstein, Paweł Horodecki, and Ryszard Horodecki. Generally covariant N -particle dynamics.
*Journal of Geometry and Physics*, 160:103990, feb 2021. doi:10.1016/j.geomphys.2020.103990

[BibTeX] [Download PDF]`@Article{miller_generally_2021, author = {Miller, Tomasz and Eckstein, Michał and Horodecki, Paweł and Horodecki, Ryszard}, journal = {Journal of {G}eometry and {P}hysics}, title = {Generally covariant {N} -particle dynamics}, year = {2021}, issn = {03930440}, month = feb, pages = {103990}, volume = {160}, doi = {10.1016/j.geomphys.2020.103990}, groups = {Pawel_H, Michal_H}, language = {en}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0393044020302539}, urldate = {2021-05-10}, }`

- B. Ahmadi, S. Salimi, and A. S. Khorashad. Irreversible work and Maxwell demon in terms of quantum thermodynamic force.
*Scientific Reports*, 11(1):2301, dec 2021. doi:10.1038/s41598-021-81737-z

[BibTeX] [Abstract] [Download PDF]

Abstract The second law of classical equilibrium thermodynamics, based on the positivity of entropy production, asserts that any process occurs only in a direction that some information may be lost (flow out of the system) due to the irreversibility inside the system. However, any thermodynamic system can exhibit fluctuations in which negative entropy production may be observed. In particular, in stochastic quantum processes due to quantum correlations and also memory effects we may see the reversal energy flow (heat flow from the cold system to the hot system) and the backflow of information into the system that leads to the negativity of the entropy production which is an apparent violation of the Second Law. In order to resolve this apparent violation, we will try to properly extend the Second Law to quantum processes by incorporating information explicitly into the Second Law. We will also provide a thermodynamic operational meaning for the flow and backflow of information. Finally, it is shown that negative and positive entropy production can be described by a quantum thermodynamic force.

`@Article{ahmadi_irreversible_2021, author = {Ahmadi, B. and Salimi, S. and Khorashad, A. S.}, journal = {Scientific {R}eports}, title = {Irreversible work and {Maxwell} demon in terms of quantum thermodynamic force}, year = {2021}, issn = {2045-2322}, month = dec, number = {1}, pages = {2301}, volume = {11}, abstract = {Abstract The second law of classical equilibrium thermodynamics, based on the positivity of entropy production, asserts that any process occurs only in a direction that some information may be lost (flow out of the system) due to the irreversibility inside the system. However, any thermodynamic system can exhibit fluctuations in which negative entropy production may be observed. In particular, in stochastic quantum processes due to quantum correlations and also memory effects we may see the reversal energy flow (heat flow from the cold system to the hot system) and the backflow of information into the system that leads to the negativity of the entropy production which is an apparent violation of the Second Law. In order to resolve this apparent violation, we will try to properly extend the Second Law to quantum processes by incorporating information explicitly into the Second Law. We will also provide a thermodynamic operational meaning for the flow and backflow of information. Finally, it is shown that negative and positive entropy production can be described by a quantum thermodynamic force.}, doi = {10.1038/s41598-021-81737-z}, language = {en}, url = {http://www.nature.com/articles/s41598-021-81737-z}, urldate = {2021-05-10}, }`

- Qihao Guo, Yuan-Yuan Zhao, Markus Grassl, Xinfang Nie, Guo-Yong Xiang, Tao Xin, Zhang-Qi Yin, and Bei Zeng. Testing a quantum error-correcting code on various platforms.
*Science Bulletin*, 66(1):29–35, jan 2021. doi:10.1016/j.scib.2020.07.033

[BibTeX] [Download PDF]`@Article{guo_testing_2021, author = {Guo, Qihao and Zhao, Yuan-Yuan and Grassl, Markus and Nie, Xinfang and Xiang, Guo-Yong and Xin, Tao and Yin, Zhang-Qi and Zeng, Bei}, journal = {Science {B}ulletin}, title = {Testing a quantum error-correcting code on various platforms}, year = {2021}, issn = {20959273}, month = jan, number = {1}, pages = {29--35}, volume = {66}, doi = {10.1016/j.scib.2020.07.033}, language = {en}, url = {https://linkinghub.elsevier.com/retrieve/pii/S2095927320305168}, urldate = {2021-05-10}, }`

- Piotr Mironowicz, Gustavo Cañas, Jaime Cariñe, Esteban S. Gómez, Johanna F. Barra, Adán Cabello, Guilherme B. Xavier, Gustavo Lima, and Marcin Pawłowski. Quantum randomness protected against detection loophole attacks.
*Quantum Information Processing*, 20(1):39, jan 2021. doi:10.1007/s11128-020-02948-3

[BibTeX] [Download PDF]`@Article{mironowicz_quantum_2021, author = {Mironowicz, Piotr and Cañas, Gustavo and Cariñe, Jaime and Gómez, Esteban S. and Barra, Johanna F. and Cabello, Adán and Xavier, Guilherme B. and Lima, Gustavo and Pawłowski, Marcin}, journal = {Quantum {I}nformation {P}rocessing}, title = {Quantum randomness protected against detection loophole attacks}, year = {2021}, issn = {1570-0755, 1573-1332}, month = jan, number = {1}, pages = {39}, volume = {20}, doi = {10.1007/s11128-020-02948-3}, language = {en}, url = {http://link.springer.com/10.1007/s11128-020-02948-3}, urldate = {2021-05-10}, }`

- Matthias Christandl, Roberto Ferrara, and Karol Horodecki. Upper Bounds on Device-Independent Quantum Key Distribution.
*Physical Review Letters*, 126(16):160501, apr 2021. doi:10.1103/PhysRevLett.126.160501

[BibTeX] [Download PDF]`@Article{christandl_upper_2021, author = {Christandl, Matthias and Ferrara, Roberto and Horodecki, Karol}, journal = {Physical {R}eview {L}etters}, title = {Upper {Bounds} on {Device}-{Independent} {Quantum} {Key} {Distribution}}, year = {2021}, issn = {0031-9007, 1079-7114}, month = apr, number = {16}, pages = {160501}, volume = {126}, doi = {10.1103/PhysRevLett.126.160501}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.126.160501}, urldate = {2021-05-10}, }`

- Robert Alicki, David Gelbwaser-Klimovsky, Alejandro Jenkins, and Elizabeth von Hauff. Dynamical theory for the battery’s electromotive force.
*Physical Chemistry Chemical Physics*, 23(15):9428–9439, 2021. doi:10.1039/D1CP00196E

[BibTeX] [Abstract] [Download PDF]

We propose a dynamical theory of how the chemical energy stored in a battery generates the electromotive force (emf). , We propose a dynamical theory of how the chemical energy stored in a battery generates the electromotive force (emf). In this picture, the battery’s half-cell acts as an engine, cyclically extracting work from its underlying chemical disequilibrium. We show that the double layer at the electrode–electrolyte interface can exhibit a rapid self-oscillation that pumps an electric current, thus accounting for the persistent conversion of chemical energy into electrical work equal to the emf times the separated charge. We suggest a connection between this mechanism and the slow self-oscillations observed in various electrochemical cells, including batteries, as well as the enhancement of the current observed when ultrasound is applied to the half-cell. Finally, we propose more direct experimental tests of the predictions of this dynamical theory.

`@Article{alicki_dynamical_2021, author = {Alicki, Robert and Gelbwaser-Klimovsky, David and Jenkins, Alejandro and von Hauff, Elizabeth}, journal = {Physical {C}hemistry {C}hemical {P}hysics}, title = {Dynamical theory for the battery's electromotive force}, year = {2021}, issn = {1463-9076, 1463-9084}, number = {15}, pages = {9428--9439}, volume = {23}, abstract = {We propose a dynamical theory of how the chemical energy stored in a battery generates the electromotive force (emf). , We propose a dynamical theory of how the chemical energy stored in a battery generates the electromotive force (emf). In this picture, the battery's half-cell acts as an engine, cyclically extracting work from its underlying chemical disequilibrium. We show that the double layer at the electrode–electrolyte interface can exhibit a rapid self-oscillation that pumps an electric current, thus accounting for the persistent conversion of chemical energy into electrical work equal to the emf times the separated charge. We suggest a connection between this mechanism and the slow self-oscillations observed in various electrochemical cells, including batteries, as well as the enhancement of the current observed when ultrasound is applied to the half-cell. Finally, we propose more direct experimental tests of the predictions of this dynamical theory.}, doi = {10.1039/D1CP00196E}, language = {en}, url = {http://xlink.rsc.org/?DOI=D1CP00196E}, urldate = {2021-05-10}, }`

- Marek Żukowski and Marcin Markiewicz. Physics and Metaphysics of Wigner’s Friends: Even Performed Premeasurements Have No Results.
*Physical Review Letters*, 126(13):130402, apr 2021. doi:10.1103/PhysRevLett.126.130402

[BibTeX] [Download PDF]`@Article{zukowski_physics_2021, author = {Żukowski, Marek and Markiewicz, Marcin}, journal = {Physical {R}eview {L}etters}, title = {Physics and {Metaphysics} of {Wigner}’s {Friends}: {Even} {Performed} {Premeasurements} {Have} {No} {Results}}, year = {2021}, issn = {0031-9007, 1079-7114}, month = apr, number = {13}, pages = {130402}, volume = {126}, doi = {10.1103/PhysRevLett.126.130402}, language = {en}, shorttitle = {Physics and {Metaphysics} of {Wigner}’s {Friends}}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.126.130402}, urldate = {2021-05-10}, }`

- John H. Selby, Carlo Maria Scandolo, and Bob Coecke. Reconstructing quantum theory from diagrammatic postulates.
*Quantum*, 5:445, apr 2021. doi:10.22331/q-2021-04-28-445

[BibTeX] [Abstract] [Download PDF]

A reconstruction of quantum theory refers to both a mathematical and a conceptual paradigm that allows one to derive the usual formulation of quantum theory from a set of primitive assumptions. The motivation for doing so is a discomfort with the usual formulation of quantum theory, a discomfort that started with its originator John von Neumann. We present a reconstruction of finite-dimensional quantum theory where all of the postulates are stated in diagrammatic terms, making them intuitive. Equivalently, they are stated in category-theoretic terms, making them mathematically appealing. Again equivalently, they are stated in process-theoretic terms, establishing that the conceptual backbone of quantum theory concerns the manner in which systems and processes compose. Aside from the diagrammatic form, the key novel aspect of this reconstruction is the introduction of a new postulate, symmetric purification. Unlike the ordinary purification postulate, symmetric purification applies equally well to classical theory as well as quantum theory. Therefore we first reconstruct the full process theoretic description of quantum theory, consisting of composite classical-quantum systems and their interactions, before restricting ourselves to just the ‘fully quantum’ systems as the final step. We propose two novel alternative manners of doing so, ‘no-leaking’ (roughly that information gain causes disturbance) and ‘purity of cups’ (roughly the existence of entangled states). Interestingly, these turn out to be equivalent in any process theory with cups & caps. Additionally, we show how the standard purification postulate can be seen as an immediate consequence of the symmetric purification postulate and purity of cups. Other tangential results concern the specific frameworks of generalised probabilistic theories (GPTs) and process theories (a.k.a. CQM). Firstly, we provide a diagrammatic presentation of GPTs, which, henceforth, can be subsumed under process theories. Secondly, we argue that the ‘sharp dagger’ is indeed the right choice of a dagger structure as this sharpness is vital to the reconstruction.

`@article{selby_reconstructing_2021, title = {Reconstructing quantum theory from diagrammatic postulates}, volume = {5}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2021-04-28-445/}, doi = {10.22331/q-2021-04-28-445}, abstract = {A reconstruction of quantum theory refers to both a mathematical and a conceptual paradigm that allows one to derive the usual formulation of quantum theory from a set of primitive assumptions. The motivation for doing so is a discomfort with the usual formulation of quantum theory, a discomfort that started with its originator John von Neumann. We present a reconstruction of finite-dimensional quantum theory where all of the postulates are stated in diagrammatic terms, making them intuitive. Equivalently, they are stated in category-theoretic terms, making them mathematically appealing. Again equivalently, they are stated in process-theoretic terms, establishing that the conceptual backbone of quantum theory concerns the manner in which systems and processes compose. Aside from the diagrammatic form, the key novel aspect of this reconstruction is the introduction of a new postulate, symmetric purification. Unlike the ordinary purification postulate, symmetric purification applies equally well to classical theory as well as quantum theory. Therefore we first reconstruct the full process theoretic description of quantum theory, consisting of composite classical-quantum systems and their interactions, before restricting ourselves to just the ‘fully quantum’ systems as the final step. We propose two novel alternative manners of doing so, ‘no-leaking’ (roughly that information gain causes disturbance) and ‘purity of cups’ (roughly the existence of entangled states). Interestingly, these turn out to be equivalent in any process theory with cups \& caps. Additionally, we show how the standard purification postulate can be seen as an immediate consequence of the symmetric purification postulate and purity of cups. Other tangential results concern the specific frameworks of generalised probabilistic theories (GPTs) and process theories (a.k.a. CQM). Firstly, we provide a diagrammatic presentation of GPTs, which, henceforth, can be subsumed under process theories. Secondly, we argue that the ‘sharp dagger’ is indeed the right choice of a dagger structure as this sharpness is vital to the reconstruction.}, language = {en}, urldate = {2021-05-10}, journal = {Quantum}, author = {Selby, John H. and Scandolo, Carlo Maria and Coecke, Bob}, month = apr, year = {2021}, pages = {445}, }`

- H. S. Karthik, H. Akshata Shenoy, and Usha A. R. Devi. Leggett-Garg inequalities and temporal correlations for a qubit under PT -symmetric dynamics.
*Physical Review A*, 103(3):32420, mar 2021. doi:10.1103/PhysRevA.103.032420

[BibTeX] [Download PDF]`@Article{karthik_leggett-garg_2021, author = {Karthik, H. S. and Akshata Shenoy, H. and Devi, A. R. Usha}, journal = {Physical {R}eview {A}}, title = {Leggett-{Garg} inequalities and temporal correlations for a qubit under {PT} -symmetric dynamics}, year = {2021}, issn = {2469-9926, 2469-9934}, month = mar, number = {3}, pages = {032420}, volume = {103}, doi = {10.1103/PhysRevA.103.032420}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.103.032420}, urldate = {2021-05-10}, }`

- Nikolai Miklin and Michał Oszmaniec. A universal scheme for robust self-testing in the prepare-and-measure scenario.
*Quantum*, 5:424, apr 2021. doi:10.22331/q-2021-04-06-424

[BibTeX] [Abstract] [Download PDF]

We consider the problem of certification of arbitrary ensembles of pure states and projective measurements solely from the experimental statistics in the prepare-and-measure scenario assuming the upper bound on the dimension of the Hilbert space. To this aim, we propose a universal and intuitive scheme based on establishing perfect correlations between target states and suitably-chosen projective measurements. The method works in all finite dimensions and allows for robust certification of the overlaps between arbitrary preparation states and between the corresponding measurement operators. Finally, we prove that for qubits, our technique can be used to robustly self-test arbitrary configurations of pure quantum states and projective measurements. These results pave the way towards the practical application of the prepare-and-measure paradigm to certification of quantum devices.

`@article{miklin_universal_2021, title = {A universal scheme for robust self-testing in the prepare-and-measure scenario}, volume = {5}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2021-04-06-424/}, doi = {10.22331/q-2021-04-06-424}, abstract = {We consider the problem of certification of arbitrary ensembles of pure states and projective measurements solely from the experimental statistics in the prepare-and-measure scenario assuming the upper bound on the dimension of the Hilbert space. To this aim, we propose a universal and intuitive scheme based on establishing perfect correlations between target states and suitably-chosen projective measurements. The method works in all finite dimensions and allows for robust certification of the overlaps between arbitrary preparation states and between the corresponding measurement operators. Finally, we prove that for qubits, our technique can be used to robustly self-test arbitrary configurations of pure quantum states and projective measurements. These results pave the way towards the practical application of the prepare-and-measure paradigm to certification of quantum devices.}, language = {en}, urldate = {2021-05-10}, journal = {Quantum}, author = {Miklin, Nikolai and Oszmaniec, Michał}, month = apr, year = {2021}, pages = {424}, }`

- David Schmid, John H. Selby, Elie Wolfe, Ravi Kunjwal, and Robert W. Spekkens. Characterization of Noncontextuality in the Framework of Generalized Probabilistic Theories.
*PRX Quantum*, 2(1):10331, feb 2021. doi:10.1103/PRXQuantum.2.010331

[BibTeX] [Download PDF]`@Article{schmid_characterization_2021, author = {Schmid, David and Selby, John H. and Wolfe, Elie and Kunjwal, Ravi and Spekkens, Robert W.}, journal = {P{RX} {Q}uantum}, title = {Characterization of {Noncontextuality} in the {Framework} of {Generalized} {Probabilistic} {Theories}}, year = {2021}, issn = {2691-3399}, month = feb, number = {1}, pages = {010331}, volume = {2}, doi = {10.1103/PRXQuantum.2.010331}, language = {en}, url = {https://link.aps.org/doi/10.1103/PRXQuantum.2.010331}, urldate = {2021-05-10}, }`

- Patryk Lipka-Bartosik, Paweł Mazurek, and Michał Horodecki. Second law of thermodynamics for batteries with vacuum state.
*Quantum*, 5:408, mar 2021. doi:10.22331/q-2021-03-10-408

[BibTeX] [Abstract] [Download PDF]

In stochastic thermodynamics work is a random variable whose average is bounded by the change in the free energy of the system. In most treatments, however, the work reservoir that absorbs this change is either tacitly assumed or modelled using unphysical systems with unbounded Hamiltonians (i.e. the ideal weight). In this work we describe the consequences of introducing the ground state of the battery and hence — of breaking its translational symmetry. The most striking consequence of this shift is the fact that the Jarzynski identity is replaced by a family of inequalities. Using these inequalities we obtain corrections to the second law of thermodynamics which vanish exponentially with the distance of the initial state of the battery to the bottom of its spectrum. Finally, we study an exemplary thermal operation which realizes the approximate Landauer erasure and demonstrate the consequences which arise when the ground state of the battery is explicitly introduced. In particular, we show that occupation of the vacuum state of any physical battery sets a lower bound on fluctuations of work, while batteries without vacuum state allow for fluctuation-free erasure.

`@Article{lipka-bartosik_second_2021, author = {Lipka-Bartosik, Patryk and Mazurek, Paweł and Horodecki, Michał}, journal = {Quantum}, title = {Second law of thermodynamics for batteries with vacuum state}, year = {2021}, issn = {2521-327X}, month = mar, pages = {408}, volume = {5}, abstract = {In stochastic thermodynamics work is a random variable whose average is bounded by the change in the free energy of the system. In most treatments, however, the work reservoir that absorbs this change is either tacitly assumed or modelled using unphysical systems with unbounded Hamiltonians (i.e. the ideal weight). In this work we describe the consequences of introducing the ground state of the battery and hence — of breaking its translational symmetry. The most striking consequence of this shift is the fact that the Jarzynski identity is replaced by a family of inequalities. Using these inequalities we obtain corrections to the second law of thermodynamics which vanish exponentially with the distance of the initial state of the battery to the bottom of its spectrum. Finally, we study an exemplary thermal operation which realizes the approximate Landauer erasure and demonstrate the consequences which arise when the ground state of the battery is explicitly introduced. In particular, we show that occupation of the vacuum state of any physical battery sets a lower bound on fluctuations of work, while batteries without vacuum state allow for fluctuation-free erasure.}, doi = {10.22331/q-2021-03-10-408}, groups = {Michal_H}, language = {en}, url = {https://quantum-journal.org/papers/q-2021-03-10-408/}, urldate = {2021-05-10}, }`

- Aaron Z. Goldberg, Pablo de la Hoz, Gunnar Björk, Andrei B. Klimov, Markus Grassl, Gerd Leuchs, and Luis L. Sánchez-Soto. Quantum concepts in optical polarization.
*Advances in Optics and Photonics*, 13(1):1, mar 2021. doi:10.1364/AOP.404175

[BibTeX] [Download PDF]`@Article{goldberg_quantum_2021, author = {Goldberg, Aaron Z. and de la Hoz, Pablo and Björk, Gunnar and Klimov, Andrei B. and Grassl, Markus and Leuchs, Gerd and Sánchez-Soto, Luis L.}, journal = {Advances in {O}ptics and {P}hotonics}, title = {Quantum concepts in optical polarization}, year = {2021}, issn = {1943-8206}, month = mar, number = {1}, pages = {1}, volume = {13}, doi = {10.1364/AOP.404175}, language = {en}, url = {https://www.osapublishing.org/abstract.cfm?URI=aop-13-1-1}, urldate = {2021-05-10}, }`

- Roope Uola, Tristan Kraft, Sébastien Designolle, Nikolai Miklin, Armin Tavakoli, Juha-Pekka Pellonpää, Otfried Gühne, and Nicolas Brunner. Quantum measurement incompatibility in subspaces.
*Physical Review A*, 103(2):22203, feb 2021. doi:10.1103/PhysRevA.103.022203

[BibTeX] [Download PDF]`@Article{uola_quantum_2021, author = {Uola, Roope and Kraft, Tristan and Designolle, Sébastien and Miklin, Nikolai and Tavakoli, Armin and Pellonpää, Juha-Pekka and Gühne, Otfried and Brunner, Nicolas}, journal = {Physical {R}eview {A}}, title = {Quantum measurement incompatibility in subspaces}, year = {2021}, issn = {2469-9926, 2469-9934}, month = feb, number = {2}, pages = {022203}, volume = {103}, doi = {10.1103/PhysRevA.103.022203}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.103.022203}, urldate = {2021-05-10}, }`

- Armin Tavakoli, Máté. Farkas, Denis Rosset, Jean-Daniel Bancal, and Jedrzej Kaniewski. Mutually unbiased bases and symmetric informationally complete measurements in Bell experiments.
*Science Advances*, 7(7):eabc3847, feb 2021. doi:10.1126/sciadv.abc3847

[BibTeX] [Abstract] [Download PDF]

Mutually unbiased bases (MUBs) and symmetric informationally complete projectors (SICs) are crucial to many conceptual and practical aspects of quantum theory. Here, we develop their role in quantum nonlocality by (i) introducing families of Bell inequalities that are maximally violated by d-dimensional MUBs and SICs, respectively, (ii) proving device-independent certification of natural operational notions of MUBs and SICs, and (iii) using MUBs and SICs to develop optimal-rate and nearly optimal-rate protocols for device-independent quantum key distribution and device-independent quantum random number generation, respectively. Moreover, we also present the first example of an extremal point of the quantum set of correlations that admits physically inequivalent quantum realizations. Our results elaborately demonstrate the foundational and practical relevance of the two most important discrete Hilbert space structures to the field of quantum nonlocality.

`@Article{tavakoli_mutually_2021, author = {Tavakoli, Armin and Farkas, Máté and Rosset, Denis and Bancal, Jean-Daniel and Kaniewski, Jedrzej}, journal = {Science {A}dvances}, title = {Mutually unbiased bases and symmetric informationally complete measurements in {Bell} experiments}, year = {2021}, issn = {2375-2548}, month = feb, number = {7}, pages = {eabc3847}, volume = {7}, abstract = {Mutually unbiased bases (MUBs) and symmetric informationally complete projectors (SICs) are crucial to many conceptual and practical aspects of quantum theory. Here, we develop their role in quantum nonlocality by (i) introducing families of Bell inequalities that are maximally violated by d-dimensional MUBs and SICs, respectively, (ii) proving device-independent certification of natural operational notions of MUBs and SICs, and (iii) using MUBs and SICs to develop optimal-rate and nearly optimal-rate protocols for device-independent quantum key distribution and device-independent quantum random number generation, respectively. Moreover, we also present the first example of an extremal point of the quantum set of correlations that admits physically inequivalent quantum realizations. Our results elaborately demonstrate the foundational and practical relevance of the two most important discrete Hilbert space structures to the field of quantum nonlocality.}, doi = {10.1126/sciadv.abc3847}, language = {en}, url = {https://advances.sciencemag.org/lookup/doi/10.1126/sciadv.abc3847}, urldate = {2021-05-10}, }`

- Máté. Farkas, Nayda Guerrero, Jaime Cariñe, Gustavo Cañas, and Gustavo Lima. Self-Testing Mutually Unbiased Bases in Higher Dimensions with Space-Division Multiplexing Optical Fiber Technology.
*Physical Review Applied*, 15(1):14028, jan 2021. doi:10.1103/PhysRevApplied.15.014028

[BibTeX] [Download PDF]`@Article{farkas_self-testing_2021, author = {Farkas, Máté and Guerrero, Nayda and Cariñe, Jaime and Cañas, Gustavo and Lima, Gustavo}, journal = {Physical {R}eview {A}pplied}, title = {Self-{Testing} {Mutually} {Unbiased} {Bases} in {Higher} {Dimensions} with {Space}-{Division} {Multiplexing} {Optical} {Fiber} {Technology}}, year = {2021}, issn = {2331-7019}, month = jan, number = {1}, pages = {014028}, volume = {15}, doi = {10.1103/PhysRevApplied.15.014028}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevApplied.15.014028}, urldate = {2021-05-10}, }`

- Konrad Schlichtholz, Bianka Woloncewicz, and Marek Żukowski. Nonclassicality of bright Greenberger-Horne-Zeilinger–like radiation of an optical parametric source.
*Physical Review A*, 103(4):42226, apr 2021. doi:10.1103/PhysRevA.103.042226

[BibTeX] [Download PDF]`@Article{schlichtholz_nonclassicality_2021, author = {Schlichtholz, Konrad and Woloncewicz, Bianka and Żukowski, Marek}, journal = {Physical {R}eview {A}}, title = {Nonclassicality of bright {Greenberger}-{Horne}-{Zeilinger}–like radiation of an optical parametric source}, year = {2021}, issn = {2469-9926, 2469-9934}, month = apr, number = {4}, pages = {042226}, volume = {103}, doi = {10.1103/PhysRevA.103.042226}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.103.042226}, urldate = {2021-07-28}, }`

- Roberto Salazar, Tanmoy Biswas, Jakub Czartowski, Karol Życzkowski, and Paweł Horodecki. Optimal allocation of quantum resources.
*Quantum*, 5:407, mar 2021. doi:10.22331/q-2021-03-10-407

[BibTeX] [Abstract] [Download PDF]

The optimal allocation of resources is a crucial task for their efficient use in a wide range of practical applications in science and engineering. This paper investigates the optimal allocation of resources in multipartite quantum systems. In particular, we show the relevance of proportional fairness and optimal reliability criteria for the application of quantum resources. Moreover, we present optimal allocation solutions for an arbitrary number of qudits using measurement incompatibility as an exemplary resource theory. Besides, we study the criterion of optimal equitability and demonstrate its relevance to scenarios involving several resource theories such as nonlocality vs local contextuality. Finally, we highlight the potential impact of our results for quantum networks and other multi-party quantum information processing, in particular to the future Quantum Internet.

`@Article{salazar_optimal_2021, author = {Salazar, Roberto and Biswas, Tanmoy and Czartowski, Jakub and Życzkowski, Karol and Horodecki, Paweł}, journal = {Quantum}, title = {Optimal allocation of quantum resources}, year = {2021}, issn = {2521-327X}, month = mar, pages = {407}, volume = {5}, abstract = {The optimal allocation of resources is a crucial task for their efficient use in a wide range of practical applications in science and engineering. This paper investigates the optimal allocation of resources in multipartite quantum systems. In particular, we show the relevance of proportional fairness and optimal reliability criteria for the application of quantum resources. Moreover, we present optimal allocation solutions for an arbitrary number of qudits using measurement incompatibility as an exemplary resource theory. Besides, we study the criterion of optimal equitability and demonstrate its relevance to scenarios involving several resource theories such as nonlocality vs local contextuality. Finally, we highlight the potential impact of our results for quantum networks and other multi-party quantum information processing, in particular to the future Quantum Internet.}, doi = {10.22331/q-2021-03-10-407}, groups = {Pawel_H}, language = {en}, url = {https://quantum-journal.org/papers/q-2021-03-10-407/}, urldate = {2021-07-28}, }`

- Chandan Datta, Tanmoy Biswas, Debashis Saha, and Remigiusz Augusiak. Perfect discrimination of quantum measurements using entangled systems.
*New Journal of Physics*, 23(4):43021, apr 2021. doi:10.1088/1367-2630/abecaf

[BibTeX] [Download PDF]`@Article{datta_perfect_2021, author = {Datta, Chandan and Biswas, Tanmoy and Saha, Debashis and Augusiak, Remigiusz}, journal = {New {J}ournal of {P}hysics}, title = {Perfect discrimination of quantum measurements using entangled systems}, year = {2021}, issn = {1367-2630}, month = apr, number = {4}, pages = {043021}, volume = {23}, doi = {10.1088/1367-2630/abecaf}, url = {https://iopscience.iop.org/article/10.1088/1367-2630/abecaf}, urldate = {2021-07-28}, }`

- R. Horodecki. Quantum Information.
*Acta Physica Polonica A*, 139(3):197–2018, mar 2021. doi:10.12693/APhysPolA.139.197

[BibTeX] [Download PDF]`@Article{horodecki_quantum_2021, author = {Horodecki, R.}, journal = {Acta {P}hysica {P}olonica {A}}, title = {Quantum {Information}}, year = {2021}, issn = {1898-794X, 0587-4246}, month = mar, number = {3}, pages = {197--2018}, volume = {139}, doi = {10.12693/APhysPolA.139.197}, url = {http://przyrbwn.icm.edu.pl/APP/PDF/139/app139z3p01.pdf}, urldate = {2021-07-28}, }`

- Marcin Wieśniak. Symmetrized persistency of Bell correlations for Dicke states and GHZ-based mixtures.
*Scientific Reports*, 11(1):14333, dec 2021. doi:10.1038/s41598-021-93786-5

[BibTeX] [Abstract] [Download PDF]

Abstract Quantum correlations, in particular those, which enable to violate a Bell inequality, open a way to advantage in certain communication tasks. However, the main difficulty in harnessing quantumness is its fragility to, e.g, noise or loss of particles. We study the persistency of Bell correlations of GHZ based mixtures and Dicke states. For the former, we consider quantum communication complexity reduction (QCCR) scheme, and propose new Bell inequalities (BIs), which can be used in that scheme for higher persistency in the limit of large number of particles N . In case of Dicke states, we show that persistency can reach 0.482 N , significantly more than reported in previous studies.

`@Article{wiesniak_symmetrized_2021, author = {Wieśniak, Marcin}, journal = {Scientific {R}eports}, title = {Symmetrized persistency of {Bell} correlations for {Dicke} states and {GHZ}-based mixtures}, year = {2021}, issn = {2045-2322}, month = dec, number = {1}, pages = {14333}, volume = {11}, abstract = {Abstract Quantum correlations, in particular those, which enable to violate a Bell inequality, open a way to advantage in certain communication tasks. However, the main difficulty in harnessing quantumness is its fragility to, e.g, noise or loss of particles. We study the persistency of Bell correlations of GHZ based mixtures and Dicke states. For the former, we consider quantum communication complexity reduction (QCCR) scheme, and propose new Bell inequalities (BIs), which can be used in that scheme for higher persistency in the limit of large number of particles N . In case of Dicke states, we show that persistency can reach 0.482 N , significantly more than reported in previous studies.}, doi = {10.1038/s41598-021-93786-5}, language = {en}, url = {http://www.nature.com/articles/s41598-021-93786-5}, urldate = {2021-07-28}, }`

- K. Anjali, Akshata Shenoy Hejamadi, H. S. Karthik, S. Sahu, Sudha, and Usha A. R. Devi. Characterizing nonlocality of pure symmetric three-qubit states.
*Quantum Information Processing*, 20(5):187, may 2021. doi:10.1007/s11128-021-03124-x

[BibTeX] [Download PDF]`@Article{anjali_characterizing_2021, author = {Anjali, K. and Hejamadi, Akshata Shenoy and Karthik, H. S. and Sahu, S. and {Sudha} and Devi, A. R. Usha}, journal = {Quantum {I}nformation {P}rocessing}, title = {Characterizing nonlocality of pure symmetric three-qubit states}, year = {2021}, issn = {1570-0755, 1573-1332}, month = may, number = {5}, pages = {187}, volume = {20}, doi = {10.1007/s11128-021-03124-x}, language = {en}, url = {https://link.springer.com/10.1007/s11128-021-03124-x}, urldate = {2021-07-28}, }`

- Michał Banacki, Ricard Ravell Rodríguez, and Paweł Horodecki. On the edge of the set of no-signaling assemblages.
*Physical Review A*, 103(5):52434, may 2021. arXiv: 2008.12325 doi:10.1103/PhysRevA.103.052434

[BibTeX] [Abstract] [Download PDF]

Following recent advancements, we consider a scenario of multipartite postquantum steering and general no-signaling assemblages. We introduce the notion of the edge of the set of no-signaling assemblages and we present its characterization. Next, we use this concept to construct witnesses for no-signaling assemblages without an LHS model. Finally, in the simplest nontrivial case of steering with two untrusted subsystems, we discuss the possibility of quantum realization of assemblages on the edge. In particular, for three-qubit states, we obtain a no-go type result, which states that it is impossible to produce assemblage on the edge using measurements described by POVMs as long as the rank of a given state is greater than or equal to 3.

`@Article{banacki_edge_2021, author = {Banacki, Michał and Rodríguez, Ricard Ravell and Horodecki, Paweł}, journal = {Physical {R}eview {A}}, title = {On the edge of the set of no-signaling assemblages}, year = {2021}, issn = {2469-9926, 2469-9934}, month = may, note = {arXiv: 2008.12325}, number = {5}, pages = {052434}, volume = {103}, abstract = {Following recent advancements, we consider a scenario of multipartite postquantum steering and general no-signaling assemblages. We introduce the notion of the edge of the set of no-signaling assemblages and we present its characterization. Next, we use this concept to construct witnesses for no-signaling assemblages without an LHS model. Finally, in the simplest nontrivial case of steering with two untrusted subsystems, we discuss the possibility of quantum realization of assemblages on the edge. In particular, for three-qubit states, we obtain a no-go type result, which states that it is impossible to produce assemblage on the edge using measurements described by POVMs as long as the rank of a given state is greater than or equal to 3.}, doi = {10.1103/PhysRevA.103.052434}, groups = {Pawel_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2008.12325}, urldate = {2021-07-28}, }`

- Stefano Cusumano and Łukasz Rudnicki. Comment on “Fluctuations in Extractable Work Bound the Charging Power of Quantum Batteries”.
*Physical Review Letters*, 127(2):28901, jul 2021. arXiv: 2102.05627 doi:10.1103/PhysRevLett.127.028901

[BibTeX] [Abstract] [Download PDF]

In the abstract of{\textasciitilde}[Phys. Rev. Lett. \{{\textbackslash}bf 125\}, 040601 (2020)] one can read that: […]\{{\textbackslash}it to have a nonzero rate of change of the extractable work, the state \${\textbackslash}rho_{\textbackslash}mathcal\{W\}\$ of the battery cannot be an eigenstate of a “free energy operator”, defined by \${\textbackslash}mathcal\{F\}=H_{\textbackslash}mathcal\{W\}+{\textbackslash}beta{\textasciicircum}\{-1\}{\textbackslash}log {\textbackslash}rho_{\textbackslash}mathcal\{W\}\$, where \$H_{\textbackslash}mathcal\{W\}\$ is the Hamiltonian of the battery and \${\textbackslash}beta\$ is the inverse temperature\} […]. Contrarily to what is presented below Eq.{\textasciitilde}(17) of the paper, we observe that the above conclusion does not hold when the battery is subject to nonunitary dynamics.

`@Article{cusumano_comment_2021, author = {Cusumano, Stefano and Rudnicki, Łukasz}, journal = {Physical {R}eview {L}etters}, title = {Comment on "{Fluctuations} in {Extractable} {Work} {Bound} the {Charging} {Power} of {Quantum} {Batteries}"}, year = {2021}, issn = {0031-9007, 1079-7114}, month = jul, note = {arXiv: 2102.05627}, number = {2}, pages = {028901}, volume = {127}, abstract = {In the abstract of{\textasciitilde}[Phys. Rev. Lett. \{{\textbackslash}bf 125\}, 040601 (2020)] one can read that: [...]\{{\textbackslash}it to have a nonzero rate of change of the extractable work, the state \${\textbackslash}rho\_{\textbackslash}mathcal\{W\}\$ of the battery cannot be an eigenstate of a "free energy operator", defined by \${\textbackslash}mathcal\{F\}=H\_{\textbackslash}mathcal\{W\}+{\textbackslash}beta{\textasciicircum}\{-1\}{\textbackslash}log {\textbackslash}rho\_{\textbackslash}mathcal\{W\}\$, where \$H\_{\textbackslash}mathcal\{W\}\$ is the Hamiltonian of the battery and \${\textbackslash}beta\$ is the inverse temperature\} [...]. Contrarily to what is presented below Eq.{\textasciitilde}(17) of the paper, we observe that the above conclusion does not hold when the battery is subject to nonunitary dynamics.}, doi = {10.1103/PhysRevLett.127.028901}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2102.05627}, urldate = {2021-07-28}, }`

- Nikolai Miklin and Marcin Pawłowski. Information Causality without concatenation.
*Physical Review Letters*, 126(22):220403, jun 2021. arXiv: 2101.12710 doi:10.1103/PhysRevLett.126.220403

[BibTeX] [Abstract] [Download PDF]

Information Causality is a physical principle which states that the amount of randomly accessible data over a classical communication channel cannot exceed its capacity, even if the sender and the receiver have access to a source of nonlocal correlations. This principle can be used to bound the nonlocality of quantum mechanics without resorting to its full formalism, with a notable example of reproducing the Tsirelson’s bound of the Clauser-Horne-Shimony-Holt inequality. Despite being promising, the latter result found little generalization to other Bell inequalities because of the limitations imposed by the process of concatenation, in which several nonsignaling resources are put together to produce tighter bounds. In this work, we show that concatenation can be successfully replaced by limits on the communication channel capacity. It allows us to re-derive and, in some cases, significantly improve all the previously known results in a simpler manner and apply the Information Causality principle to previously unapproachable Bell scenarios.

`@Article{miklin_information_2021, author = {Miklin, Nikolai and Pawłowski, Marcin}, journal = {Physical {R}eview {L}etters}, title = {Information {Causality} without concatenation}, year = {2021}, issn = {0031-9007, 1079-7114}, month = jun, note = {arXiv: 2101.12710}, number = {22}, pages = {220403}, volume = {126}, abstract = {Information Causality is a physical principle which states that the amount of randomly accessible data over a classical communication channel cannot exceed its capacity, even if the sender and the receiver have access to a source of nonlocal correlations. This principle can be used to bound the nonlocality of quantum mechanics without resorting to its full formalism, with a notable example of reproducing the Tsirelson's bound of the Clauser-Horne-Shimony-Holt inequality. Despite being promising, the latter result found little generalization to other Bell inequalities because of the limitations imposed by the process of concatenation, in which several nonsignaling resources are put together to produce tighter bounds. In this work, we show that concatenation can be successfully replaced by limits on the communication channel capacity. It allows us to re-derive and, in some cases, significantly improve all the previously known results in a simpler manner and apply the Information Causality principle to previously unapproachable Bell scenarios.}, doi = {10.1103/PhysRevLett.126.220403}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2101.12710}, urldate = {2021-07-28}, }`

- Shin-Liang Chen, Nikolai Miklin, Costantino Budroni, and Yueh-Nan Chen. Device-independent quantification of measurement incompatibility.
*Physical Review Research*, 3(2):23143, may 2021. arXiv: 2010.08456 doi:10.1103/PhysRevResearch.3.023143

[BibTeX] [Abstract] [Download PDF]

Incompatible measurements, i.e., measurements that cannot be simultaneously performed, are necessary to observe nonlocal correlations. It is natural to ask, e.g., how incompatible the measurements have to be to achieve a certain violation of a Bell inequality. In this work, we provide the direct link between Bell nonlocality and the quantification of measurement incompatibility. This includes quantifiers for both incompatible and genuine-multipartite incompatible measurements. Our method straightforwardly generalizes to include constraints on the system’s dimension (semi-device-independent approach) and on projective measurements, providing improved bounds on incompatibility quantifiers, and to include the prepare-and-measure scenario.

`@Article{chen_device-independent_2021, author = {Chen, Shin-Liang and Miklin, Nikolai and Budroni, Costantino and Chen, Yueh-Nan}, journal = {Physical {R}eview {R}esearch}, title = {Device-independent quantification of measurement incompatibility}, year = {2021}, issn = {2643-1564}, month = may, note = {arXiv: 2010.08456}, number = {2}, pages = {023143}, volume = {3}, abstract = {Incompatible measurements, i.e., measurements that cannot be simultaneously performed, are necessary to observe nonlocal correlations. It is natural to ask, e.g., how incompatible the measurements have to be to achieve a certain violation of a Bell inequality. In this work, we provide the direct link between Bell nonlocality and the quantification of measurement incompatibility. This includes quantifiers for both incompatible and genuine-multipartite incompatible measurements. Our method straightforwardly generalizes to include constraints on the system's dimension (semi-device-independent approach) and on projective measurements, providing improved bounds on incompatibility quantifiers, and to include the prepare-and-measure scenario.}, doi = {10.1103/PhysRevResearch.3.023143}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2010.08456}, urldate = {2021-07-28}, }`

- Markus Grassl. Entanglement-Assisted Quantum Communication Beating the Quantum Singleton Bound.
*Physical Review A*, 103(2):L020601, feb 2021. arXiv: 2007.01249 doi:10.1103/PhysRevA.103.L020601

[BibTeX] [Abstract] [Download PDF]

Brun, Devetak, and Hsieh [Science 314, 436 (2006)] demonstrated that pre-shared entanglement between sender and receiver enables quantum communication protocols that have better parameters than schemes without the assistance of entanglement. Subsequently, the same authors derived a version of the so-called quantum Singleton bound that relates the parameters of the entanglement-assisted quantum-error correcting codes proposed by them. We present a new entanglement-assisted quantum communication scheme with parameters violating this bound in certain ranges.

`@Article{grassl_entanglement-assisted_2021, author = {Grassl, Markus}, journal = {Physical {R}eview {A}}, title = {Entanglement-{Assisted} {Quantum} {Communication} {Beating} the {Quantum} {Singleton} {Bound}}, year = {2021}, issn = {2469-9926, 2469-9934}, month = feb, note = {arXiv: 2007.01249}, number = {2}, pages = {L020601}, volume = {103}, abstract = {Brun, Devetak, and Hsieh [Science 314, 436 (2006)] demonstrated that pre-shared entanglement between sender and receiver enables quantum communication protocols that have better parameters than schemes without the assistance of entanglement. Subsequently, the same authors derived a version of the so-called quantum Singleton bound that relates the parameters of the entanglement-assisted quantum-error correcting codes proposed by them. We present a new entanglement-assisted quantum communication scheme with parameters violating this bound in certain ranges.}, doi = {10.1103/PhysRevA.103.L020601}, keywords = {Quantum Physics, Computer Science - Information Theory}, url = {http://arxiv.org/abs/2007.01249}, urldate = {2021-07-28}, }`

- Wooyeong Song, Youngrong Lim, Hyukjoon Kwon, Gerardo Adesso, Marcin Wieśniak, Marcin Pawłowski, Jaewan Kim, and Jeongho Bang. Quantum secure learning with classical samples.
*Physical Review A*, 103(4):42409, apr 2021. arXiv: 1912.10594 doi:10.1103/PhysRevA.103.042409

[BibTeX] [Abstract] [Download PDF]

Studies addressing the question “Can a learner complete the learning securely?” have recently been spurred from the standpoints of fundamental theory and potential applications. In the relevant context of this question, we present a classical-quantum hybrid sampling protocol and define a security condition that allows only legitimate learners to prepare a finite set of samples that guarantees the success of the learning; the security condition excludes intruders. We do this by combining our security concept with the bound of the so-called probably approximately correct (PAC) learning. We show that while the lower bound on the learning samples guarantees PAC learning, an upper bound can be derived to rule out adversarial learners. Such a secure learning condition is appealing, because it is defined only by the size of samples required for the successful learning and is independent of the algorithm employed. Notably, the security stems from the fundamental quantum no-broadcasting principle. No such condition can thus occur in any classical regime, where learning samples can be copied. Owing to the hybrid architecture, our scheme also offers a practical advantage for implementation in noisy intermediate-scale quantum devices.

`@Article{song_quantum_2021, author = {Song, Wooyeong and Lim, Youngrong and Kwon, Hyukjoon and Adesso, Gerardo and Wieśniak, Marcin and Pawłowski, Marcin and Kim, Jaewan and Bang, Jeongho}, journal = {Physical {R}eview {A}}, title = {Quantum secure learning with classical samples}, year = {2021}, issn = {2469-9926, 2469-9934}, month = apr, note = {arXiv: 1912.10594}, number = {4}, pages = {042409}, volume = {103}, abstract = {Studies addressing the question "Can a learner complete the learning securely?" have recently been spurred from the standpoints of fundamental theory and potential applications. In the relevant context of this question, we present a classical-quantum hybrid sampling protocol and define a security condition that allows only legitimate learners to prepare a finite set of samples that guarantees the success of the learning; the security condition excludes intruders. We do this by combining our security concept with the bound of the so-called probably approximately correct (PAC) learning. We show that while the lower bound on the learning samples guarantees PAC learning, an upper bound can be derived to rule out adversarial learners. Such a secure learning condition is appealing, because it is defined only by the size of samples required for the successful learning and is independent of the algorithm employed. Notably, the security stems from the fundamental quantum no-broadcasting principle. No such condition can thus occur in any classical regime, where learning samples can be copied. Owing to the hybrid architecture, our scheme also offers a practical advantage for implementation in noisy intermediate-scale quantum devices.}, doi = {10.1103/PhysRevA.103.042409}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/1912.10594}, urldate = {2021-07-28}, }`

- Marcin Markiewicz, Marcin Karczewski, and Pawel Kurzynski. Borromean states in discrete-time quantum walks.
*Quantum*, 5:523, aug 2021. doi:10.22331/q-2021-08-16-523

[BibTeX] [Download PDF]`@Article{Markiewicz2021borromeanstatesin, author = {Markiewicz, Marcin and Karczewski, Marcin and Kurzynski, Pawel}, journal = {{Quantum}}, title = {Borromean states in discrete-time quantum walks}, year = {2021}, issn = {2521-327X}, month = aug, pages = {523}, volume = {5}, doi = {10.22331/q-2021-08-16-523}, publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}}, url = {https://doi.org/10.22331/q-2021-08-16-523}, }`

- Robert Alicki, David Gelbwaser-Klimovsky, and Alejandro Jenkins. The problem of engines in statistical physics.
*Entropy*, 23(8), 2021. doi:10.3390/e23081095

[BibTeX] [Abstract] [Download PDF]

Engines are open systems that can generate work cyclically at the expense of an external disequilibrium. They are ubiquitous in nature and technology, but the course of mathematical physics over the last 300 years has tended to make their dynamics in time a theoretical blind spot. This has hampered the usefulness of statistical mechanics applied to active systems, including living matter. We argue that recent advances in the theory of open quantum systems, coupled with renewed interest in understanding how active forces result from positive feedback between different macroscopic degrees of freedom in the presence of dissipation, point to a more realistic description of autonomous engines. We propose a general conceptualization of an engine that helps clarify the distinction between its heat and work outputs. Based on this, we show how the external loading force and the thermal noise may be incorporated into the relevant equations of motion. This modifies the usual Fokker–Planck and Langevin equations, offering a thermodynamically complete formulation of the irreversible dynamics of simple oscillating and rotating engines.

`@Article{AlickiAugust2021, author = {Alicki, Robert and Gelbwaser-Klimovsky, David and Jenkins, Alejandro}, journal = {Entropy}, title = {The Problem of Engines in Statistical Physics}, year = {2021}, issn = {1099-4300}, number = {8}, volume = {23}, abstract = {Engines are open systems that can generate work cyclically at the expense of an external disequilibrium. They are ubiquitous in nature and technology, but the course of mathematical physics over the last 300 years has tended to make their dynamics in time a theoretical blind spot. This has hampered the usefulness of statistical mechanics applied to active systems, including living matter. We argue that recent advances in the theory of open quantum systems, coupled with renewed interest in understanding how active forces result from positive feedback between different macroscopic degrees of freedom in the presence of dissipation, point to a more realistic description of autonomous engines. We propose a general conceptualization of an engine that helps clarify the distinction between its heat and work outputs. Based on this, we show how the external loading force and the thermal noise may be incorporated into the relevant equations of motion. This modifies the usual Fokker–Planck and Langevin equations, offering a thermodynamically complete formulation of the irreversible dynamics of simple oscillating and rotating engines.}, article-number = {1095}, doi = {10.3390/e23081095}, pubmedid = {34441235}, url = {https://www.mdpi.com/1099-4300/23/8/1095}, }`

- Maciej Stankiewicz, Karol Horodecki, Omer Sakarya, and Danuta Makowiec. Private Weakly-Random Sequences from Human Heart Rate for Quantum Amplification.
*Entropy*, 23(9):1182, sep 2021. doi:10.3390/e23091182

[BibTeX] [Abstract] [Download PDF]

We investigate whether the heart rate can be treated as a semi-random source with the aim of amplification by quantum devices. We use a semi-random source model called $\epsilon$-Santha-Vazirani source, which can be amplified via quantum protocols to obtain fully private random sequence. We analyze time intervals between consecutive heartbeats obtained from Holter electrocardiogram (ECG) recordings of people of different sex and age. We propose several transformations of the original time series into binary sequences. We have performed different statistical randomness tests and estimated quality parameters. We find that the heart can be treated as good enough, and private by its nature, source of randomness, that every human possesses. As such, in principle it can be used as input to quantum device-independent randomness amplification protocols. The properly interpreted $\epsilon$ parameter can potentially serve as a new characteristic of the human’s heart from the perspective of medicine.

`@Article{Stankiewicz2021, author = {Stankiewicz, Maciej and Horodecki, Karol and Sakarya, Omer and Makowiec, Danuta}, journal = {Entropy}, title = {Private {W}eakly-{R}andom {S}equences from {H}uman {H}eart {R}ate for {Q}uantum {A}mplification}, year = {2021}, month = sep, number = {9}, pages = {1182}, volume = {23}, abstract = {We investigate whether the heart rate can be treated as a semi-random source with the aim of amplification by quantum devices. We use a semi-random source model called $\epsilon$-Santha-Vazirani source, which can be amplified via quantum protocols to obtain fully private random sequence. We analyze time intervals between consecutive heartbeats obtained from Holter electrocardiogram (ECG) recordings of people of different sex and age. We propose several transformations of the original time series into binary sequences. We have performed different statistical randomness tests and estimated quality parameters. We find that the heart can be treated as good enough, and private by its nature, source of randomness, that every human possesses. As such, in principle it can be used as input to quantum device-independent randomness amplification protocols. The properly interpreted $\epsilon$ parameter can potentially serve as a new characteristic of the human's heart from the perspective of medicine.}, archiveprefix = {arXiv}, doi = {10.3390/e23091182}, eprint = {2107.14630}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021Entrp..23.1182S}, }`

- Anubhav Chaturvedi, Máté. Farkas, and Victoria J. Wright. Characterising and bounding the set of quantum behaviours in contextuality scenarios.
*Quantum*, 5:484, 06 2021. doi:10.22331/q-2021-06-29-484

[BibTeX] [Abstract] [Download PDF]

The predictions of quantum theory resist generalised noncontextual explanations. In addition to the foundational relevance of this fact, the particular extent to which quantum theory violates noncontextuality limits available quantum advantage in communication and information processing. In the first part of this work, we formally define contextuality scenarios via prepare-and-measure experiments, along with the polytope of general contextual behaviours containing the set of quantum contextual behaviours. This framework allows us to recover several properties of set of quantum behaviours in these scenarios, including contextuality scenarios and associated noncontextuality inequalities that require for their violation the individual quantum preparation and measurement procedures to be mixed states and unsharp measurements. With the framework in place, we formulate novel semidefinite programming relaxations for bounding these sets of quantum contextual behaviours. Most significantly, to circumvent the inadequacy of pure states and projective measurements in contextuality scenarios, we present a novel unitary operator based semidefinite relaxation technique. We demonstrate the efficacy of these relaxations by obtaining tight upper bounds on the quantum violation of several noncontextuality inequalities and identifying novel maximally contextual quantum strategies. To further illustrate the versatility of these relaxations, we demonstrate $\textit{monogamy of preparation contextuality}$ in a tripartite setting, and present a secure semi-device independent quantum key distribution scheme powered by quantum advantage in parity oblivious random access codes.

`@Article{Chaturvedi2021, author = {Anubhav Chaturvedi and Máté Farkas and Victoria J Wright}, journal = {Quantum}, title = {Characterising and bounding the set of quantum behaviours in contextuality scenarios}, year = {2021}, issn = {2521-327X}, month = {06}, pages = {484}, volume = {5}, abstract = {The predictions of quantum theory resist generalised noncontextual explanations. In addition to the foundational relevance of this fact, the particular extent to which quantum theory violates noncontextuality limits available quantum advantage in communication and information processing. In the first part of this work, we formally define contextuality scenarios via prepare-and-measure experiments, along with the polytope of general contextual behaviours containing the set of quantum contextual behaviours. This framework allows us to recover several properties of set of quantum behaviours in these scenarios, including contextuality scenarios and associated noncontextuality inequalities that require for their violation the individual quantum preparation and measurement procedures to be mixed states and unsharp measurements. With the framework in place, we formulate novel semidefinite programming relaxations for bounding these sets of quantum contextual behaviours. Most significantly, to circumvent the inadequacy of pure states and projective measurements in contextuality scenarios, we present a novel unitary operator based semidefinite relaxation technique. We demonstrate the efficacy of these relaxations by obtaining tight upper bounds on the quantum violation of several noncontextuality inequalities and identifying novel maximally contextual quantum strategies. To further illustrate the versatility of these relaxations, we demonstrate $\textit{monogamy of preparation contextuality}$ in a tripartite setting, and present a secure semi-device independent quantum key distribution scheme powered by quantum advantage in parity oblivious random access codes.}, doi = {10.22331/q-2021-06-29-484}, groups = {Sainz}, publisher = {Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften}, url = {https://quantum-journal.org/papers/q-2021-06-29-484/pdf/}, }`

- Artur Barasiński, Antonín Černoch, Wiesław Laskowski, Karel Lemr, Tamás Vértesi, and Jan Soubusta. Experimentally friendly approach towards nonlocal correlations in multisetting N-partite Bell scenarios.
*Quantum*, 5:430, apr 2021. doi:10.22331/q-2021-04-14-430

[BibTeX] [Abstract] [Download PDF]

In this work, we study a recently proposed operational measure of nonlocality by Fonseca and Parisio [Phys. Rev. A 92, 030101(R) (2015)] which describes the probability of violation of local realism under randomly sampled observables, and the strength of such violation as described by resistance to white noise admixture. While our knowledge concerning these quantities is well established from a theoretical point of view, the experimental counterpart is a considerably harder task and very little has been done in this field. It is caused by the lack of complete knowledge about the facets of the local polytope required for the analysis. In this paper, we propose a simple procedure towards experimentally determining both quantities for N -qubit pure states, based on the incomplete set of tight Bell inequalities. We show that the imprecision arising from this approach is of similar magnitude as the potential measurement errors. We also show that even with both a randomly chosen N -qubit pure state and randomly chosen measurement bases, a violation of local realism can be detected experimentally almost 100 \% of the time. Among other applications, our work provides a feasible alternative for the witnessing of genuine multipartite entanglement without aligned reference frames.

`@Article{Barasinski2021, author = {Barasiński, Artur and Černoch, Antonín and Laskowski, Wiesław and Lemr, Karel and Vértesi, Tamás and Soubusta, Jan}, journal = {Quantum}, title = {Experimentally friendly approach towards nonlocal correlations in multisetting {N}-partite {Bell} scenarios}, year = {2021}, issn = {2521-327X}, month = apr, pages = {430}, volume = {5}, abstract = {In this work, we study a recently proposed operational measure of nonlocality by Fonseca and Parisio [Phys. Rev. A 92, 030101(R) (2015)] which describes the probability of violation of local realism under randomly sampled observables, and the strength of such violation as described by resistance to white noise admixture. While our knowledge concerning these quantities is well established from a theoretical point of view, the experimental counterpart is a considerably harder task and very little has been done in this field. It is caused by the lack of complete knowledge about the facets of the local polytope required for the analysis. In this paper, we propose a simple procedure towards experimentally determining both quantities for N -qubit pure states, based on the incomplete set of tight Bell inequalities. We show that the imprecision arising from this approach is of similar magnitude as the potential measurement errors. We also show that even with both a randomly chosen N -qubit pure state and randomly chosen measurement bases, a violation of local realism can be detected experimentally almost 100 \% of the time. Among other applications, our work provides a feasible alternative for the witnessing of genuine multipartite entanglement without aligned reference frames.}, doi = {10.22331/q-2021-04-14-430}, language = {en}, url = {https://quantum-journal.org/papers/q-2021-04-14-430/}, urldate = {2021-10-11}, }`

- Pawel Blasiak, Ewa Borsuk, Marcin Markiewicz, and Yong-Su Kim. Efficient linear-optical generation of a multipartite W state.
*Physical Review A*, 104(2):23701, aug 2021. doi:10.1103/PhysRevA.104.023701

[BibTeX] [Download PDF]`@Article{Blasiak2021, author = {Blasiak, Pawel and Borsuk, Ewa and Markiewicz, Marcin and Kim, Yong-Su}, journal = {Physical {R}eview {A}}, title = {Efficient linear-optical generation of a multipartite {W} state}, year = {2021}, issn = {2469-9926, 2469-9934}, month = aug, number = {2}, pages = {023701}, volume = {104}, doi = {10.1103/PhysRevA.104.023701}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.104.023701}, urldate = {2021-10-11}, }`

- Erik Aurell, Michał Eckstein, and Paweł Horodecki. Quantum Black Holes as Solvents.
*Foundations of Physics*, 51(2):54, apr 2021. doi:10.1007/s10701-021-00456-7

[BibTeX] [Abstract] [Download PDF]

Abstract Almost all of the entropy in the universe is in the form of Bekenstein–Hawking (BH) entropy of super-massive black holes. This entropy, if it satisfies Boltzmann’s equation \$\$S={\textbackslash}log {\textbackslash}mathcal\{N\}\$\$ S = log N , hence represents almost all the accessible phase space of the Universe, somehow associated to objects which themselves fill out a very small fraction of ordinary three-dimensional space. Although time scales are very long, it is believed that black holes will eventually evaporate by emitting Hawking radiation, which is thermal when counted mode by mode. A pure quantum state collapsing to a black hole will hence eventually re-emerge as a state with strictly positive entropy, which constitutes the famous black hole information paradox. Expanding on a remark by Hawking we posit that BH entropy is a thermodynamic entropy, which must be distinguished from information-theoretic entropy. The paradox can then be explained by information return in Hawking radiation. The novel perspective advanced here is that if BH entropy counts the number of accessible physical states in a quantum black hole, then the paradox can be seen as an instance of the fundamental problem of statistical mechanics. We suggest a specific analogy to the increase of the entropy in a solvation process. We further show that the huge phase volume ( \$\${\textbackslash}mathcal\{N\}\$\$ N ), which must be made available to the universe in a gravitational collapse, cannot originate from the entanglement between ordinary matter and/or radiation inside and outside the black hole. We argue that, instead, the quantum degrees of freedom of the gravitational field must get activated near the singularity, resulting in a final state of the ‘entangled entanglement’ form involving both matter and gravity.

`@Article{Aurell2021, author = {Aurell, Erik and Eckstein, Michał and Horodecki, Paweł}, journal = {Foundations of {P}hysics}, title = {Quantum {Black} {Holes} as {Solvents}}, year = {2021}, issn = {0015-9018, 1572-9516}, month = apr, number = {2}, pages = {54}, volume = {51}, abstract = {Abstract Almost all of the entropy in the universe is in the form of Bekenstein–Hawking (BH) entropy of super-massive black holes. This entropy, if it satisfies Boltzmann’s equation \$\$S={\textbackslash}log {\textbackslash}mathcal\{N\}\$\$ S = log N , hence represents almost all the accessible phase space of the Universe, somehow associated to objects which themselves fill out a very small fraction of ordinary three-dimensional space. Although time scales are very long, it is believed that black holes will eventually evaporate by emitting Hawking radiation, which is thermal when counted mode by mode. A pure quantum state collapsing to a black hole will hence eventually re-emerge as a state with strictly positive entropy, which constitutes the famous black hole information paradox. Expanding on a remark by Hawking we posit that BH entropy is a thermodynamic entropy, which must be distinguished from information-theoretic entropy. The paradox can then be explained by information return in Hawking radiation. The novel perspective advanced here is that if BH entropy counts the number of accessible physical states in a quantum black hole, then the paradox can be seen as an instance of the fundamental problem of statistical mechanics. We suggest a specific analogy to the increase of the entropy in a solvation process. We further show that the huge phase volume ( \$\${\textbackslash}mathcal\{N\}\$\$ N ), which must be made available to the universe in a gravitational collapse, cannot originate from the entanglement between ordinary matter and/or radiation inside and outside the black hole. We argue that, instead, the quantum degrees of freedom of the gravitational field must get activated near the singularity, resulting in a final state of the ‘entangled entanglement’ form involving both matter and gravity.}, doi = {10.1007/s10701-021-00456-7}, groups = {Pawel_H}, language = {en}, url = {https://link.springer.com/10.1007/s10701-021-00456-7}, urldate = {2021-10-11}, }`

- Robert Alicki, David Gelbwaser-Klimovsky, and Alejandro Jenkins. Leaking elastic capacitor as model for active matter.
*Physical Review E*, 103(5):52131, may 2021. doi:10.1103/PhysRevE.103.052131

[BibTeX] [Abstract] [Download PDF]

We introduce the “leaking elastic capacitor” (LEC) model, a nonconservative dynamical system that combines simple electrical and mechanical degrees of freedom. We show that an LEC connected to an external voltage source can be destabilized (Hopf bifurcation) due to positive feedback between the mechanical separation of the plates and their electrical charging. Numerical simulation finds regimes in which the LEC exhibits a limit cycle (regular self-oscillation) or strange attractors (chaos). The LEC acts as an autonomous engine, cyclically performing work at the expense of the constant voltage source. We show that this mechanical work can be used to pump current, generating an electromotive force without any time-varying magnetic flux and in a thermodynamically irreversible way. We consider how this mechanism can sustain electromechanical waves propagating along flexible plates. We argue that the LEC model can offer a qualitatively new and more realistic description of important properties of active systems with electrical double layers in condensed-matter physics, chemistry, and biology.

`@Article{Alicki2021, author = {Alicki, Robert and Gelbwaser-Klimovsky, David and Jenkins, Alejandro}, journal = {Physical {R}eview {E}}, title = {Leaking elastic capacitor as model for active matter}, year = {2021}, month = may, number = {5}, pages = {052131}, volume = {103}, abstract = {We introduce the ``leaking elastic capacitor'' (LEC) model, a nonconservative dynamical system that combines simple electrical and mechanical degrees of freedom. We show that an LEC connected to an external voltage source can be destabilized (Hopf bifurcation) due to positive feedback between the mechanical separation of the plates and their electrical charging. Numerical simulation finds regimes in which the LEC exhibits a limit cycle (regular self-oscillation) or strange attractors (chaos). The LEC acts as an autonomous engine, cyclically performing work at the expense of the constant voltage source. We show that this mechanical work can be used to pump current, generating an electromotive force without any time-varying magnetic flux and in a thermodynamically irreversible way. We consider how this mechanism can sustain electromechanical waves propagating along flexible plates. We argue that the LEC model can offer a qualitatively new and more realistic description of important properties of active systems with electrical double layers in condensed-matter physics, chemistry, and biology.}, archiveprefix = {arXiv}, doi = {10.1103/PhysRevE.103.052131}, eid = {052131}, eprint = {2010.05534}, keywords = {Physics - Classical Physics, Condensed Matter - Soft Condensed Matter, Physics - Biological Physics, Physics - Chemical Physics}, primaryclass = {physics.class-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021PhRvE.103e2131A}, }`

- Stefano Cusumano and Łukasz Rudnicki. Thermodynamics of Reduced State of the Field.
*Entropy. An International and Interdisciplinary Journal of Entropy and Information Studies*, 23(9):Paper No. 1198, 2021. doi:10.3390/e23091198

[BibTeX] [Download PDF]`@Article{Cusumano2021, author = {Cusumano, Stefano and Rudnicki, Łukasz}, journal = {Entropy. {A}n {I}nternational and {I}nterdisciplinary {J}ournal of {E}ntropy and {I}nformation {S}tudies}, title = {Thermodynamics of {R}educed {S}tate of the {F}ield}, year = {2021}, number = {9}, pages = {Paper No. 1198}, volume = {23}, doi = {10.3390/e23091198}, keywords = {81}, mrnumber = {4320432}, url = {https://www.mdpi.com/1099-4300/23/9/1198/pdf}, }`

- Marcin Markiewicz and Janusz Przewocki. On construction of finite averaging sets for sl(2,C) via its Cartan decomposition.
*Journal of Physics. A. Mathematical and Theoretical*, 54(23):Paper No. 235302, 20, 2021. doi:10.1088/1751-8121/abfa44

[BibTeX] [Download PDF]`@Article{Markiewicz2021a, author = {Markiewicz, Marcin and Przewocki, Janusz}, journal = {Journal of {P}hysics. {A}. {M}athematical and {T}heoretical}, title = {On construction of finite averaging sets for SL(2,{C}) via its {C}artan decomposition}, year = {2021}, issn = {1751-8113}, number = {23}, pages = {Paper No. 235302, 20}, volume = {54}, doi = {10.1088/1751-8121/abfa44}, keywords = {22E30 (05E16 28C10 81P45)}, mrnumber = {4271309}, url = {https://iopscience.iop.org/article/10.1088/1751-8121/abfa44/pdf}, }`

- Łukasz Rudnicki. Quantum speed limit and geometric measure of entanglement.
*Physical Review A*, 104(3):32417, sep 2021. doi:10.1103/PhysRevA.104.032417

[BibTeX] [Abstract] [Download PDF]

Using the approach offered by quantum speed limit, we show that geometric measure of multipartite entanglement for pure states [T.-C. Wei and P. M. Goldbart, Phys. Rev. A 68, 042307 (2003), 10.1103/PhysRevA.68.042307] can be interpreted as the minimal time necessary to unitarily evolve a given quantum state to a separable one.

`@Article{Rudnicki2021, author = {Rudnicki, Łukasz}, journal = {Physical {R}eview {A}}, title = {Quantum speed limit and geometric measure of entanglement}, year = {2021}, month = sep, number = {3}, pages = {032417}, volume = {104}, abstract = {Using the approach offered by quantum speed limit, we show that geometric measure of multipartite entanglement for pure states [T.-C. Wei and P. M. Goldbart, Phys. Rev. A 68, 042307 (2003), 10.1103/PhysRevA.68.042307] can be interpreted as the minimal time necessary to unitarily evolve a given quantum state to a separable one.}, archiveprefix = {arXiv}, doi = {10.1103/PhysRevA.104.032417}, eid = {032417}, eprint = {2107.11877}, groups = {Rudnicki}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021PhRvA.104c2417R}, }`

- Carlo Maria Scandolo, Roberto Salazar, Jarosław K. Korbicz, and Paweł Horodecki. Universal structure of objective states in all fundamental causal theories.
*Physical Review Research*, 3(3):33148, aug 2021. doi:10.1103/PhysRevResearch.3.033148

[BibTeX] [Abstract] [Download PDF]

A crucial question is how objective and classical behavior arises from a fundamental physical theory. Here we provide a natural definition of a decoherence process valid in all causal theories and show how its behavior can be extremely different from the quantum one. Remarkably, despite this, we prove that the so- called spectrum broadcast structure characterizes all objective states in every fundamental causal theory, exactly as in quantum mechanics. Our results show a stark contrast between the extraordinarily diverse decoherence behavior and the universal features of objectivity.

`@Article{Scandolo2021, author = {Scandolo, Carlo Maria and Salazar, Roberto and Korbicz, Jarosław K. and Horodecki, Paweł}, journal = {Physical {R}eview {R}esearch}, title = {Universal structure of objective states in all fundamental causal theories}, year = {2021}, month = aug, number = {3}, pages = {033148}, volume = {3}, abstract = {A crucial question is how objective and classical behavior arises from a fundamental physical theory. Here we provide a natural definition of a decoherence process valid in all causal theories and show how its behavior can be extremely different from the quantum one. Remarkably, despite this, we prove that the so- called spectrum broadcast structure characterizes all objective states in every fundamental causal theory, exactly as in quantum mechanics. Our results show a stark contrast between the extraordinarily diverse decoherence behavior and the universal features of objectivity.}, doi = {10.1103/PhysRevResearch.3.033148}, eid = {033148}, groups = {Pawel_H}, url = {https://ui.adsabs.harvard.edu/abs/2021PhRvR...3c3148S}, }`

- Markus Grassl. Comment on “an encryption protocol for neqr images based on one-particle quantum walks on a circle”.
*Quantum Information Processing*, 20(5):183, may 2021. doi:10.1007/s11128-021-03094-0

[BibTeX] [Abstract] [Download PDF]

In [1], the authors propose a protocol to encrypt quantum images. Below we show that the protocol is incorrect.

`@Article{Grassl2021, author = {Grassl, Markus}, journal = {Quantum {I}nformation {P}rocessing}, title = {Comment on "An encryption protocol for NEQR images based on one-particle quantum walks on a circle"}, year = {2021}, month = may, number = {5}, pages = {183}, volume = {20}, abstract = {In [1], the authors propose a protocol to encrypt quantum images. Below we show that the protocol is incorrect.}, doi = {10.1007/s11128-021-03094-0}, eid = {183}, keywords = {Entanglement, Quantum circuits, Quantum image encryption}, url = {https://ui.adsabs.harvard.edu/abs/2021QuIP...20..183G}, }`

- Klaus Liegener and Łukasz Rudnicki. Algorithmic approach to cosmological coherent state expectation values in loop quantum gravity.
*Classical and Quantum Gravity*, 38(20):Paper No. 205001, 39, 2021. doi:10.1088/1361-6382/ac226f

[BibTeX] [Download PDF]`@Article{Liegener2021, author = {Liegener, Klaus and Rudnicki, Łukasz}, journal = {Classical and {Q}uantum {G}ravity}, title = {Algorithmic approach to cosmological coherent state expectation values in loop quantum gravity}, year = {2021}, issn = {0264-9381}, number = {20}, pages = {Paper No. 205001, 39}, volume = {38}, doi = {10.1088/1361-6382/ac226f}, keywords = {83C45 (83C27)}, mrnumber = {4318548}, url = {https://iopscience.iop.org/article/10.1088/1361-6382/ac226f/pdf}, }`

- Łukasz Rudnicki and Stephen P. Walborn. Entropic uncertainty relations for mutually unbiased periodic coarse-grained observables resembling their discrete counterparts.
*Physical Review A*, 104(4):Paper No. 042210, 2021. doi:10.1103/physreva.104.042210

[BibTeX] [Download PDF]`@Article{Rudnicki2021a, author = {Rudnicki, Łukasz and Walborn, Stephen P.}, journal = {Physical {R}eview {A}}, title = {Entropic uncertainty relations for mutually unbiased periodic coarse-grained observables resembling their discrete counterparts}, year = {2021}, issn = {2469-9926}, number = {4}, pages = {Paper No. 042210}, volume = {104}, doi = {10.1103/physreva.104.042210}, keywords = {81S07}, mrnumber = {4339485}, url = {https://journals.aps.org/pra/pdf/10.1103/PhysRevA.104.042210}, }`

- Wooyeong Song, Marcin Wieśniak, Nana Liu, Marcin Pawłowski, Jinhyoung Lee, Jaewan Kim, and Jeongho Bang. Tangible reduction in learning sample complexity with large classical samples and small quantum system.
*Quantum Information Processing*, 20(8):Paper No. 275, 18, 2021. doi:10.1007/s11128-021-03217-7

[BibTeX] [Download PDF]`@Article{Song2021, author = {Song, Wooyeong and Wieśniak, Marcin and Liu, Nana and Pawłowski, Marcin and Lee, Jinhyoung and Kim, Jaewan and Bang, Jeongho}, journal = {Quantum {I}nformation {P}rocessing}, title = {Tangible reduction in learning sample complexity with large classical samples and small quantum system}, year = {2021}, issn = {1570-0755}, number = {8}, pages = {Paper No. 275, 18}, volume = {20}, doi = {10.1007/s11128-021-03217-7}, keywords = {81P68}, mrnumber = {4303621}, url = {https://link.springer.com/content/pdf/10.1007/s11128-021-03217-7.pdf}, }`

- Tamal Guha, Mir Alimuddin, Sumit Rout, Amit Mukherjee, Some Sankar Bhattacharya, and Manik Banik. Quantum advantage for shared randomness generation.
*Quantum*, 5:569, 2021. doi:10.22331/q-2021-10-27-569

[BibTeX] [Download PDF]`@Article{Guha2021, author = {Guha, Tamal and Alimuddin, Mir and Rout, Sumit and Mukherjee, Amit and Bhattacharya, Some Sankar and Banik, Manik}, journal = {Quantum}, title = {Quantum Advantage for Shared Randomness Generation}, year = {2021}, pages = {569}, volume = {5}, archiveprefix = {arXiv}, doi = {10.22331/q-2021-10-27-569}, eprint = {2001.01889}, primaryclass = {quant-ph}, url = {https://quantum-journal.org/papers/q-2021-10-27-569/pdf/}, }`

- Pawel Blasiak, Ewa Borsuk, and Marcin Markiewicz. On safe post-selection for bell tests with ideal detectors: causal diagram approach.
*Quantum*, 5:575, 11 2021. doi:10.22331/q-2021-11-11-575

[BibTeX] [Abstract] [Download PDF]

Reasoning about Bell nonlocality from the correlations observed in post-selected data is always a matter of concern. This is because conditioning on the outcomes is a source of non-causal correlations, known as a $\textit{selection bias}$, rising doubts whether the conclusion concerns the actual causal process or maybe it is just an effect of processing the data. Yet, even in the idealised case without detection inefficiencies, post-selection is an integral part of experimental designs, not least because it is a part of the entanglement generation process itself. In this paper we discuss a broad class of scenarios with post-selection on multiple spatially distributed outcomes. A simple criterion is worked out, called the $\textit{all-but-one}$ principle, showing when the conclusions about nonlocality from breaking Bell inequalities with post-selected data remain in force. Generality of this result, attained by adopting the high-level diagrammatic tools of causal inference, provides safe grounds for systematic reasoning based on the standard form of multipartite Bell inequalities in a wide array of entanglement generation schemes, without worrying about the dangers of selection bias. In particular, it can be applied to post-selection defined by single-particle events in each detection chanel when the number of particles in the system is conserved.

`@Article{Blasiak2021b, author = {Pawel Blasiak and Ewa Borsuk and Marcin Markiewicz}, journal = {Quantum}, title = {On safe post-selection for Bell tests with ideal detectors: Causal diagram approach}, year = {2021}, issn = {2521-327X}, month = {11}, pages = {575}, volume = {5}, abstract = {Reasoning about Bell nonlocality from the correlations observed in post-selected data is always a matter of concern. This is because conditioning on the outcomes is a source of non-causal correlations, known as a $\textit{selection bias}$, rising doubts whether the conclusion concerns the actual causal process or maybe it is just an effect of processing the data. Yet, even in the idealised case without detection inefficiencies, post-selection is an integral part of experimental designs, not least because it is a part of the entanglement generation process itself. In this paper we discuss a broad class of scenarios with post-selection on multiple spatially distributed outcomes. A simple criterion is worked out, called the $\textit{all-but-one}$ principle, showing when the conclusions about nonlocality from breaking Bell inequalities with post-selected data remain in force. Generality of this result, attained by adopting the high-level diagrammatic tools of causal inference, provides safe grounds for systematic reasoning based on the standard form of multipartite Bell inequalities in a wide array of entanglement generation schemes, without worrying about the dangers of selection bias. In particular, it can be applied to post-selection defined by single-particle events in each detection chanel when the number of particles in the system is conserved.}, doi = {10.22331/q-2021-11-11-575}, publisher = {Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften}, url = {https://quantum-journal.org/papers/q-2021-11-11-575/pdf/}, }`

- Michał‚ Oszmaniec, Adam Sawicki, and Michał‚ Horodecki. Epsilon-Nets, Unitary Designs and Random Quantum Circuits.
*Ieee transactions on information Theory*, PP:1–1, 2021. doi:10.1109/TIT.2021.3128110

[BibTeX] [Abstract] [Download PDF]

Epsilon-nets and approximate unitary t-designs are natural notions that capture properties of unitary operations relevant for numerous applications in quantum information and quantum computing. In this work we study quantitative connections between these two notions. Specifically, we prove that, for d dimensional Hilbert space, unitaries constituting Î´-approximate t-expanders form Ďµ-nets for t â‰� d5/2/Ďµ and Î´ â‰� (Ďµ3/2/d)d2. We also show that for arbitrary t, Ďµ-nets can be used to construct Î´-approximate unitary t-designs for Î´ â‰� Ďµt, where the notion of approximation is based on the diamond norm. Finally, we prove that the degree of an exact unitary t design necessary to obtain an Ďµ-net must grow at least as fast as 1/Ďµ (for fixed dimension) and not slower than d2 (for fixed Ďµ). This shows near optimality of our result connecting t-designs and Ďµ-nets. We apply our findings in the context of quantum computing. First, we show that that approximate t-designs can be generated by shallow random circuits formed from a set of universal two-qudit gates in the parallel and sequential local architectures considered in [1]. Importantly, our gate sets need not to be symmetric (i.e. contains gates together with their inverses) or consist of gates with algebraic entries. Second, we consider compilation of quantum gates and prove a non-constructive Solovay-Kitaev theorem for general universal gate sets. Our main technical contribution is a new construction of efficient polynomial approximations to the Dirac delta in the space of quantum channels, which can be of independent interest.

`@Article{Oszmaniec2021, author = {Michał‚ Oszmaniec and Adam Sawicki and Michał‚ Horodecki}, journal = {IEEE Transactions on Information {T}heory}, title = {Epsilon-{N}ets, {U}nitary {D}esigns and {R}andom {Q}uantum {C}ircuits}, year = {2021}, issn = {1557-9654}, pages = {1--1}, volume = {PP}, abstract = {Epsilon-nets and approximate unitary t-designs are natural notions that capture properties of unitary operations relevant for numerous applications in quantum information and quantum computing. In this work we study quantitative connections between these two notions. Specifically, we prove that, for d dimensional Hilbert space, unitaries constituting Î´-approximate t-expanders form Ďµ-nets for t â‰� d5/2/Ďµ and Î´ â‰� (Ďµ3/2/d)d2. We also show that for arbitrary t, Ďµ-nets can be used to construct Î´-approximate unitary t-designs for Î´ â‰� Ďµt, where the notion of approximation is based on the diamond norm. Finally, we prove that the degree of an exact unitary t design necessary to obtain an Ďµ-net must grow at least as fast as 1/Ďµ (for fixed dimension) and not slower than d2 (for fixed Ďµ). This shows near optimality of our result connecting t-designs and Ďµ-nets. We apply our findings in the context of quantum computing. First, we show that that approximate t-designs can be generated by shallow random circuits formed from a set of universal two-qudit gates in the parallel and sequential local architectures considered in [1]. Importantly, our gate sets need not to be symmetric (i.e. contains gates together with their inverses) or consist of gates with algebraic entries. Second, we consider compilation of quantum gates and prove a non-constructive Solovay-Kitaev theorem for general universal gate sets. Our main technical contribution is a new construction of efficient polynomial approximations to the Dirac delta in the space of quantum channels, which can be of independent interest.}, doi = {10.1109/TIT.2021.3128110}, groups = {Michal_H}, issue = {99}, keywords = {Logic gates, Quantum channels, Quantum computing, Quantum circuit, Layout, Hilbert space, Diamond, unitary designs, epsilon nets, random quantum circuits, compilation of quantum gates, unitary channels}, publisher = {IEEE}, url = {https://arxiv.org/pdf/2007.10885}, }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. Can single photon excitation of two spatially separated modes lead to a violation of Bell inequality via weak-field homodyne measurements?.
*New journal of physics*, 23(7):73042, jul 2021. doi:10.1088/1367-2630/ac0ffe

[BibTeX] [Abstract] [Download PDF]

We reconsider the all-optical weak homodyne-measurement based experimental schemes aimed at revealing Bell nonclassicality (‘nonlocality’) of a single photon. We focus on the schemes put forward by Tan et al (TWC, 1991) and Hardy (1994). In our previous work we show that the TWC experiment can be described by a local hidden variable model, hence the claimed nonclassicality is apparent. The nonclassicality proof proposed by Hardy remains impeccable. We investigate which feature of the Hardy’s approach is crucial to disclose the nonclassicality. There are consequential differences between TWC and Hardy setups: (i) the initial state of Hardy is a superposition of a single photon excitation with vacuum in one of the input modes of a 50-50 beamsplitter. In the TWC case there is no vacuum component. (ii) In the final measurements of Hardy’s proposal the local settings are specified by the presence or absence of a local oscillator field (on/off). In the TWC case the auxiliary fields are constant, only phases are varied. We show that in Hardy’s setup the violation of local realism occurs due to the varying strength of the local oscillators. Still, one does not need to operate in the fully on/off detection scheme. Thus, the nonclassicality in a Hardy-like setup cannot be attributed to the single-photon state alone. It is a consequence of its interference with the photons from auxiliary local fields. Neither can it be attributed to the joint state of the single photon excitation and the local oscillator modes, as this state is measurement setting dependent. Despite giving spurious violations of local realism, the TWC scheme can serve as an entanglement indicator, for the TWC state. Nevertheless an analogue indicator based on intensity rates rather than just intensities overperforms it.

`@Article{Das2021a, author = {Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Woloncewicz, Bianka and Żukowski, Marek}, journal = {New Journal of Physics}, title = {Can single photon excitation of two spatially separated modes lead to a violation of {B}ell inequality via weak-field homodyne measurements?}, year = {2021}, month = jul, number = {7}, pages = {073042}, volume = {23}, abstract = {We reconsider the all-optical weak homodyne-measurement based experimental schemes aimed at revealing Bell nonclassicality ('nonlocality') of a single photon. We focus on the schemes put forward by Tan et al (TWC, 1991) and Hardy (1994). In our previous work we show that the TWC experiment can be described by a local hidden variable model, hence the claimed nonclassicality is apparent. The nonclassicality proof proposed by Hardy remains impeccable. We investigate which feature of the Hardy's approach is crucial to disclose the nonclassicality. There are consequential differences between TWC and Hardy setups: (i) the initial state of Hardy is a superposition of a single photon excitation with vacuum in one of the input modes of a 50-50 beamsplitter. In the TWC case there is no vacuum component. (ii) In the final measurements of Hardy's proposal the local settings are specified by the presence or absence of a local oscillator field (on/off). In the TWC case the auxiliary fields are constant, only phases are varied. We show that in Hardy's setup the violation of local realism occurs due to the varying strength of the local oscillators. Still, one does not need to operate in the fully on/off detection scheme. Thus, the nonclassicality in a Hardy-like setup cannot be attributed to the single-photon state alone. It is a consequence of its interference with the photons from auxiliary local fields. Neither can it be attributed to the joint state of the single photon excitation and the local oscillator modes, as this state is measurement setting dependent. Despite giving spurious violations of local realism, the TWC scheme can serve as an entanglement indicator, for the TWC state. Nevertheless an analogue indicator based on intensity rates rather than just intensities overperforms it.}, archiveprefix = {arXiv}, doi = {10.1088/1367-2630/ac0ffe}, eid = {073042}, eprint = {2102.06689}, keywords = {nonclassicality of single photon excitation, Bell inequalities, homodyne measurement, mode entanglement, entanglement witness, operators based on rates, Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021NJPh...23g3042D}, }`

- Kamil Korzekwa and Matteo Lostaglio. Quantum advantage in simulating stochastic processes.
*Physical review x*, 11(2):21019, 4 2021. doi:10.1103/PhysRevX.11.021019

[BibTeX] [Abstract] [Download PDF]

We investigate the problem of simulating classical stochastic processes through quantum dynamics and present three scenarios where memory or time quantum advantages arise. First, by introducing and analyzing a quantum version of the embeddability problem for stochastic matrices, we show that quantum memoryless dynamics can simulate classical processes that necessarily require memory. Second, by extending the notion of space-time cost of a stochastic process P to the quantum domain, we prove an advantage of the quantum cost of simulating P over the classical cost. Third, we demonstrate that the set of classical states accessible via Markovian master equations with quantum controls is larger than the set of those accessible with classical controls, leading, e.g., to a potential advantage in cooling protocols.

`@Article{Korzekwa2021, author = {Kamil Korzekwa and Matteo Lostaglio}, journal = {Physical Review X}, title = {Quantum Advantage in Simulating Stochastic Processes}, year = {2021}, issn = {2160-3308}, month = {4}, number = {2}, pages = {021019}, volume = {11}, abstract = {We investigate the problem of simulating classical stochastic processes through quantum dynamics and present three scenarios where memory or time quantum advantages arise. First, by introducing and analyzing a quantum version of the embeddability problem for stochastic matrices, we show that quantum memoryless dynamics can simulate classical processes that necessarily require memory. Second, by extending the notion of space-time cost of a stochastic process P to the quantum domain, we prove an advantage of the quantum cost of simulating P over the classical cost. Third, we demonstrate that the set of classical states accessible via Markovian master equations with quantum controls is larger than the set of those accessible with classical controls, leading, e.g., to a potential advantage in cooling protocols.}, doi = {10.1103/PhysRevX.11.021019}, publisher = {American Physical Society}, url = {https://journals.aps.org/prx/pdf/10.1103/PhysRevX.11.021019}, }`

- Karol Horodecki, Michał{} Studziński, Ryszard P. Kostecki, Omer Sakarya, and Dong Yang. Upper bounds on the leakage of private data and operational approach to markovianity.
*Phys. Rev. A*, 104:52422, 2021. doi:10.1103/PhysRevA.104.052422

[BibTeX] [Download PDF]`@Article{Horodecki2021, author = {Horodecki, Karol and Studzi\'nski, Micha\l{} and Kostecki, Ryszard P. and Sakarya, Omer and Yang, Dong}, journal = {Phys. {R}ev. {A}}, title = {Upper bounds on the leakage of private data and operational approach to markovianity}, year = {2021}, pages = {052422}, volume = {104}, archiveprefix = {arXiv}, doi = {10.1103/PhysRevA.104.052422}, eprint = {2107.10737}, primaryclass = {quant-ph}, url = {https://journals.aps.org/pra/pdf/10.1103/PhysRevA.104.052422}, }`

- Zbigniew Pucha{l}a, Kamil Korzekwa, Roberto Salazar, Pawe{ł} Horodecki, and Karol Życzkowski. Dephasing superchannels.
*Physical Review A*, pages arXiv:2107.06585, jul 2021.

[BibTeX] [Abstract] [Download PDF]

We characterise a class of environmental noises that decrease coherent properties of quantum channels by introducing and analysing the properties of dephasing superchannels. These are defined as superchannels that affect only non-classical properties of a quantum channel $\mathcal{E}$, i.e., they leave invariant the transition probabilities induced by $\mathcal{E}$ in the distinguished basis. We prove that such superchannels $\Xi_C$ form a particular subclass of Schur-product supermaps that act on the Jamiolkowski state $J(\mathcal{E})$ of a channel $\mathcal{E}$ via a Schur product, $J’=J\circ C$. We also find physical realizations of general $\Xi_C$ through a pre- and post-processing employing dephasing channels with memory, and show that memory plays a non-trivial role for quantum systems of dimension $d>2$. Moreover, we prove that coherence generating power of a general quantum channel is a monotone under dephasing superchannels. Finally, we analyse the effect dephasing noise can have on a quantum channel $\mathcal{E}$ by investigating the number of distinguishable channels that $\mathcal{E}$ can be mapped to by a family of dephasing superchannels. More precisely, we upper bound this number in terms of hypothesis testing channel divergence between $\mathcal{E}$ and its fully dephased version, and also relate it to the robustness of coherence of $\mathcal{E}$.

`@Article{Puchala2021, author = {Pucha{\l}a, Zbigniew and Korzekwa, Kamil and Salazar, Roberto and Horodecki, Pawe{\l} and {\.Z}yczkowski, Karol}, journal = {Physical {R}eview {A}}, title = {Dephasing superchannels}, year = {2021}, month = jul, pages = {arXiv:2107.06585}, abstract = {We characterise a class of environmental noises that decrease coherent properties of quantum channels by introducing and analysing the properties of dephasing superchannels. These are defined as superchannels that affect only non-classical properties of a quantum channel $\mathcal{E}$, i.e., they leave invariant the transition probabilities induced by $\mathcal{E}$ in the distinguished basis. We prove that such superchannels $\Xi_C$ form a particular subclass of Schur-product supermaps that act on the Jamiolkowski state $J(\mathcal{E})$ of a channel $\mathcal{E}$ via a Schur product, $J'=J\circ C$. We also find physical realizations of general $\Xi_C$ through a pre- and post-processing employing dephasing channels with memory, and show that memory plays a non-trivial role for quantum systems of dimension $d>2$. Moreover, we prove that coherence generating power of a general quantum channel is a monotone under dephasing superchannels. Finally, we analyse the effect dephasing noise can have on a quantum channel $\mathcal{E}$ by investigating the number of distinguishable channels that $\mathcal{E}$ can be mapped to by a family of dephasing superchannels. More precisely, we upper bound this number in terms of hypothesis testing channel divergence between $\mathcal{E}$ and its fully dephased version, and also relate it to the robustness of coherence of $\mathcal{E}$.}, archiveprefix = {arXiv}, eid = {arXiv:2107.06585}, eprint = {2107.06585}, groups = {Pawel_H}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv210706585P}, }`

### 2020

- Paweł Mazurek, Máté. Farkas, Andrzej Grudka, Michał Horodecki, and Michał Studziński. Quantum error-correction codes and absolutely maximally entangled states.
*Physical Review A*, 101(4):42305, 2020. doi:10.1103/PhysRevA.101.042305

[BibTeX] [Download PDF]`@Article{mazurek_quantum_2020, author = {Mazurek, Paweł and Farkas, Máté and Grudka, Andrzej and Horodecki, Michał and Studziński, Michał}, journal = {Physical {R}eview {A}}, title = {Quantum error-correction codes and absolutely maximally entangled states}, year = {2020}, issn = {2469-9926, 2469-9934}, month = apr, number = {4}, pages = {042305}, volume = {101}, doi = {10.1103/PhysRevA.101.042305}, groups = {Michal_H}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.101.042305}, urldate = {2020-04-22}, }`

- Karol Horodecki and Maciej Stankiewicz. Semi-device-independent quantum money.
*New Journal of Physics*, 22(2):23007, 2020. doi:10.1088/1367-2630/ab6872

[BibTeX] [Download PDF]`@Article{horodecki_semi-device-independent_2020, author = {Horodecki, Karol and Stankiewicz, Maciej}, journal = {New {J}ournal of {P}hysics}, title = {Semi-device-independent quantum money}, year = {2020}, issn = {1367-2630}, month = feb, number = {2}, pages = {023007}, volume = {22}, doi = {10.1088/1367-2630/ab6872}, url = {https://iopscience.iop.org/article/10.1088/1367-2630/ab6872}, urldate = {2020-04-22}, }`

- Tomasz Linowski, Grzegorz Rajchel-Mieldzioć, and Karol Życzkowski. Entangling power of multipartite unitary gates.
*Journal of Physics A: Mathematical and Theoretical*, 53(12):125303, 2020. doi:10.1088/1751-8121/ab749a

[BibTeX] [Download PDF]`@Article{linowski_entangling_2020, author = {Linowski, Tomasz and Rajchel-Mieldzioć, Grzegorz and Życzkowski, Karol}, journal = {Journal of {P}hysics {A}: {M}athematical and {T}heoretical}, title = {Entangling power of multipartite unitary gates}, year = {2020}, issn = {1751-8113, 1751-8121}, month = mar, number = {12}, pages = {125303}, volume = {53}, doi = {10.1088/1751-8121/ab749a}, url = {https://iopscience.iop.org/article/10.1088/1751-8121/ab749a}, urldate = {2020-04-22}, }`

- Marcin Pawłowski. Entropy in Foundations of Quantum Physics.
*Entropy*, 22(3):371, mar 2020. doi:10.3390/e22030371

[BibTeX] [Abstract] [Download PDF]

Entropy can be used in studies on foundations of quantum physics in many different ways, each of them using different properties of this mathematical object […]

`@article{pawlowski_entropy_2020, title = {Entropy in {Foundations} of {Quantum} {Physics}}, volume = {22}, issn = {1099-4300}, url = {https://www.mdpi.com/1099-4300/22/3/371}, doi = {10.3390/e22030371}, abstract = {Entropy can be used in studies on foundations of quantum physics in many different ways, each of them using different properties of this mathematical object [...]}, language = {en}, number = {3}, urldate = {2020-04-22}, journal = {Entropy}, author = {Pawłowski, Marcin}, month = mar, year = {2020}, pages = {371}, }`

- Massimiliano Smania, Piotr Mironowicz, Mohamed Nawareg, Marcin Pawłowski, Adán Cabello, and Mohamed Bourennane. Experimental certification of an informationally complete quantum measurement in a device-independent protocol.
*Optica*, 7(2):123, feb 2020. doi:10.1364/OPTICA.377959

[BibTeX] [Download PDF]`@article{smania_experimental_2020, title = {Experimental certification of an informationally complete quantum measurement in a device-independent protocol}, volume = {7}, issn = {2334-2536}, url = {https://www.osapublishing.org/abstract.cfm?URI=optica-7-2-123}, doi = {10.1364/OPTICA.377959}, language = {en}, number = {2}, urldate = {2020-04-22}, journal = {Optica}, author = {Smania, Massimiliano and Mironowicz, Piotr and Nawareg, Mohamed and Pawłowski, Marcin and Cabello, Adán and Bourennane, Mohamed}, month = feb, year = {2020}, pages = {123}, }`

- Simon Milz, Fattah Sakuldee, Felix A. Pollock, and Kavan Modi. Kolmogorov extension theorem for (quantum) causal modelling and general probabilistic theories.
*Quantum*, 4:255, apr 2020. doi:10.22331/q-2020-04-20-255

[BibTeX] [Abstract] [Download PDF]

In classical physics, the Kolmogorov extension theorem lays the foundation for the theory of stochastic processes. It has been known for a long time that, in its original form, this theorem does not hold in quantum mechanics. More generally, it does not hold in any theory of stochastic processes – classical, quantum or beyond – that does not just describe passive observations, but allows for active interventions. Such processes form the basis of the study of causal modelling across the sciences, including in the quantum domain. To date, these frameworks have lacked a conceptual underpinning similar to that provided by Kolmogorov’s theorem for classical stochastic processes. We prove a generalized extension theorem that applies to all theories of stochastic processes, putting them on equally firm mathematical ground as their classical counterpart. Additionally, we show that quantum causal modelling and quantum stochastic processes are equivalent. This provides the correct framework for the description of experiments involving continuous control, which play a crucial role in the development of quantum technologies. Furthermore, we show that the original extension theorem follows from the generalized one in the correct limit, and elucidate how a comprehensive understanding of general stochastic processes allows one to unambiguously define the distinction between those that are classical and those that are quantum.

`@Article{milz_kolmogorov_2020, author = {Milz, Simon and Sakuldee, Fattah and Pollock, Felix A. and Modi, Kavan}, journal = {Quantum}, title = {Kolmogorov extension theorem for (quantum) causal modelling and general probabilistic theories}, year = {2020}, issn = {2521-327X}, month = apr, pages = {255}, volume = {4}, abstract = {In classical physics, the Kolmogorov extension theorem lays the foundation for the theory of stochastic processes. It has been known for a long time that, in its original form, this theorem does not hold in quantum mechanics. More generally, it does not hold in any theory of stochastic processes -- classical, quantum or beyond -- that does not just describe passive observations, but allows for active interventions. Such processes form the basis of the study of causal modelling across the sciences, including in the quantum domain. To date, these frameworks have lacked a conceptual underpinning similar to that provided by Kolmogorov’s theorem for classical stochastic processes. We prove a generalized extension theorem that applies to all theories of stochastic processes, putting them on equally firm mathematical ground as their classical counterpart. Additionally, we show that quantum causal modelling and quantum stochastic processes are equivalent. This provides the correct framework for the description of experiments involving continuous control, which play a crucial role in the development of quantum technologies. Furthermore, we show that the original extension theorem follows from the generalized one in the correct limit, and elucidate how a comprehensive understanding of general stochastic processes allows one to unambiguously define the distinction between those that are classical and those that are quantum.}, doi = {10.22331/q-2020-04-20-255}, language = {en}, url = {https://quantum-journal.org/papers/q-2020-04-20-255/}, urldate = {2020-04-22}, }`

- Krzysztof Szczygielski and Robert Alicki. On Howland time-independent formulation of CP-divisible quantum evolutions.
*Reviews in Mathematical Physics*, page 2050021, 2020. doi:10.1142/S0129055X2050021X

[BibTeX] [Abstract] [Download PDF]

We extend Howland time-independent formalism to the case of completely positive and trace preserving dynamics of finite-dimensional open quantum systems governed by periodic, time-dependent Lindbladian in Weak Coupling Limit, expanding our result from previous papers. We propose the Bochner space of periodic, square integrable matrix-valued functions, as well as its tensor product representation, as the generalized space of states within the time-independent formalism. We examine some densely defined operators on this space, together with their Fourier-like expansions and address some problems related to their convergence by employing general results on Banach space-valued Fourier series, such as the generalized Carleson–Hunt theorem. We formulate Markovian dynamics in the generalized space of states by constructing appropriate time-independent Lindbladian in standard (Lindblad–Gorini–Kossakowski–Sudarshan) form, as well as one-parameter semigroup of bounded evolution maps. We show their similarity with Markovian generators and dynamical maps defined on matrix space, i.e. the generator still possesses a standard form (extended by closed perturbation) and the resulting semigroup is also completely positive, trace preserving and a contraction.

`@Article{szczygielski_howland_2020, author = {Szczygielski, Krzysztof and Alicki, Robert}, journal = {Reviews in {M}athematical {P}hysics}, title = {On {Howland} time-independent formulation of {CP}-divisible quantum evolutions}, year = {2020}, issn = {0129-055X, 1793-6659}, month = jan, pages = {2050021}, abstract = {We extend Howland time-independent formalism to the case of completely positive and trace preserving dynamics of finite-dimensional open quantum systems governed by periodic, time-dependent Lindbladian in Weak Coupling Limit, expanding our result from previous papers. We propose the Bochner space of periodic, square integrable matrix-valued functions, as well as its tensor product representation, as the generalized space of states within the time-independent formalism. We examine some densely defined operators on this space, together with their Fourier-like expansions and address some problems related to their convergence by employing general results on Banach space-valued Fourier series, such as the generalized Carleson–Hunt theorem. We formulate Markovian dynamics in the generalized space of states by constructing appropriate time-independent Lindbladian in standard (Lindblad–Gorini–Kossakowski–Sudarshan) form, as well as one-parameter semigroup of bounded evolution maps. We show their similarity with Markovian generators and dynamical maps defined on matrix space, i.e. the generator still possesses a standard form (extended by closed perturbation) and the resulting semigroup is also completely positive, trace preserving and a contraction.}, doi = {10.1142/S0129055X2050021X}, language = {en}, url = {https://www.worldscientific.com/doi/abs/10.1142/S0129055X2050021X}, urldate = {2020-05-13}, }`

- Monika Rosicka, Paweł Mazurek, Andrzej Grudka, and Michał Horodecki. Generalized XOR non-locality games with graph description on a square lattice.
*Journal of Physics A: Mathematical and Theoretical*, 53(26):265302, jul 2020. doi:10.1088/1751-8121/ab8f3e

[BibTeX] [Download PDF]`@Article{rosicka_generalized_2020, author = {Rosicka, Monika and Mazurek, Paweł and Grudka, Andrzej and Horodecki, Michał}, journal = {Journal of {P}hysics {A}: {M}athematical and {T}heoretical}, title = {Generalized {XOR} non-locality games with graph description on a square lattice}, year = {2020}, issn = {1751-8113, 1751-8121}, month = jul, number = {26}, pages = {265302}, volume = {53}, doi = {10.1088/1751-8121/ab8f3e}, groups = {Michal_H}, url = {https://iopscience.iop.org/article/10.1088/1751-8121/ab8f3e}, urldate = {2020-06-24}, }`

- Felix Huber and Markus Grassl. Quantum Codes of Maximal Distance and Highly Entangled Subspaces.
*Quantum*, 4:284, jun 2020. doi:10.22331/q-2020-06-18-284

[BibTeX] [Download PDF]`@Article{huber_quantum_2020, author = {Huber, Felix and Grassl, Markus}, journal = {Quantum}, title = {Quantum {Codes} of {Maximal} {Distance} and {Highly} {Entangled} {Subspaces}}, year = {2020}, issn = {2521-327X}, month = jun, pages = {284}, volume = {4}, doi = {10.22331/q-2020-06-18-284}, language = {en}, url = {https://quantum-journal.org/papers/q-2020-06-18-284/}, urldate = {2020-06-24}, }`

- Paul Skrzypczyk, Matty J. Hoban, Ana Belén Sainz, and Noah Linden. Complexity of compatible measurements.
*Physical Review Research*, 2(2):23292, jun 2020. doi:10.1103/PhysRevResearch.2.023292

[BibTeX] [Download PDF]`@Article{skrzypczyk_complexity_2020, author = {Skrzypczyk, Paul and Hoban, Matty J. and Sainz, Ana Belén and Linden, Noah}, journal = {Physical {R}eview {R}esearch}, title = {Complexity of compatible measurements}, year = {2020}, issn = {2643-1564}, month = jun, number = {2}, pages = {023292}, volume = {2}, doi = {10.1103/PhysRevResearch.2.023292}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevResearch.2.023292}, urldate = {2020-06-24}, }`

- Armin Tavakoli, Marek Żukowski, and Časlav Brukner. Does violation of a Bell inequality always imply quantum advantage in a communication complexity problem?.
*Quantum*, 4:316, sep 2020. doi:10.22331/q-2020-09-07-316

[BibTeX] [Abstract] [Download PDF]

Quantum correlations which violate a Bell inequality are presumed to power better-than-classical protocols for solving communication complexity problems (CCPs). How general is this statement? We show that violations of correlation-type Bell inequalities allow advantages in CCPs, when communication protocols are tailored to emulate the Bell no-signaling constraint (by not communicating measurement settings). Abandonment of this restriction on classical models allows us to disprove the main result of, inter alia, {\textbackslash}cite\{BZ02\}; we show that quantum correlations obtained from these communication strategies assisted by a small quantum violation of the CGLMP Bell inequalities do not imply advantages in any CCP in the input/output scenario considered in the reference. More generally, we show that there exists quantum correlations, with nontrivial local marginal probabilities, which violate the I 3322 Bell inequality, but do not enable a quantum advantange in any CCP, regardless of the communication strategy employed in the quantum protocol, for a scenario with a fixed number of inputs and outputs

`@Article{tavakoli_does_2020, author = {Tavakoli, Armin and Żukowski, Marek and Brukner, Časlav}, journal = {Quantum}, title = {Does violation of a {Bell} inequality always imply quantum advantage in a communication complexity problem?}, year = {2020}, issn = {2521-327X}, month = sep, pages = {316}, volume = {4}, abstract = {Quantum correlations which violate a Bell inequality are presumed to power better-than-classical protocols for solving communication complexity problems (CCPs). How general is this statement? We show that violations of correlation-type Bell inequalities allow advantages in CCPs, when communication protocols are tailored to emulate the Bell no-signaling constraint (by not communicating measurement settings). Abandonment of this restriction on classical models allows us to disprove the main result of, inter alia, {\textbackslash}cite\{BZ02\}; we show that quantum correlations obtained from these communication strategies assisted by a small quantum violation of the CGLMP Bell inequalities do not imply advantages in any CCP in the input/output scenario considered in the reference. More generally, we show that there exists quantum correlations, with nontrivial local marginal probabilities, which violate the I 3322 Bell inequality, but do not enable a quantum advantange in any CCP, regardless of the communication strategy employed in the quantum protocol, for a scenario with a fixed number of inputs and outputs}, doi = {10.22331/q-2020-09-07-316}, language = {en}, url = {https://quantum-journal.org/papers/q-2020-09-07-316/}, urldate = {2021-05-10}, }`

- Ł. Rudnicki, L. L. Sánchez-Soto, G. Leuchs, and R. W. Boyd. Fundamental quantum limits in ellipsometry.
*Optics Letters*, 45(16):4607, 2020. doi:10.1364/OL.392955

[BibTeX] [Download PDF]`@Article{rudnicki_fundamental_2020, author = {Rudnicki, Ł. and Sánchez-Soto, L. L. and Leuchs, G. and Boyd, R. W.}, journal = {Optics {L}etters}, title = {Fundamental quantum limits in ellipsometry}, year = {2020}, issn = {0146-9592, 1539-4794}, month = aug, number = {16}, pages = {4607}, volume = {45}, doi = {10.1364/OL.392955}, language = {en}, url = {https://www.osapublishing.org/abstract.cfm?URI=ol-45-16-4607}, urldate = {2021-05-10}, }`

- Ana Belén Sainz, Matty J. Hoban, Paul Skrzypczyk, and Leandro Aolita. Bipartite Postquantum Steering in Generalized Scenarios.
*Physical Review Letters*, 125(5):50404, jul 2020. doi:10.1103/PhysRevLett.125.050404

[BibTeX] [Download PDF]`@Article{sainz_bipartite_2020, author = {Sainz, Ana Belén and Hoban, Matty J. and Skrzypczyk, Paul and Aolita, Leandro}, journal = {Physical {R}eview {L}etters}, title = {Bipartite {Postquantum} {Steering} in {Generalized} {Scenarios}}, year = {2020}, issn = {0031-9007, 1079-7114}, month = jul, number = {5}, pages = {050404}, volume = {125}, doi = {10.1103/PhysRevLett.125.050404}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.125.050404}, urldate = {2021-05-10}, }`

- Sandu Popescu, Ana Belén Sainz, Anthony J. Short, and Andreas Winter. Reference Frames Which Separately Store Noncommuting Conserved Quantities.
*Physical Review Letters*, 125(9):90601, aug 2020. doi:10.1103/PhysRevLett.125.090601

[BibTeX] [Download PDF]`@Article{popescu_reference_2020, author = {Popescu, Sandu and Sainz, Ana Belén and Short, Anthony J. and Winter, Andreas}, journal = {Physical {R}eview {L}etters}, title = {Reference {Frames} {Which} {Separately} {Store} {Noncommuting} {Conserved} {Quantities}}, year = {2020}, issn = {0031-9007, 1079-7114}, month = aug, number = {9}, pages = {090601}, volume = {125}, doi = {10.1103/PhysRevLett.125.090601}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.125.090601}, urldate = {2021-05-10}, }`

- Debashis Saha, Michał Oszmaniec, Łukasz Czekaj, Michał Horodecki, and Ryszard Horodecki. Operational foundations for complementarity and uncertainty relations.
*Physical Review A*, 101(5):52104, 2020. doi:10.1103/PhysRevA.101.052104

[BibTeX] [Download PDF]`@Article{saha_operational_2020, author = {Saha, Debashis and Oszmaniec, Michał and Czekaj, Łukasz and Horodecki, Michał and Horodecki, Ryszard}, journal = {Physical {R}eview {A}}, title = {Operational foundations for complementarity and uncertainty relations}, year = {2020}, issn = {2469-9926, 2469-9934}, month = may, number = {5}, pages = {052104}, volume = {101}, doi = {10.1103/PhysRevA.101.052104}, groups = {Michal_H}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.101.052104}, urldate = {2021-05-10}, }`

- Géza Tóth, Tamás Vértesi, Paweł Horodecki, and Ryszard Horodecki. Activating Hidden Metrological Usefulness.
*Physical Review Letters*, 125(2):20402, jul 2020. doi:10.1103/PhysRevLett.125.020402

[BibTeX] [Download PDF]`@Article{toth_activating_2020, author = {Tóth, Géza and Vértesi, Tamás and Horodecki, Paweł and Horodecki, Ryszard}, journal = {Physical {R}eview {L}etters}, title = {Activating {Hidden} {Metrological} {Usefulness}}, year = {2020}, issn = {0031-9007, 1079-7114}, month = jul, number = {2}, pages = {020402}, volume = {125}, doi = {10.1103/PhysRevLett.125.020402}, groups = {Pawel_H}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.125.020402}, urldate = {2021-05-10}, }`

- Karol Horodecki, Ryszard P. Kostecki, Roberto Salazar, and Michał Studziński. Limitations for private randomness repeaters.
*Physical Review A*, 102(1):12615, jul 2020. doi:10.1103/PhysRevA.102.012615

[BibTeX] [Download PDF]`@Article{horodecki_limitations_2020, author = {Horodecki, Karol and Kostecki, Ryszard P. and Salazar, Roberto and Studziński, Michał}, journal = {Physical {R}eview {A}}, title = {Limitations for private randomness repeaters}, year = {2020}, issn = {2469-9926, 2469-9934}, month = jul, number = {1}, pages = {012615}, volume = {102}, doi = {10.1103/PhysRevA.102.012615}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.012615}, urldate = {2021-05-10}, }`

- Anna de Rosier, Jacek Gruca, Fernando Parisio, Tamás Vértesi, and Wiesław Laskowski. Strength and typicality of nonlocality in multisetting and multipartite Bell scenarios.
*Physical Review A*, 101(1):12116, jan 2020. doi:10.1103/PhysRevA.101.012116

[BibTeX] [Download PDF]`@Article{de_rosier_strength_2020, author = {de Rosier, Anna and Gruca, Jacek and Parisio, Fernando and Vértesi, Tamás and Laskowski, Wiesław}, journal = {Physical {R}eview {A}}, title = {Strength and typicality of nonlocality in multisetting and multipartite {Bell} scenarios}, year = {2020}, issn = {2469-9926, 2469-9934}, month = jan, number = {1}, pages = {012116}, volume = {101}, doi = {10.1103/PhysRevA.101.012116}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.101.012116}, urldate = {2021-05-10}, }`

- Lukas Knips, Jan Dziewior, Waldemar Kłobus, Wiesław Laskowski, Tomasz Paterek, Peter J. Shadbolt, Harald Weinfurter, and Jasmin D. A. Meinecke. Multipartite entanglement analysis from random correlations.
*Npj Quantum Information*, 6(1):51, dec 2020. doi:10.1038/s41534-020-0281-5

[BibTeX] [Abstract] [Download PDF]

Abstract Quantum entanglement is usually revealed via a well aligned, carefully chosen set of measurements. Yet, under a number of experimental conditions, for example in communication within multiparty quantum networks, noise along the channels or fluctuating orientations of reference frames may ruin the quality of the distributed states. Here, we show that even for strong fluctuations one can still gain detailed information about the state and its entanglement using random measurements. Correlations between all or subsets of the measurement outcomes and especially their distributions provide information about the entanglement structure of a state. We analytically derive an entanglement criterion for two-qubit states and provide strong numerical evidence for witnessing genuine multipartite entanglement of three and four qubits. Our methods take the purity of the states into account and are based on only the second moments of measured correlations. Extended features of this theory are demonstrated experimentally with four photonic qubits. As long as the rate of entanglement generation is sufficiently high compared to the speed of the fluctuations, this method overcomes any type and strength of localized unitary noise.

`@Article{knips_multipartite_2020, author = {Knips, Lukas and Dziewior, Jan and Kłobus, Waldemar and Laskowski, Wiesław and Paterek, Tomasz and Shadbolt, Peter J. and Weinfurter, Harald and Meinecke, Jasmin D. A.}, journal = {npj {Q}uantum {I}nformation}, title = {Multipartite entanglement analysis from random correlations}, year = {2020}, issn = {2056-6387}, month = dec, number = {1}, pages = {51}, volume = {6}, abstract = {Abstract Quantum entanglement is usually revealed via a well aligned, carefully chosen set of measurements. Yet, under a number of experimental conditions, for example in communication within multiparty quantum networks, noise along the channels or fluctuating orientations of reference frames may ruin the quality of the distributed states. Here, we show that even for strong fluctuations one can still gain detailed information about the state and its entanglement using random measurements. Correlations between all or subsets of the measurement outcomes and especially their distributions provide information about the entanglement structure of a state. We analytically derive an entanglement criterion for two-qubit states and provide strong numerical evidence for witnessing genuine multipartite entanglement of three and four qubits. Our methods take the purity of the states into account and are based on only the second moments of measured correlations. Extended features of this theory are demonstrated experimentally with four photonic qubits. As long as the rate of entanglement generation is sufficiently high compared to the speed of the fluctuations, this method overcomes any type and strength of localized unitary noise.}, doi = {10.1038/s41534-020-0281-5}, language = {en}, url = {http://www.nature.com/articles/s41534-020-0281-5}, urldate = {2021-05-10}, }`

- Saptarshi Roy, Tamoghna Das, and Aditi Sen(De). Computable genuine multimode entanglement measure: Gaussian versus non-Gaussian.
*Physical Review A*, 102(1):12421, jul 2020. doi:10.1103/PhysRevA.102.012421

[BibTeX] [Download PDF]`@Article{roy_computable_2020, author = {Roy, Saptarshi and Das, Tamoghna and Sen(De), Aditi}, journal = {Physical {R}eview {A}}, title = {Computable genuine multimode entanglement measure: {Gaussian} versus non-{Gaussian}}, year = {2020}, issn = {2469-9926, 2469-9934}, month = jul, number = {1}, pages = {012421}, volume = {102}, doi = {10.1103/PhysRevA.102.012421}, language = {en}, shorttitle = {Computable genuine multimode entanglement measure}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.012421}, urldate = {2021-05-10}, }`

- Elie Wolfe, David Schmid, Ana Belén Sainz, Ravi Kunjwal, and Robert W. Spekkens. Quantifying Bell: the Resource Theory of Nonclassicality of Common-Cause Boxes.
*Quantum*, 4:280, jun 2020. doi:10.22331/q-2020-06-08-280

[BibTeX] [Abstract] [Download PDF]

We take a resource-theoretic approach to the problem of quantifying nonclassicality in Bell scenarios. The resources are conceptualized as probabilistic processes from the setting variables to the outcome variables having a particular causal structure, namely, one wherein the wings are only connected by a common cause. We term them “common-cause boxes”. We define the distinction between classical and nonclassical resources in terms of whether or not a classical causal model can explain the correlations. One can then quantify the relative nonclassicality of resources by considering their interconvertibility relative to the set of operations that can be implemented using a classical common cause (which correspond to local operations and shared randomness). We prove that the set of free operations forms a polytope, which in turn allows us to derive an efficient algorithm for deciding whether one resource can be converted to another. We moreover define two distinct monotones with simple closed-form expressions in the two-party binary-setting binary-outcome scenario, and use these to reveal various properties of the pre-order of resources, including a lower bound on the cardinality of any complete set of monotones. In particular, we show that the information contained in the degrees of violation of facet-defining Bell inequalities is not sufficient for quantifying nonclassicality, even though it is sufficient for witnessing nonclassicality. Finally, we show that the continuous set of convexly extremal quantumly realizable correlations are all at the top of the pre-order of quantumly realizable correlations. In addition to providing new insights on Bell nonclassicality, our work also sets the stage for quantifying nonclassicality in more general causal networks.

`@article{wolfe_quantifying_2020, title = {Quantifying {Bell}: the {Resource} {Theory} of {Nonclassicality} of {Common}-{Cause} {Boxes}}, volume = {4}, issn = {2521-327X}, shorttitle = {Quantifying {Bell}}, url = {https://quantum-journal.org/papers/q-2020-06-08-280/}, doi = {10.22331/q-2020-06-08-280}, abstract = {We take a resource-theoretic approach to the problem of quantifying nonclassicality in Bell scenarios. The resources are conceptualized as probabilistic processes from the setting variables to the outcome variables having a particular causal structure, namely, one wherein the wings are only connected by a common cause. We term them "common-cause boxes". We define the distinction between classical and nonclassical resources in terms of whether or not a classical causal model can explain the correlations. One can then quantify the relative nonclassicality of resources by considering their interconvertibility relative to the set of operations that can be implemented using a classical common cause (which correspond to local operations and shared randomness). We prove that the set of free operations forms a polytope, which in turn allows us to derive an efficient algorithm for deciding whether one resource can be converted to another. We moreover define two distinct monotones with simple closed-form expressions in the two-party binary-setting binary-outcome scenario, and use these to reveal various properties of the pre-order of resources, including a lower bound on the cardinality of any complete set of monotones. In particular, we show that the information contained in the degrees of violation of facet-defining Bell inequalities is not sufficient for quantifying nonclassicality, even though it is sufficient for witnessing nonclassicality. Finally, we show that the continuous set of convexly extremal quantumly realizable correlations are all at the top of the pre-order of quantumly realizable correlations. In addition to providing new insights on Bell nonclassicality, our work also sets the stage for quantifying nonclassicality in more general causal networks.}, language = {en}, urldate = {2021-05-10}, journal = {Quantum}, author = {Wolfe, Elie and Schmid, David and Sainz, Ana Belén and Kunjwal, Ravi and Spekkens, Robert W.}, month = jun, year = {2020}, pages = {280}, }`

- Ravishankar Ramanathan, Monika Rosicka, Karol Horodecki, Stefano Pironio, Michał Horodecki, and Paweł Horodecki. Gadget structures in proofs of the Kochen-Specker theorem.
*Quantum*, 4:308, aug 2020. doi:10.22331/q-2020-08-14-308

[BibTeX] [Abstract] [Download PDF]

The Kochen-Specker theorem is a fundamental result in quantum foundations that has spawned massive interest since its inception. We show that within every Kochen-Specker graph, there exist interesting subgraphs which we term 01 -gadgets, that capture the essential contradiction necessary to prove the Kochen-Specker theorem, i.e,. every Kochen-Specker graph contains a 01 -gadget and from every 01 -gadget one can construct a proof of the Kochen-Specker theorem. Moreover, we show that the 01 -gadgets form a fundamental primitive that can be used to formulate state-independent and state-dependent statistical Kochen-Specker arguments as well as to give simple constructive proofs of an “extended” Kochen-Specker theorem first considered by Pitowsky in {\textbackslash}cite\{Pitowsky\}.

`@Article{ramanathan_gadget_2020, author = {Ramanathan, Ravishankar and Rosicka, Monika and Horodecki, Karol and Pironio, Stefano and Horodecki, Michał and Horodecki, Paweł}, journal = {Quantum}, title = {Gadget structures in proofs of the {Kochen}-{Specker} theorem}, year = {2020}, issn = {2521-327X}, month = aug, pages = {308}, volume = {4}, abstract = {The Kochen-Specker theorem is a fundamental result in quantum foundations that has spawned massive interest since its inception. We show that within every Kochen-Specker graph, there exist interesting subgraphs which we term 01 -gadgets, that capture the essential contradiction necessary to prove the Kochen-Specker theorem, i.e,. every Kochen-Specker graph contains a 01 -gadget and from every 01 -gadget one can construct a proof of the Kochen-Specker theorem. Moreover, we show that the 01 -gadgets form a fundamental primitive that can be used to formulate state-independent and state-dependent statistical Kochen-Specker arguments as well as to give simple constructive proofs of an ``extended'' Kochen-Specker theorem first considered by Pitowsky in {\textbackslash}cite\{Pitowsky\}.}, doi = {10.22331/q-2020-08-14-308}, groups = {Pawel_H, Michal_H}, language = {en}, url = {https://quantum-journal.org/papers/q-2020-08-14-308/}, urldate = {2021-05-10}, }`

- Filip B. Maciejewski, Zoltán Zimborás, and Michał Oszmaniec. Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography.
*Quantum*, 4:257, apr 2020. doi:10.22331/q-2020-04-24-257

[BibTeX] [Abstract] [Download PDF]

We propose a simple scheme to reduce readout errors in experiments on quantum systems with finite number of measurement outcomes. Our method relies on performing classical post-processing which is preceded by Quantum Detector Tomography, i.e., the reconstruction of a Positive-Operator Valued Measure (POVM) describing the given quantum measurement device. If the measurement device is affected only by an invertible classical noise, it is possible to correct the outcome statistics of future experiments performed on the same device. To support the practical applicability of this scheme for near-term quantum devices, we characterize measurements implemented in IBM’s and Rigetti’s quantum processors. We find that for these devices, based on superconducting transmon qubits, classical noise is indeed the dominant source of readout errors. Moreover, we analyze the influence of the presence of coherent errors and finite statistics on the performance of our error-mitigation procedure. Applying our scheme on the IBM’s 5-qubit device, we observe a significant improvement of the results of a number of single- and two-qubit tasks including Quantum State Tomography (QST), Quantum Process Tomography (QPT), the implementation of non-projective measurements, and certain quantum algorithms (Grover’s search and the Bernstein-Vazirani algorithm). Finally, we present results showing improvement for the implementation of certain probability distributions in the case of five qubits.

`@article{maciejewski_mitigation_2020, title = {Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography}, volume = {4}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2020-04-24-257/}, doi = {10.22331/q-2020-04-24-257}, abstract = {We propose a simple scheme to reduce readout errors in experiments on quantum systems with finite number of measurement outcomes. Our method relies on performing classical post-processing which is preceded by Quantum Detector Tomography, i.e., the reconstruction of a Positive-Operator Valued Measure (POVM) describing the given quantum measurement device. If the measurement device is affected only by an invertible classical noise, it is possible to correct the outcome statistics of future experiments performed on the same device. To support the practical applicability of this scheme for near-term quantum devices, we characterize measurements implemented in IBM's and Rigetti's quantum processors. We find that for these devices, based on superconducting transmon qubits, classical noise is indeed the dominant source of readout errors. Moreover, we analyze the influence of the presence of coherent errors and finite statistics on the performance of our error-mitigation procedure. Applying our scheme on the IBM's 5-qubit device, we observe a significant improvement of the results of a number of single- and two-qubit tasks including Quantum State Tomography (QST), Quantum Process Tomography (QPT), the implementation of non-projective measurements, and certain quantum algorithms (Grover's search and the Bernstein-Vazirani algorithm). Finally, we present results showing improvement for the implementation of certain probability distributions in the case of five qubits.}, language = {en}, urldate = {2021-05-10}, journal = {Quantum}, author = {Maciejewski, Filip B. and Zimborás, Zoltán and Oszmaniec, Michał}, month = apr, year = {2020}, pages = {257}, }`

- K. Rosołek, M. Wieśniak, and L. Knips. Quadratic Entanglement Criteria for Qutrits.
*Acta Physica Polonica A*, 137(3):374–378, mar 2020. doi:10.12693/APhysPolA.137.374

[BibTeX] [Download PDF]`@Article{rosolek_quadratic_2020, author = {Rosołek, K. and Wieśniak, M. and Knips, L.}, journal = {Acta {P}hysica {P}olonica {A}}, title = {Quadratic {Entanglement} {Criteria} for {Qutrits}}, year = {2020}, issn = {1898-794X, 0587-4246}, month = mar, number = {3}, pages = {374--378}, volume = {137}, doi = {10.12693/APhysPolA.137.374}, url = {http://przyrbwn.icm.edu.pl/APP/PDF/137/app137z3p18.pdf}, urldate = {2021-05-10}, }`

- Marcin Wieśniak, Palash Pandya, Omer Sakarya, and Bianka Woloncewicz. Distance between Bound Entangled States from Unextendible Product Bases and Separable States.
*Quantum Reports*, 2(1):49–56, jan 2020. doi:10.3390/quantum2010004

[BibTeX] [Abstract] [Download PDF]

We discuss the use of the Gilbert algorithm to tailor entanglement witnesses for unextendible product basis bound entangled states (UPB BE states). The method relies on the fact that an optimal entanglement witness is given by a plane perpendicular to a line between the reference state, entanglement of which is to be witnessed, and its closest separable state (CSS). The Gilbert algorithm finds an approximation of CSS. In this article, we investigate if this approximation can be good enough to yield a valid entanglement witness. We compare witnesses found with Gilbert algorithm and those given by Bandyopadhyay–Ghosh–Roychowdhury (BGR) construction. This comparison allows us to learn about the amount of entanglement and we find a relationship between it and a feature of the construction of UPBBE states, namely the size of their central tile. We show that in most studied cases, witnesses found with the Gilbert algorithm in this work are more optimal than ones obtained by Bandyopadhyay, Ghosh, and Roychowdhury. This result implies the increased tolerance to experimental imperfections in a realization of the state.

`@Article{wiesniak_distance_2020, author = {Wieśniak, Marcin and Pandya, Palash and Sakarya, Omer and Woloncewicz, Bianka}, journal = {Quantum {R}eports}, title = {Distance between {Bound} {Entangled} {States} from {Unextendible} {Product} {Bases} and {Separable} {States}}, year = {2020}, issn = {2624-960X}, month = jan, number = {1}, pages = {49--56}, volume = {2}, abstract = {We discuss the use of the Gilbert algorithm to tailor entanglement witnesses for unextendible product basis bound entangled states (UPB BE states). The method relies on the fact that an optimal entanglement witness is given by a plane perpendicular to a line between the reference state, entanglement of which is to be witnessed, and its closest separable state (CSS). The Gilbert algorithm finds an approximation of CSS. In this article, we investigate if this approximation can be good enough to yield a valid entanglement witness. We compare witnesses found with Gilbert algorithm and those given by Bandyopadhyay–Ghosh–Roychowdhury (BGR) construction. This comparison allows us to learn about the amount of entanglement and we find a relationship between it and a feature of the construction of UPBBE states, namely the size of their central tile. We show that in most studied cases, witnesses found with the Gilbert algorithm in this work are more optimal than ones obtained by Bandyopadhyay, Ghosh, and Roychowdhury. This result implies the increased tolerance to experimental imperfections in a realization of the state.}, doi = {10.3390/quantum2010004}, language = {en}, url = {https://www.mdpi.com/2624-960X/2/1/4}, urldate = {2021-05-10}, }`

- Tomasz Linowski, Clemens Gneiting, and Łukasz Rudnicki. Stabilizing entanglement in two-mode Gaussian states.
*Physical Review A*, 102(4):42405, oct 2020. doi:10.1103/PhysRevA.102.042405

[BibTeX] [Download PDF]`@Article{linowski_stabilizing_2020, author = {Linowski, Tomasz and Gneiting, Clemens and Rudnicki, Łukasz}, journal = {Physical {R}eview {A}}, title = {Stabilizing entanglement in two-mode {Gaussian} states}, year = {2020}, issn = {2469-9926, 2469-9934}, month = oct, number = {4}, pages = {042405}, volume = {102}, doi = {10.1103/PhysRevA.102.042405}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.042405}, urldate = {2021-05-10}, }`

- Michał Eckstein and Paweł Horodecki. The Experiment Paradox in Physics.
*Foundations of Science*, oct 2020. doi:10.1007/s10699-020-09711-y

[BibTeX] [Abstract] [Download PDF]

Abstract Modern physics is founded on two mainstays: mathematical modelling and empirical verification. These two assumptions are prerequisite for the objectivity of scientific discourse. Here we show, however, that they are contradictory, leading to the ‘experiment paradox’. We reveal that any experiment performed on a physical system is—by necessity—invasive and thus establishes inevitable limits to the accuracy of any mathematical model. We track its manifestations in both classical and quantum physics and show how it is overcome ‘in practice’ via the concept of environment. We argue that the unravelled paradox induces a new type of ‘ontic’ underdetermination, which has deep consequences for the methodological foundations of physics.

`@Article{eckstein_experiment_2020, author = {Eckstein, Michał and Horodecki, Paweł}, journal = {Foundations of {S}cience}, title = {The {Experiment} {Paradox} in {Physics}}, year = {2020}, issn = {1233-1821, 1572-8471}, month = oct, abstract = {Abstract Modern physics is founded on two mainstays: mathematical modelling and empirical verification. These two assumptions are prerequisite for the objectivity of scientific discourse. Here we show, however, that they are contradictory, leading to the ‘experiment paradox’. We reveal that any experiment performed on a physical system is—by necessity—invasive and thus establishes inevitable limits to the accuracy of any mathematical model. We track its manifestations in both classical and quantum physics and show how it is overcome ‘in practice’ via the concept of environment. We argue that the unravelled paradox induces a new type of ‘ontic’ underdetermination, which has deep consequences for the methodological foundations of physics.}, doi = {10.1007/s10699-020-09711-y}, groups = {Pawel_H}, language = {en}, url = {http://link.springer.com/10.1007/s10699-020-09711-y}, urldate = {2021-05-10}, }`

- Aaron Z. Goldberg, Andrei B. Klimov, Markus Grassl, Gerd Leuchs, and Luis L. Sánchez-Soto. Extremal quantum states.
*Avs Quantum Science*, 2(4):44701, dec 2020. doi:10.1116/5.0025819

[BibTeX] [Download PDF]`@Article{goldberg_extremal_2020, author = {Goldberg, Aaron Z. and Klimov, Andrei B. and Grassl, Markus and Leuchs, Gerd and Sánchez-Soto, Luis L.}, journal = {AVS {Q}uantum {S}cience}, title = {Extremal quantum states}, year = {2020}, issn = {2639-0213}, month = dec, number = {4}, pages = {044701}, volume = {2}, doi = {10.1116/5.0025819}, language = {en}, url = {http://avs.scitation.org/doi/10.1116/5.0025819}, urldate = {2021-05-10}, }`

- Berry Groisman, Michael Mc Gettrick, Mehdi Mhalla, and Marcin Pawlowski. How Quantum Information Can Improve Social Welfare.
*IEEE Journal on Selected Areas in Information Theory*, 1(2):445–453, aug 2020. doi:10.1109/JSAIT.2020.3012922

[BibTeX] [Download PDF]`@Article{groisman_how_2020, author = {Groisman, Berry and Mc Gettrick, Michael and Mhalla, Mehdi and Pawlowski, Marcin}, journal = {I{EEE} {J}ournal on {S}elected {A}reas in {I}nformation {T}heory}, title = {How {Quantum} {Information} {Can} {Improve} {Social} {Welfare}}, year = {2020}, issn = {2641-8770}, month = aug, number = {2}, pages = {445--453}, volume = {1}, doi = {10.1109/JSAIT.2020.3012922}, url = {https://ieeexplore.ieee.org/document/9173538/}, urldate = {2021-05-10}, }`

- Omer Sakarya, Marek Winczewski, Adam Rutkowski, and Karol Horodecki. Hybrid quantum network design against unauthorized secret-key generation, and its memory cost.
*Physical Review Research*, 2(4):43022, oct 2020. doi:10.1103/PhysRevResearch.2.043022

[BibTeX] [Download PDF]`@Article{sakarya_hybrid_2020, author = {Sakarya, Omer and Winczewski, Marek and Rutkowski, Adam and Horodecki, Karol}, journal = {Physical {R}eview {R}esearch}, title = {Hybrid quantum network design against unauthorized secret-key generation, and its memory cost}, year = {2020}, issn = {2643-1564}, month = oct, number = {4}, pages = {043022}, volume = {2}, doi = {10.1103/PhysRevResearch.2.043022}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevResearch.2.043022}, urldate = {2021-05-10}, }`

- John H. Selby and Ciarán M. Lee. Compositional resource theories of coherence.
*Quantum*, 4:319, sep 2020. doi:10.22331/q-2020-09-11-319

[BibTeX] [Abstract] [Download PDF]

Quantum coherence is one of the most important resources in quantum information theory. Indeed, preventing the loss of coherence is one of the most important technical challenges obstructing the development of large-scale quantum computers. Recently, there has been substantial progress in developing mathematical resource theories of coherence, paving the way towards its quantification and control. To date however, these resource theories have only been mathematically formalised within the realms of convex-geometry, information theory, and linear algebra. This approach is limited in scope, and makes it difficult to generalise beyond resource theories of coherence for single system quantum states. In this paper we take a complementary perspective, showing that resource theories of coherence can instead be defined purely compositionally, that is, working with the mathematics of process theories, string diagrams and category theory. This new perspective offers several advantages: i) it unifies various existing approaches to the study of coherence, for example, subsuming both speakable and unspeakable coherence; ii) it provides a general treatment of the compositional multi-system setting; iii) it generalises immediately to the case of quantum channels, measurements, instruments, and beyond rather than just states; iv) it can easily be generalised to the setting where there are multiple distinct sources of decoherence; and, iv) it directly extends to arbitrary process theories, for example, generalised probabilistic theories and Spekkens toy model–-providing the ability to operationally characterise coherence rather than relying on specific mathematical features of quantum theory for its description. More importantly, by providing a new, complementary, perspective on the resource of coherence, this work opens the door to the development of novel tools which would not be accessible from the linear algebraic mind set.

`@article{selby_compositional_2020, title = {Compositional resource theories of coherence}, volume = {4}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2020-09-11-319/}, doi = {10.22331/q-2020-09-11-319}, abstract = {Quantum coherence is one of the most important resources in quantum information theory. Indeed, preventing the loss of coherence is one of the most important technical challenges obstructing the development of large-scale quantum computers. Recently, there has been substantial progress in developing mathematical resource theories of coherence, paving the way towards its quantification and control. To date however, these resource theories have only been mathematically formalised within the realms of convex-geometry, information theory, and linear algebra. This approach is limited in scope, and makes it difficult to generalise beyond resource theories of coherence for single system quantum states. In this paper we take a complementary perspective, showing that resource theories of coherence can instead be defined purely compositionally, that is, working with the mathematics of process theories, string diagrams and category theory. This new perspective offers several advantages: i) it unifies various existing approaches to the study of coherence, for example, subsuming both speakable and unspeakable coherence; ii) it provides a general treatment of the compositional multi-system setting; iii) it generalises immediately to the case of quantum channels, measurements, instruments, and beyond rather than just states; iv) it can easily be generalised to the setting where there are multiple distinct sources of decoherence; and, iv) it directly extends to arbitrary process theories, for example, generalised probabilistic theories and Spekkens toy model---providing the ability to operationally characterise coherence rather than relying on specific mathematical features of quantum theory for its description. More importantly, by providing a new, complementary, perspective on the resource of coherence, this work opens the door to the development of novel tools which would not be accessible from the linear algebraic mind set.}, language = {en}, urldate = {2021-05-10}, journal = {Quantum}, author = {Selby, John H. and Lee, Ciarán M.}, month = sep, year = {2020}, pages = {319}, }`

- Palash Pandya, Omer Sakarya, and Marcin Wieśniak. Hilbert-Schmidt distance and entanglement witnessing.
*Physical Review A*, 102(1):12409, jul 2020. doi:10.1103/PhysRevA.102.012409

[BibTeX] [Download PDF]`@Article{pandya_hilbert-schmidt_2020, author = {Pandya, Palash and Sakarya, Omer and Wieśniak, Marcin}, journal = {Physical {R}eview {A}}, title = {Hilbert-{Schmidt} distance and entanglement witnessing}, year = {2020}, issn = {2469-9926, 2469-9934}, month = jul, number = {1}, pages = {012409}, volume = {102}, doi = {10.1103/PhysRevA.102.012409}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.012409}, urldate = {2021-05-10}, }`

- Jamie Sikora and John H. Selby. Impossibility of coin flipping in generalized probabilistic theories via discretizations of semi-infinite programs.
*Physical Review Research*, 2(4):43128, oct 2020. doi:10.1103/PhysRevResearch.2.043128

[BibTeX] [Download PDF]`@Article{sikora_impossibility_2020, author = {Sikora, Jamie and Selby, John H.}, journal = {Physical {R}eview {R}esearch}, title = {Impossibility of coin flipping in generalized probabilistic theories via discretizations of semi-infinite programs}, year = {2020}, issn = {2643-1564}, month = oct, number = {4}, pages = {043128}, volume = {2}, doi = {10.1103/PhysRevResearch.2.043128}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevResearch.2.043128}, urldate = {2021-05-10}, }`

- Alley Hameedi, Breno Marques, Piotr Mironowicz, Debashis Saha, Marcin Pawłowski, and Mohamed Bourennane. Experimental test of nonclassicality with arbitrarily low detection efficiency.
*Physical Review A*, 102(3):32621, sep 2020. doi:10.1103/PhysRevA.102.032621

[BibTeX] [Download PDF]`@Article{hameedi_experimental_2020, author = {Hameedi, Alley and Marques, Breno and Mironowicz, Piotr and Saha, Debashis and Pawłowski, Marcin and Bourennane, Mohamed}, journal = {Physical {R}eview {A}}, title = {Experimental test of nonclassicality with arbitrarily low detection efficiency}, year = {2020}, issn = {2469-9926, 2469-9934}, month = sep, number = {3}, pages = {032621}, volume = {102}, doi = {10.1103/PhysRevA.102.032621}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.032621}, urldate = {2021-05-10}, }`

- Thao P. Le, Piotr Mironowicz, and Paweł Horodecki. Blurred quantum Darwinism across quantum reference frames.
*Physical Review A*, 102(6):62420, dec 2020. doi:10.1103/PhysRevA.102.062420

[BibTeX] [Download PDF]`@Article{le_blurred_2020, author = {Le, Thao P. and Mironowicz, Piotr and Horodecki, Paweł}, journal = {Physical {R}eview {A}}, title = {Blurred quantum {Darwinism} across quantum reference frames}, year = {2020}, issn = {2469-9926, 2469-9934}, month = dec, number = {6}, pages = {062420}, volume = {102}, doi = {10.1103/PhysRevA.102.062420}, groups = {Pawel_H}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.062420}, urldate = {2021-05-10}, }`

- Sudha, H. S. Karthik, Rajarshi Pal, K. S. Akhilesh, Sibasish Ghosh, K. S. Mallesh, and A. R. Usha Devi. Canonical forms of two-qubit states under local operations.
*Physical Review A*, 102(5):52419, nov 2020. doi:10.1103/PhysRevA.102.052419

[BibTeX] [Download PDF]`@Article{sudha_canonical_2020, author = {{Sudha} and Karthik, H. S. and Pal, Rajarshi and Akhilesh, K. S. and Ghosh, Sibasish and Mallesh, K. S. and Usha Devi, A. R.}, journal = {Physical {R}eview {A}}, title = {Canonical forms of two-qubit states under local operations}, year = {2020}, issn = {2469-9926, 2469-9934}, month = nov, number = {5}, pages = {052419}, volume = {102}, doi = {10.1103/PhysRevA.102.052419}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.052419}, urldate = {2021-05-10}, }`

- Anubhav Chaturvedi and Debashis Saha. Quantum prescriptions are more ontologically distinct than they are operationally distinguishable.
*Quantum*, 4:345, oct 2020. doi:10.22331/q-2020-10-21-345

[BibTeX] [Abstract] [Download PDF]

Based on an intuitive generalization of the Leibniz principle of `the identity of indiscernibles’, we introduce a novel ontological notion of classicality, called bounded ontological distinctness. Formulated as a principle, bounded ontological distinctness equates the distinguishability of a set of operational physical entities to the distinctness of their ontological counterparts. Employing three instances of two-dimensional quantum preparations, we demonstrate the violation of bounded ontological distinctness or excess ontological distinctness of quantum preparations, without invoking any additional assumptions. Moreover, our methodology enables the inference of tight lower bounds on the extent of excess ontological distinctness of quantum preparations. Similarly, we demonstrate excess ontological distinctness of quantum transformations, using three two-dimensional unitary transformations. However, to demonstrate excess ontological distinctness of quantum measurements, an additional assumption such as outcome determinism or bounded ontological distinctness of preparations is required. Moreover, we show that quantum violations of other well-known ontological principles implicate quantum excess ontological distinctness. Finally, to showcase the operational vitality of excess ontological distinctness, we introduce two distinct classes of communication tasks powered by excess ontological distinctness.

`@article{chaturvedi_quantum_2020, title = {Quantum prescriptions are more ontologically distinct than they are operationally distinguishable}, volume = {4}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2020-10-21-345/}, doi = {10.22331/q-2020-10-21-345}, abstract = {Based on an intuitive generalization of the Leibniz principle of `the identity of indiscernibles', we introduce a novel ontological notion of classicality, called bounded ontological distinctness. Formulated as a principle, bounded ontological distinctness equates the distinguishability of a set of operational physical entities to the distinctness of their ontological counterparts. Employing three instances of two-dimensional quantum preparations, we demonstrate the violation of bounded ontological distinctness or excess ontological distinctness of quantum preparations, without invoking any additional assumptions. Moreover, our methodology enables the inference of tight lower bounds on the extent of excess ontological distinctness of quantum preparations. Similarly, we demonstrate excess ontological distinctness of quantum transformations, using three two-dimensional unitary transformations. However, to demonstrate excess ontological distinctness of quantum measurements, an additional assumption such as outcome determinism or bounded ontological distinctness of preparations is required. Moreover, we show that quantum violations of other well-known ontological principles implicate quantum excess ontological distinctness. Finally, to showcase the operational vitality of excess ontological distinctness, we introduce two distinct classes of communication tasks powered by excess ontological distinctness.}, language = {en}, urldate = {2021-05-10}, journal = {Quantum}, author = {Chaturvedi, Anubhav and Saha, Debashis}, month = oct, year = {2020}, pages = {345}, }`

- Michał Eckstein, Paweł Horodecki, Ryszard Horodecki, and Tomasz Miller. Operational causality in spacetime.
*Physical Review A*, 101(4):42128, apr 2020. arXiv: 1902.05002 doi:10.1103/PhysRevA.101.042128

[BibTeX] [Abstract] [Download PDF]

The no-signalling principle preventing superluminal communication is a limiting paradigm for physical theories. Within the information-theoretic framework it is commonly understood in terms of admissible correlations in composite systems. Here we unveil its complementary incarnation –- the ‘dynamical no-signalling principle’ –-, which forbids superluminal signalling via measurements on simple physical objects (e.g. particles) evolving in time. We show that it imposes strong constraints on admissible models of dynamics. The posited principle is universal –- it can be applied to any theory (classical, quantum or post-quantum) with well-defined rules of calculating detection statistics in spacetime. As an immediate application we show how one could exploit the Schr{\textbackslash}”odinger equation to establish a fully operational superluminal protocol in the Minkowski spacetime. This example illustrates how the principle can be used to identify the limits of applicability of a given model of quantum or post-quantum dynamics.

`@Article{eckstein_operational_2020, author = {Eckstein, Michał and Horodecki, Paweł and Horodecki, Ryszard and Miller, Tomasz}, journal = {Physical {R}eview {A}}, title = {Operational causality in spacetime}, year = {2020}, issn = {2469-9926, 2469-9934}, month = apr, note = {arXiv: 1902.05002}, number = {4}, pages = {042128}, volume = {101}, abstract = {The no-signalling principle preventing superluminal communication is a limiting paradigm for physical theories. Within the information-theoretic framework it is commonly understood in terms of admissible correlations in composite systems. Here we unveil its complementary incarnation --- the 'dynamical no-signalling principle' ---, which forbids superluminal signalling via measurements on simple physical objects (e.g. particles) evolving in time. We show that it imposes strong constraints on admissible models of dynamics. The posited principle is universal --- it can be applied to any theory (classical, quantum or post-quantum) with well-defined rules of calculating detection statistics in spacetime. As an immediate application we show how one could exploit the Schr{\textbackslash}"odinger equation to establish a fully operational superluminal protocol in the Minkowski spacetime. This example illustrates how the principle can be used to identify the limits of applicability of a given model of quantum or post-quantum dynamics.}, doi = {10.1103/PhysRevA.101.042128}, groups = {Pawel_H}, keywords = {Quantum Physics, General Relativity and Quantum Cosmology, Mathematical Physics, 81P16 (Primary), 81P15, 28E99, 60B05 (Secondary)}, url = {http://arxiv.org/abs/1902.05002}, urldate = {2021-05-11}, }`

- Marcin Łobejko, Paweł Mazurek, and Michał Horodecki. Thermodynamics of Minimal Coupling Quantum Heat Engines.
*Quantum*, 4:375, dec 2020. arXiv: 2003.05788 doi:10.22331/q-2020-12-23-375

[BibTeX] [Abstract] [Download PDF]

The minimal-coupling quantum heat engine is a thermal machine consisting of an explicit energy storage system, heat baths, and a working body, which alternatively couples to subsystems through discrete strokes – energy-conserving two-body quantum operations. Within this paradigm, we present a general framework of quantum thermodynamics, where a work extraction process is fundamentally limited by a flow of non-passive energy (ergotropy), while energy dissipation is expressed through a flow of passive energy. It turns out that small dimensionality of the working body and a restriction only to two-body operations make the engine fundamentally irreversible. Our main result is finding the optimal efficiency and work production per cycle within the whole class of irreversible minimal-coupling engines composed of three strokes and with the two-level working body, where we take into account all possible quantum correlations between the working body and the battery. One of the key new tools is the introduced “control-marginal state” – one which acts only on a working body Hilbert space, but encapsulates all features regarding work extraction of the total working body-battery system. In addition, we propose a generalization of the many-stroke engine, and we analyze efficiency vs extracted work trade-offs, as well as work fluctuations after many cycles of the running of the engine.

`@Article{lobejko_thermodynamics_2020, author = {Łobejko, Marcin and Mazurek, Paweł and Horodecki, Michał}, journal = {Quantum}, title = {Thermodynamics of {Minimal} {Coupling} {Quantum} {Heat} {Engines}}, year = {2020}, issn = {2521-327X}, month = dec, note = {arXiv: 2003.05788}, pages = {375}, volume = {4}, abstract = {The minimal-coupling quantum heat engine is a thermal machine consisting of an explicit energy storage system, heat baths, and a working body, which alternatively couples to subsystems through discrete strokes -- energy-conserving two-body quantum operations. Within this paradigm, we present a general framework of quantum thermodynamics, where a work extraction process is fundamentally limited by a flow of non-passive energy (ergotropy), while energy dissipation is expressed through a flow of passive energy. It turns out that small dimensionality of the working body and a restriction only to two-body operations make the engine fundamentally irreversible. Our main result is finding the optimal efficiency and work production per cycle within the whole class of irreversible minimal-coupling engines composed of three strokes and with the two-level working body, where we take into account all possible quantum correlations between the working body and the battery. One of the key new tools is the introduced "control-marginal state" -- one which acts only on a working body Hilbert space, but encapsulates all features regarding work extraction of the total working body-battery system. In addition, we propose a generalization of the many-stroke engine, and we analyze efficiency vs extracted work trade-offs, as well as work fluctuations after many cycles of the running of the engine.}, doi = {10.22331/q-2020-12-23-375}, groups = {Michal_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2003.05788}, urldate = {2021-07-28}, }`

- Cristina Cirstoiu, Kamil Korzekwa, and David Jennings. Robustness of Noether’s principle: Maximal disconnects between conservation laws and symmetries in quantum theory.
*Physical Review X*, 10(4):41035, nov 2020. arXiv: 1908.04254 doi:10.1103/PhysRevX.10.041035

[BibTeX] [Abstract] [Download PDF]

To what extent does Noether’s principle apply to quantum channels? Here, we quantify the degree to which imposing a symmetry constraint on quantum channels implies a conservation law, and show that this relates to physically impossible transformations in quantum theory, such as time-reversal and spin-inversion. In this analysis, the convex structure and extremal points of the set of quantum channels symmetric under the action of a Lie group \$G\$ becomes essential. It allows us to derive bounds on the deviation from conservation laws under any symmetric quantum channel in terms of the deviation from closed dynamics as measured by the unitarity of the channel. In particular, we investigate in detail the \$U(1)\$ and \$SU(2)\$ symmetries related to energy and angular momentum conservation laws. In the latter case, we provide fundamental limits on how much a spin-\$j_A\$ system can be used to polarise a larger spin-\$j_B\$ system, and on how much one can invert spin polarisation using a rotationally-symmetric operation. Finally, we also establish novel links between unitarity, complementary channels and purity that are of independent interest.

`@Article{cirstoiu_robustness_2020, author = {Cirstoiu, Cristina and Korzekwa, Kamil and Jennings, David}, journal = {Physical {R}eview {X}}, title = {Robustness of {Noether}'s principle: {Maximal} disconnects between conservation laws and symmetries in quantum theory}, year = {2020}, issn = {2160-3308}, month = nov, note = {arXiv: 1908.04254}, number = {4}, pages = {041035}, volume = {10}, abstract = {To what extent does Noether's principle apply to quantum channels? Here, we quantify the degree to which imposing a symmetry constraint on quantum channels implies a conservation law, and show that this relates to physically impossible transformations in quantum theory, such as time-reversal and spin-inversion. In this analysis, the convex structure and extremal points of the set of quantum channels symmetric under the action of a Lie group \$G\$ becomes essential. It allows us to derive bounds on the deviation from conservation laws under any symmetric quantum channel in terms of the deviation from closed dynamics as measured by the unitarity of the channel. In particular, we investigate in detail the \$U(1)\$ and \$SU(2)\$ symmetries related to energy and angular momentum conservation laws. In the latter case, we provide fundamental limits on how much a spin-\$j\_A\$ system can be used to polarise a larger spin-\$j\_B\$ system, and on how much one can invert spin polarisation using a rotationally-symmetric operation. Finally, we also establish novel links between unitarity, complementary channels and purity that are of independent interest.}, doi = {10.1103/PhysRevX.10.041035}, keywords = {Quantum Physics}, shorttitle = {Robustness of {Noether}'s principle}, url = {http://arxiv.org/abs/1908.04254}, urldate = {2021-07-28}, }`

- Daniel Jost Brod and Michał Oszmaniec. Classical simulation of linear optics subject to nonuniform losses.
*Quantum*, 4:267, may 2020. arXiv: 1906.06696 doi:10.22331/q-2020-05-14-267

[BibTeX] [Abstract] [Download PDF]

We present a comprehensive study of the impact of non-uniform, i.e.{\textbackslash} path-dependent, photonic losses on the computational complexity of linear-optical processes. Our main result states that, if each beam splitter in a network induces some loss probability, non-uniform network designs cannot circumvent the efficient classical simulations based on losses. To achieve our result we obtain new intermediate results that can be of independent interest. First, we show that, for any network of lossy beam-splitters, it is possible to extract a layer of non-uniform losses that depends on the network geometry. We prove that, for every input mode of the network it is possible to commute \$s_i\$ layers of losses to the input, where \$s_i\$ is the length of the shortest path connecting the \$i\$th input to any output. We then extend a recent classical simulation algorithm due to P. Clifford and R. Clifford to allow for arbitrary \$n\$-photon input Fock states (i.e. to include collision states). Consequently, we identify two types of input states where boson sampling becomes classically simulable: (A) when \$n\$ input photons occupy a constant number of input modes; (B) when all but \$O({\textbackslash}log n)\$ photons are concentrated on a single input mode, while an additional \$O({\textbackslash}log n)\$ modes contain one photon each.

`@Article{brod_classical_2020, author = {Brod, Daniel Jost and Oszmaniec, Michał}, journal = {Quantum}, title = {Classical simulation of linear optics subject to nonuniform losses}, year = {2020}, issn = {2521-327X}, month = may, note = {arXiv: 1906.06696}, pages = {267}, volume = {4}, abstract = {We present a comprehensive study of the impact of non-uniform, i.e.{\textbackslash} path-dependent, photonic losses on the computational complexity of linear-optical processes. Our main result states that, if each beam splitter in a network induces some loss probability, non-uniform network designs cannot circumvent the efficient classical simulations based on losses. To achieve our result we obtain new intermediate results that can be of independent interest. First, we show that, for any network of lossy beam-splitters, it is possible to extract a layer of non-uniform losses that depends on the network geometry. We prove that, for every input mode of the network it is possible to commute \$s\_i\$ layers of losses to the input, where \$s\_i\$ is the length of the shortest path connecting the \$i\$th input to any output. We then extend a recent classical simulation algorithm due to P. Clifford and R. Clifford to allow for arbitrary \$n\$-photon input Fock states (i.e. to include collision states). Consequently, we identify two types of input states where boson sampling becomes classically simulable: (A) when \$n\$ input photons occupy a constant number of input modes; (B) when all but \$O({\textbackslash}log n)\$ photons are concentrated on a single input mode, while an additional \$O({\textbackslash}log n)\$ modes contain one photon each.}, doi = {10.22331/q-2020-05-14-267}, keywords = {Quantum Physics, Mathematical Physics}, url = {http://arxiv.org/abs/1906.06696}, urldate = {2021-07-28}, }`

- Michał Eckstein, Paweł Horodecki, Ryszard Horodecki, and Tomasz Miller. Operational causality in spacetime.
*Physical Review A*, 101(4):42128, apr 2020. arXiv: 1902.05002 doi:10.1103/PhysRevA.101.042128

[BibTeX] [Abstract] [Download PDF]

The no-signalling principle preventing superluminal communication is a limiting paradigm for physical theories. Within the information-theoretic framework it is commonly understood in terms of admissible correlations in composite systems. Here we unveil its complementary incarnation –- the ‘dynamical no-signalling principle’ –-, which forbids superluminal signalling via measurements on simple physical objects (e.g. particles) evolving in time. We show that it imposes strong constraints on admissible models of dynamics. The posited principle is universal –- it can be applied to any theory (classical, quantum or post-quantum) with well-defined rules of calculating detection statistics in spacetime. As an immediate application we show how one could exploit the Schr{\textbackslash}”odinger equation to establish a fully operational superluminal protocol in the Minkowski spacetime. This example illustrates how the principle can be used to identify the limits of applicability of a given model of quantum or post-quantum dynamics.

`@Article{eckstein_operational_2020-1, author = {Eckstein, Michał and Horodecki, Paweł and Horodecki, Ryszard and Miller, Tomasz}, journal = {Physical {R}eview {A}}, title = {Operational causality in spacetime}, year = {2020}, issn = {2469-9926, 2469-9934}, month = apr, note = {arXiv: 1902.05002}, number = {4}, pages = {042128}, volume = {101}, abstract = {The no-signalling principle preventing superluminal communication is a limiting paradigm for physical theories. Within the information-theoretic framework it is commonly understood in terms of admissible correlations in composite systems. Here we unveil its complementary incarnation --- the 'dynamical no-signalling principle' ---, which forbids superluminal signalling via measurements on simple physical objects (e.g. particles) evolving in time. We show that it imposes strong constraints on admissible models of dynamics. The posited principle is universal --- it can be applied to any theory (classical, quantum or post-quantum) with well-defined rules of calculating detection statistics in spacetime. As an immediate application we show how one could exploit the Schr{\textbackslash}"odinger equation to establish a fully operational superluminal protocol in the Minkowski spacetime. This example illustrates how the principle can be used to identify the limits of applicability of a given model of quantum or post-quantum dynamics.}, doi = {10.1103/PhysRevA.101.042128}, groups = {Pawel_H}, keywords = {Quantum Physics, General Relativity and Quantum Cosmology, Mathematical Physics, 81P16 (Primary), 81P15, 28E99, 60B05 (Secondary)}, url = {http://arxiv.org/abs/1902.05002}, urldate = {2021-07-28}, }`

- Jakub Czartowski, Dardo Goyeneche, Markus Grassl, and Karol Życzkowski. Isoentangled Mutually Unbiased Bases, Symmetric Quantum Measurements, and Mixed-State Designs.
*Physical Review Letters*, 124(9):90503, mar 2020. doi:10.1103/PhysRevLett.124.090503

[BibTeX] [Download PDF]`@Article{czartowski_isoentangled_2020, author = {Czartowski, Jakub and Goyeneche, Dardo and Grassl, Markus and Życzkowski, Karol}, journal = {Physical {R}eview {L}etters}, title = {Isoentangled {Mutually} {Unbiased} {Bases}, {Symmetric} {Quantum} {Measurements}, and {Mixed}-{State} {Designs}}, year = {2020}, issn = {0031-9007, 1079-7114}, month = mar, number = {9}, pages = {090503}, volume = {124}, doi = {10.1103/PhysRevLett.124.090503}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.124.090503}, urldate = {2020-04-22}, }`

- Robert Alicki and Alejandro Jenkins. Quantum theory of triboelectricity.
*Physical Review Letters*, 125(18):186101, oct 2020. doi:10.1103/PhysRevLett.125.186101

[BibTeX] [Abstract] [Download PDF]

We propose a microphysical theory of the triboelectric effect by which mechanical rubbing separates charges across the interface between two materials. Surface electrons are treated as an open system coupled to two baths, corresponding to the bulks. Extending Zel’dovich’s theory of bosonic superradiance, we show that motion-induced population inversion can generate an electromotive force. We argue that this is consistent with the basic phenomenology of triboelectrification and triboluminescence as irreversible processes, and we suggest how to carry out more precise experimental tests.

`@Article{Alicki2020, author = {Alicki, Robert and Jenkins, Alejandro}, journal = {Physical {R}eview {L}etters}, title = {Quantum Theory of Triboelectricity}, year = {2020}, month = oct, number = {18}, pages = {186101}, volume = {125}, abstract = {We propose a microphysical theory of the triboelectric effect by which mechanical rubbing separates charges across the interface between two materials. Surface electrons are treated as an open system coupled to two baths, corresponding to the bulks. Extending Zel'dovich's theory of bosonic superradiance, we show that motion-induced population inversion can generate an electromotive force. We argue that this is consistent with the basic phenomenology of triboelectrification and triboluminescence as irreversible processes, and we suggest how to carry out more precise experimental tests.}, archiveprefix = {arXiv}, doi = {10.1103/PhysRevLett.125.186101}, eid = {186101}, eprint = {1904.11997}, keywords = {Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Statistical Mechanics, High Energy Physics - Theory, Quantum Physics}, primaryclass = {cond-mat.mes-hall}, url = {https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.125.186101}, }`

- Piotr Kopszak, Marek Mozrzymas, Michał Studziński, and Michał Horodecki. Multiport based teleportation – transmission of a large amount of quantum information.
*Quantum 5, 576 (2021)*, aug 2020. doi:10.22331/q-2021-11-11-576

[BibTeX] [Abstract] [Download PDF]

We analyse the problem of transmitting a number of unknown quantum states or one composite system in one go. We derive a lower bound on the performance of such process, measured in the entanglement fidelity. The obtained bound is effectively computable and outperforms the explicit values of the entanglement fidelity calculated for the pre-existing variants of the port-based protocols, allowing for teleportation of a much larger amount of quantum information. The comparison with the exact formulas and similar analysis for the probabilistic scheme is also discussed. In particular, we present the closed-form expressions for the entanglement fidelity and for the probability of success in the probabilistic scheme in the qubit case in the picture of the spin angular momentum.

`@Article{Kopszak2020, author = {Piotr Kopszak and Marek Mozrzymas and Michał Studziński and Michał Horodecki}, journal = {Quantum 5, 576 (2021)}, title = {Multiport based teleportation -- transmission of a large amount of quantum information}, year = {2020}, month = aug, abstract = {We analyse the problem of transmitting a number of unknown quantum states or one composite system in one go. We derive a lower bound on the performance of such process, measured in the entanglement fidelity. The obtained bound is effectively computable and outperforms the explicit values of the entanglement fidelity calculated for the pre-existing variants of the port-based protocols, allowing for teleportation of a much larger amount of quantum information. The comparison with the exact formulas and similar analysis for the probabilistic scheme is also discussed. In particular, we present the closed-form expressions for the entanglement fidelity and for the probability of success in the probabilistic scheme in the qubit case in the picture of the spin angular momentum.}, archiveprefix = {arXiv}, doi = {10.22331/q-2021-11-11-576}, eprint = {2008.00856}, groups = {Michal_H}, keywords = {quant-ph}, primaryclass = {quant-ph}, url = {http://arxiv.org/pdf/2008.00856v3}, }`

### 2019

- Alejandro Pozas-Kerstjens, Rafael Rabelo, Łukasz Rudnicki, Rafael Chaves, Daniel Cavalcanti, Miguel Navascués, and Antonio Acín. Bounding the sets of classical and quantum correlations in networks.
*Physical Review Letters*, 123(14):140503, 2019. doi:10.1103/PhysRevLett.123.140503

[BibTeX] [Download PDF]`@Article{pozas-kerstjens_bounding_2019, author = {Pozas-Kerstjens, Alejandro and Rabelo, Rafael and Rudnicki, Łukasz and Chaves, Rafael and Cavalcanti, Daniel and Navascués, Miguel and Acín, Antonio}, journal = {Physical {R}eview {L}etters}, title = {Bounding the sets of classical and quantum correlations in networks}, year = {2019}, issn = {0031-9007, 1079-7114}, month = oct, number = {14}, pages = {140503}, volume = {123}, doi = {10.1103/PhysRevLett.123.140503}, groups = {Rudnicki}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.123.140503}, urldate = {2020-04-22}, }`

- Dong Yang, Karol Horodecki, and Andreas Winter. Distributed private randomness distillation.
*Physical Review Letters*, 123(17):170501, oct 2019. doi:10.1103/PhysRevLett.123.170501

[BibTeX] [Download PDF]`@Article{yang_distributed_2019, author = {Yang, Dong and Horodecki, Karol and Winter, Andreas}, journal = {Physical {R}eview {L}etters}, title = {Distributed private randomness distillation}, year = {2019}, issn = {0031-9007, 1079-7114}, month = oct, number = {17}, pages = {170501}, volume = {123}, doi = {10.1103/PhysRevLett.123.170501}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.123.170501}, urldate = {2020-04-22}, }`

- Thomas Van Himbeeck, Jonatan Bohr Brask, Stefano Pironio, Ravishankar Ramanathan, Ana Belén Sainz, and Elie Wolfe. Quantum violations in the Instrumental scenario and their relations to the Bell scenario.
*Quantum*, 3:186, 2019. doi:10.22331/q-2019-09-16-186

[BibTeX] [Abstract] [Download PDF]

The causal structure of any experiment implies restrictions on the observable correlations between measurement outcomes, which are different for experiments exploiting classical, quantum, or post-quantum resources. In the study of Bell nonlocality, these differences have been explored in great detail for more and more involved causal structures. Here, we go in the opposite direction and identify the simplest causal structure which exhibits a separation between classical, quantum, and post-quantum correlations. It arises in the so-called Instrumental scenario, known from classical causal models. We derive inequalities for this scenario and show that they are closely related to well-known Bell inequalities, such as the Clauser-Horne-Shimony-Holt inequality, which enables us to easily identify their classical, quantum, and post-quantum bounds as well as strategies violating the first two. The relations that we uncover imply that the quantum or post-quantum advantages witnessed by the violation of our Instrumental inequalities are not fundamentally different from those witnessed by the violations of standard inequalities in the usual Bell scenario. However, non-classical tests in the Instrumental scenario require fewer input choices than their Bell scenario counterpart, which may have potential implications for device-independent protocols.

`@Article{van_himbeeck_quantum_2019, author = {Van Himbeeck, Thomas and Bohr Brask, Jonatan and Pironio, Stefano and Ramanathan, Ravishankar and Sainz, Ana Belén and Wolfe, Elie}, journal = {Quantum}, title = {Quantum violations in the {Instrumental} scenario and their relations to the {Bell} scenario}, year = {2019}, issn = {2521-327X}, month = sep, pages = {186}, volume = {3}, abstract = {The causal structure of any experiment implies restrictions on the observable correlations between measurement outcomes, which are different for experiments exploiting classical, quantum, or post-quantum resources. In the study of Bell nonlocality, these differences have been explored in great detail for more and more involved causal structures. Here, we go in the opposite direction and identify the simplest causal structure which exhibits a separation between classical, quantum, and post-quantum correlations. It arises in the so-called Instrumental scenario, known from classical causal models. We derive inequalities for this scenario and show that they are closely related to well-known Bell inequalities, such as the Clauser-Horne-Shimony-Holt inequality, which enables us to easily identify their classical, quantum, and post-quantum bounds as well as strategies violating the first two. The relations that we uncover imply that the quantum or post-quantum advantages witnessed by the violation of our Instrumental inequalities are not fundamentally different from those witnessed by the violations of standard inequalities in the usual Bell scenario. However, non-classical tests in the Instrumental scenario require fewer input choices than their Bell scenario counterpart, which may have potential implications for device-independent protocols.}, doi = {10.22331/q-2019-09-16-186}, groups = {Belen}, language = {en}, url = {https://quantum-journal.org/papers/q-2019-09-16-186/}, urldate = {2020-04-22}, }`

- Piotr Mironowicz and Marcin Pawłowski. Experimentally feasible semi-device-independent certification of four-outcome positive-operator-valued measurements.
*Physical Review A*, 100(3):30301, sep 2019. doi:10.1103/PhysRevA.100.030301

[BibTeX] [Download PDF]`@Article{mironowicz_experimentally_2019, author = {Mironowicz, Piotr and Pawłowski, Marcin}, journal = {Physical {R}eview {A}}, title = {Experimentally feasible semi-device-independent certification of four-outcome positive-operator-valued measurements}, year = {2019}, issn = {2469-9926, 2469-9934}, month = sep, number = {3}, pages = {030301}, volume = {100}, doi = {10.1103/PhysRevA.100.030301}, groups = {Pawlowski}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.100.030301}, urldate = {2020-04-22} }`

- Robert Alicki. A quantum open system model of molecular battery charged by excitons.
*The Journal of Chemical Physics*, 150(21):214110, jun 2019. doi:10.1063/1.5096772

[BibTeX] [Download PDF]`@Article{alicki_quantum_2019, author = {Alicki, Robert}, journal = {The {J}ournal of {C}hemical {P}hysics}, title = {A quantum open system model of molecular battery charged by excitons}, year = {2019}, issn = {0021-9606}, month = jun, number = {21}, pages = {214110}, volume = {150}, doi = {10.1063/1.5096772}, url = {https://aip.scitation.org/doi/10.1063/1.5096772}, urldate = {2020-04-22}, }`

- Giuseppe Baio, Dariusz Chruściński, Paweł Horodecki, Antonino Messina, and Gniewomir Sarbicki. Bounds on the entanglement of two-qutrit systems from fixed marginals.
*Physical Review A*, 99(6):62312, jun 2019. doi:10.1103/PhysRevA.99.062312

[BibTeX] [Download PDF]`@Article{baio_bounds_2019, author = {Baio, Giuseppe and Chruściński, Dariusz and Horodecki, Paweł and Messina, Antonino and Sarbicki, Gniewomir}, journal = {Physical {R}eview {A}}, title = {Bounds on the entanglement of two-qutrit systems from fixed marginals}, year = {2019}, issn = {2469-9926, 2469-9934}, month = jun, number = {6}, pages = {062312}, volume = {99}, doi = {10.1103/PhysRevA.99.062312}, groups = {Pawel_H}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.99.062312}, urldate = {2020-04-22}, }`

- Robert Alicki. Quantum Features of Macroscopic Fields: Entropy and Dynamics.
*Entropy*, 21(7):705, 2019. doi:10.3390/e21070705

[BibTeX] [Abstract] [Download PDF]

Macroscopic fields such as electromagnetic, magnetohydrodynamic, acoustic or gravitational waves are usually described by classical wave equations with possible additional damping terms and coherent sources. The aim of this paper is to develop a complete macroscopic formalism including random/thermal sources, dissipation and random scattering of waves by environment. The proposed reduced state of the field combines averaged field with the two-point correlation function called single-particle density matrix. The evolution equation for the reduced state of the field is obtained by reduction of the generalized quasi-free dynamical semigroups describing irreversible evolution of bosonic quantum field and the definition of entropy for the reduced state of the field follows from the von Neumann entropy of quantum field states. The presented formalism can be applied, for example, to superradiance phenomena and allows unifying the Mueller and Jones calculi in polarization optics.

`@article{robert_alicki_quantum_2019, title = {Quantum {Features} of {Macroscopic} {Fields}: {Entropy} and {Dynamics}}, volume = {21}, issn = {1099-4300}, shorttitle = {Quantum {Features} of {Macroscopic} {Fields}}, url = {https://www.mdpi.com/1099-4300/21/7/705}, doi = {10.3390/e21070705}, abstract = {Macroscopic fields such as electromagnetic, magnetohydrodynamic, acoustic or gravitational waves are usually described by classical wave equations with possible additional damping terms and coherent sources. The aim of this paper is to develop a complete macroscopic formalism including random/thermal sources, dissipation and random scattering of waves by environment. The proposed reduced state of the field combines averaged field with the two-point correlation function called single-particle density matrix. The evolution equation for the reduced state of the field is obtained by reduction of the generalized quasi-free dynamical semigroups describing irreversible evolution of bosonic quantum field and the definition of entropy for the reduced state of the field follows from the von Neumann entropy of quantum field states. The presented formalism can be applied, for example, to superradiance phenomena and allows unifying the Mueller and Jones calculi in polarization optics.}, language = {en}, number = {7}, urldate = {2020-04-22}, journal = {Entropy}, author = {{Robert Alicki}}, month = jul, year = {2019}, pages = {705}, }`

- Paweł Horodecki and Ravishankar Ramanathan. The relativistic causality versus no-signaling paradigm for multi-party correlations.
*Nature Communications*, 10(1):1701, dec 2019. doi:10.1038/s41467-019-09505-2

[BibTeX] [Download PDF]`@Article{horodecki_relativistic_2019, author = {Horodecki, Paweł and Ramanathan, Ravishankar}, journal = {Nature {C}ommunications}, title = {The relativistic causality versus no-signaling paradigm for multi-party correlations}, year = {2019}, issn = {2041-1723}, month = dec, number = {1}, pages = {1701}, volume = {10}, doi = {10.1038/s41467-019-09505-2}, groups = {Pawel_H}, language = {en}, url = {http://www.nature.com/articles/s41467-019-09505-2}, urldate = {2020-04-22}, }`

- Junghee Ryu, Bianka Woloncewicz, Marcin Marciniak, Marcin Wieśniak, and Marek Żukowski. General mapping of multiqudit entanglement conditions to nonseparability indicators for quantum-optical fields.
*Physical Review Research*, 1(3):32041, dec 2019. doi:10.1103/PhysRevResearch.1.032041

[BibTeX] [Download PDF]`@Article{ryu_general_2019, author = {Ryu, Junghee and Woloncewicz, Bianka and Marciniak, Marcin and Wieśniak, Marcin and Żukowski, Marek}, journal = {Physical {R}eview {R}esearch}, title = {General mapping of multiqudit entanglement conditions to nonseparability indicators for quantum-optical fields}, year = {2019}, issn = {2643-1564}, month = dec, number = {3}, pages = {032041}, volume = {1}, doi = {10.1103/PhysRevResearch.1.032041}, groups = {Zukowski}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevResearch.1.032041}, urldate = {2020-05-13}, }`

- Waldemar Kłobus, Adam Burchardt, Adrian Kołodziejski, Mahasweta Pandit, Tamás Vértesi, Karol Życzkowski, and Wiesław Laskowski. K-uniform mixed states.
*Physical Review A*, 100(3):32112, sep 2019. doi:10.1103/PhysRevA.100.032112

[BibTeX] [Download PDF]`@Article{klobus_k_2019, author = {Kłobus, Waldemar and Burchardt, Adam and Kołodziejski, Adrian and Pandit, Mahasweta and Vértesi, Tamás and Życzkowski, Karol and Laskowski, Wiesław}, journal = {Physical {R}eview {A}}, title = {k-uniform mixed states}, year = {2019}, issn = {2469-9926, 2469-9934}, month = sep, number = {3}, pages = {032112}, volume = {100}, doi = {10.1103/PhysRevA.100.032112}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.100.032112}, urldate = {2021-05-10}, }`

- Máté. Farkas and Jędrzej Kaniewski. Self-testing mutually unbiased bases in the prepare-and-measure scenario.
*Physical Review A*, 99(3):32316, mar 2019. doi:10.1103/PhysRevA.99.032316

[BibTeX] [Download PDF]`@Article{farkas_self-testing_2019, author = {Farkas, Máté and Kaniewski, Jędrzej}, journal = {Physical {R}eview {A}}, title = {Self-testing mutually unbiased bases in the prepare-and-measure scenario}, year = {2019}, issn = {2469-9926, 2469-9934}, month = mar, number = {3}, pages = {032316}, volume = {99}, doi = {10.1103/PhysRevA.99.032316}, language = {en}, url = {https://link.aps.org/doi/10.1103/PhysRevA.99.032316}, urldate = {2021-05-10}, }`

- Waldemar Klobus, Adam Burchardt, Adrian Kolodziejski, Mahasweta Pandit, Tamas Vertesi, Karol Zyczkowski, and Wieslaw Laskowski. K-uniform mixed states.
*Physical Review A*, 100(3):32112, sep 2019. arXiv: 1906.01311 doi:10.1103/PhysRevA.100.032112

[BibTeX] [Abstract] [Download PDF]

We investigate the maximum purity that can be achieved by k-uniform mixed states of N parties. Such N-party states have the property that all their k-party reduced states are maximally mixed. A scheme to construct explicitly k-uniform states using a set of specific N-qubit Pauli matrices is proposed. We provide several different examples of such states and demonstrate that in some cases the state corresponds to a particular orthogonal array. The obtained states, despite being mixed, reveal strong non-classical properties such as genuine multipartite entanglement or violation of Bell inequalities.

`@Article{klobus_$k$-uniform_2019, author = {Klobus, Waldemar and Burchardt, Adam and Kolodziejski, Adrian and Pandit, Mahasweta and Vertesi, Tamas and Zyczkowski, Karol and Laskowski, Wieslaw}, journal = {Physical {R}eview {A}}, title = {k-uniform mixed states}, year = {2019}, issn = {2469-9926, 2469-9934}, month = sep, note = {arXiv: 1906.01311}, number = {3}, pages = {032112}, volume = {100}, abstract = {We investigate the maximum purity that can be achieved by k-uniform mixed states of N parties. Such N-party states have the property that all their k-party reduced states are maximally mixed. A scheme to construct explicitly k-uniform states using a set of specific N-qubit Pauli matrices is proposed. We provide several different examples of such states and demonstrate that in some cases the state corresponds to a particular orthogonal array. The obtained states, despite being mixed, reveal strong non-classical properties such as genuine multipartite entanglement or violation of Bell inequalities.}, doi = {10.1103/PhysRevA.100.032112}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/1906.01311}, urldate = {2021-07-28}, }`

- Antonio Mandarino, Karl Joulain, Melisa Domínguez Gómez, and Bruno Bellomo. Thermal transistor effect in quantum systems.
*Physcal Review Applied 16, 034026 (2021)*, feb 2019. doi:10.1103/PhysRevApplied.16.034026

[BibTeX] [Abstract] [Download PDF]

We study a quantum system composed of three interacting qubits, each coupled to a different thermal reservoir. We show how to engineer it in order to build a quantum device that is analogous to an electronic bipolar transistor. We outline how the interaction among the qubits plays a crucial role for the appearance of the effect, also linking it to the characteristics of system-bath interactions that govern the decoherence and dissipation mechanism of the system. By comparing with previous proposals, the model considered here extends the regime of parameters where the transistor effect shows up and its robustness with respect to small variations of the coupling parameters. Moreover, our model appears to be more realistic and directly connected in terms of potential implementations to feasible setups in the domain of quantum spin chains and molecular nanomagnets.

`@Article{Mandarino2019, author = {Antonio Mandarino and Karl Joulain and Melisa Domínguez Gómez and Bruno Bellomo}, journal = {Physcal {R}eview {A}pplied 16, 034026 (2021)}, title = {Thermal transistor effect in quantum systems}, year = {2019}, month = feb, abstract = {We study a quantum system composed of three interacting qubits, each coupled to a different thermal reservoir. We show how to engineer it in order to build a quantum device that is analogous to an electronic bipolar transistor. We outline how the interaction among the qubits plays a crucial role for the appearance of the effect, also linking it to the characteristics of system-bath interactions that govern the decoherence and dissipation mechanism of the system. By comparing with previous proposals, the model considered here extends the regime of parameters where the transistor effect shows up and its robustness with respect to small variations of the coupling parameters. Moreover, our model appears to be more realistic and directly connected in terms of potential implementations to feasible setups in the domain of quantum spin chains and molecular nanomagnets.}, archiveprefix = {arXiv}, doi = {10.1103/PhysRevApplied.16.034026}, eprint = {1902.01309}, file = {:Mandarino2019 - Thermal Transistor Effect in Quantum Systems.pdf:PDF}, groups = {Zukowski}, keywords = {quant-ph}, primaryclass = {quant-ph}, url = {https://journals.aps.org/prapplied/pdf/10.1103/PhysRevApplied.16.034026}, }`

### 2018

- Arijit Dutta, Tschang-Uh Nahm, Jinhyoung Lee, and Marek Żukowski. Geometric extension of Clauser–Horne inequality to more qubits.
*New Journal of Physics*, 20(9):93006, sep 2018. doi:10.1088/1367-2630/aadc78

[BibTeX] [Download PDF]`@Article{dutta_geometric_2018, author = {Dutta, Arijit and Nahm, Tschang-Uh and Lee, Jinhyoung and Żukowski, Marek}, journal = {New {J}ournal of {P}hysics}, title = {Geometric extension of {Clauser}–{Horne} inequality to more qubits}, year = {2018}, issn = {1367-2630}, month = sep, number = {9}, pages = {093006}, volume = {20}, doi = {10.1088/1367-2630/aadc78}, groups = {Zukowski}, url = {https://iopscience.iop.org/article/10.1088/1367-2630/aadc78}, urldate = {2020-04-22}, }`

## arXiv preprints

### 2022

- Gaojun Luo, Martianus Frederic Ezerman, Markus Grassl Grassl, and San Ling. How much entanglement does a quantum code need?.
*Arxiv:2207.05647 [quant-ph]*, 2022. arXiv:2207.05647 [quant-ph]

[BibTeX] [Download PDF]`@article{LuoEtAl_2022_EAQECC, title = {How Much Entanglement Does a Quantum Code Need?}, url = {https://arxiv.org/abs/2207.05647}, urldate = {2022-07-12}, abstract = {}, journal = {arXiv:2207.05647 [quant-ph]}, author = {Luo, Gaojun and Ezerman, Martianus Frederic and Grassl, Markus Grassl and Ling, San}, month = Jul, year = {2022}, note = {arXiv:2207.05647 [quant-ph]}, keywords = {} }`

- Chenfeng Cao, Chao Zhang, Zipeng Wu, Markus Grassl, and Bei Zeng. Quantum variational learning for quantum error-correcting codes.
*Arxiv:2204.03560 [quant-ph]*, 2022. arXiv:2204.03560 [quant-ph]

[BibTeX] [Download PDF]`@article{CaoEtAl_2022_VarQEC, title = {Quantum variational learning for quantum error-correcting codes}, url = {https://arxiv.org/abs/2204.03560}, urldate = {2022-04-07}, abstract = {}, journal = {arXiv:2204.03560 [quant-ph]}, author = {Cao, Chenfeng and Zhang, Chao and Wu, Zipeng and Grassl, Markus and Zeng, Bei}, month = apr, year = {2022}, note = {arXiv:2204.03560 [quant-ph]}, keywords = {} }`

- Marcin Karczewski, Giovanni Scala, Antonio Mandarino, and Marek Żukowski. Avenues to generalising Bell inequalities.
*Arxiv:2202.06606 [quant-ph]*, 2022. arXiv:2202.06606 [quant-ph]

[BibTeX] [Download PDF]`@article{karczewski_2022_avenues, title = {Avenues to generalising {Bell} inequalities}, url = {https://arxiv.org/abs/2202.06606}, urldate = {2022-03-29}, abstract = {}, journal = {arXiv:2202.06606 [quant-ph]}, author = {Karczewski, Marcin and Scala, Giovanni and Mandarino, Antonio and Żukowski, Marek}, month = feb, year = {2022}, note = {arXiv:2202.06606 [quant-ph]}, keywords = {Quantum Physics} }`

- Vinicius P. Rossi, Matty J. Hoban, and Ana Belen Sainz. On characterising assemblages in Einstein-Podolsky-Rosen scenarios.
*Arxiv:2202.05553 [quant-ph]*, feb 2022. arXiv:2202.05553 [quant-ph]

[BibTeX] [Abstract] [Download PDF]

Characterising non-classical quantum phenomena is crucial not only from a fundamental perspective, but also to better understand its capabilities for information processing and communication tasks. In this work, we focus on exploring the characterisation of Einstein-Podolsky-Rosen inference (a.k.a. steering): a signature of non-classicality manifested when one or more parties in a Bell scenario have their systems and measurements described by quantum theory, rather than being treated as black boxes. We propose a way of characterising common-cause assemblages from the correlations that arise when the trusted party performs tomographically-complete measurements on their share of the experiment, and discuss the advantages and challenges of this approach. Within this framework, we show that so-called almost quantum assemblages satisfy the principle of macroscopic noncontextuality, and demonstrate that a subset of almost quantum correlations recover almost quantum assemblages in this approach.

`@article{rossi_2022_characterising, title = {On characterising assemblages in {Einstein-Podolsky-Rosen} scenarios}, url = {https://arxiv.org/abs/2202.05553}, urldate = {2022-03-29}, abstract = {Characterising non-classical quantum phenomena is crucial not only from a fundamental perspective, but also to better understand its capabilities for information processing and communication tasks. In this work, we focus on exploring the characterisation of Einstein-Podolsky-Rosen inference (a.k.a. steering): a signature of non-classicality manifested when one or more parties in a Bell scenario have their systems and measurements described by quantum theory, rather than being treated as black boxes. We propose a way of characterising common-cause assemblages from the correlations that arise when the trusted party performs tomographically-complete measurements on their share of the experiment, and discuss the advantages and challenges of this approach. Within this framework, we show that so-called almost quantum assemblages satisfy the principle of macroscopic noncontextuality, and demonstrate that a subset of almost quantum correlations recover almost quantum assemblages in this approach.}, journal = {arXiv:2202.05553 [quant-ph]}, author = {Rossi, Vinicius P. and Hoban, Matty J. and Sainz, Ana Belen}, month = feb, year = {2022}, note = {arXiv:2202.05553 [quant-ph]}, keywords = {Quantum Physics} }`

### 2021

- Marcus Appleby, Ingemar Bengtsson, Markus Grassl, Michael Harrison, and Gary McConnell. SIC-POVMs from Stark units.
*Arxiv:2112.05552 [quant-ph]*, 2021. arXiv:2112.05552 [quant-ph]

[BibTeX] [Abstract] [Download PDF]

We propose a recipe for constructing a SIC fiducial vector in any complex Hilbert space whose dimension is of the form $d=n^2+3$, starting from Stark units in a ray class field that does not contain a complex root of unity. The recipe relies on some number theoretical theorems, a version of the Stark conjectures, and some standard conjectures for SICs. In this paper we focus on the conceptually simplest case, prime dimensions of the form $d=n^2+3$, and report that we have constructed SICs in twelve prime dimensions of this kind, the highest being $d=19603$.

`@article{Grassl_SIC-POVMS_2021, title = {{SIC-POVMs} from {Stark} Units}, url = {https://arxiv.org/abs/2112.05552}, urldate = {2021-12-13}, abstract = {We propose a recipe for constructing a SIC fiducial vector in any complex Hilbert space whose dimension is of the form $d=n^2+3$, starting from Stark units in a ray class field that does not contain a complex root of unity. The recipe relies on some number theoretical theorems, a version of the Stark conjectures, and some standard conjectures for SICs. In this paper we focus on the conceptually simplest case, prime dimensions of the form $d=n^2+3$, and report that we have constructed SICs in twelve prime dimensions of this kind, the highest being $d=19603$.}, journal = {arXiv:2112.05552 [quant-ph]}, author = {Appleby, Marcus and Bengtsson, Ingemar and Grassl, Markus and Harrison, Michael and McConnell, Gary}, month = dec, year = {2021}, note = {arXiv:2112.05552 [quant-ph]}, keywords = {Quantum Physics, Number Theory} }`

- John H. Selby, David Schmid, Elie Wolfe, Ana Belén Sainz, Ravi Kunjwal, and Robert W. Spekkens. Accessible fragments of generalized probabilistic theories, cone equivalence, and applications to witnessing nonclassicality.
*Arxiv:2112.04521 [quant-ph]*, dec 2021. arXiv: 2112.04521

[BibTeX] [Abstract] [Download PDF]

The formalism of generalized probabilistic theories (GPTs) was originally developed as a way to characterize the landscape of conceivable physical theories. Thus, the GPT describing a given physical theory necessarily includes all physically possible processes. We here consider the question of how to provide a GPT-like characterization of a particular experimental setup within a given physical theory. We show that the resulting characterization is not generally a GPT in and of itself-rather, it is described by a more general mathematical object that we introduce and term an accessible GPT fragment. We then introduce an equivalence relation, termed cone equivalence, between accessible GPT fragments (and, as a special case, between standard GPTs). We give a number of examples of experimental scenarios that are best described using accessible GPT fragments, and where moreover cone-equivalence arises naturally. We then prove that an accessible GPT fragment admits of a classical explanation if and only if every other fragment that is cone-equivalent to it also admits of a classical explanation. Finally, we leverage this result to prove several fundamental results regarding the experimental requirements for witnessing the failure of generalized noncontextuality. In particular, we prove that neither incompatibility among measurements nor the assumption of freedom of choice is necessary for witnessing failures of generalized noncontextuality, and, moreover, that such failures can be witnessed even using arbitrarily inefficient detectors.

`@article{selby_accessible_2021, title = {Accessible fragments of generalized probabilistic theories, cone equivalence, and applications to witnessing nonclassicality}, url = {https://arxiv.org/abs/2112.04521}, abstract = {The formalism of generalized probabilistic theories (GPTs) was originally developed as a way to characterize the landscape of conceivable physical theories. Thus, the GPT describing a given physical theory necessarily includes all physically possible processes. We here consider the question of how to provide a GPT-like characterization of a particular experimental setup within a given physical theory. We show that the resulting characterization is not generally a GPT in and of itself-rather, it is described by a more general mathematical object that we introduce and term an accessible GPT fragment. We then introduce an equivalence relation, termed cone equivalence, between accessible GPT fragments (and, as a special case, between standard GPTs). We give a number of examples of experimental scenarios that are best described using accessible GPT fragments, and where moreover cone-equivalence arises naturally. We then prove that an accessible GPT fragment admits of a classical explanation if and only if every other fragment that is cone-equivalent to it also admits of a classical explanation. Finally, we leverage this result to prove several fundamental results regarding the experimental requirements for witnessing the failure of generalized noncontextuality. In particular, we prove that neither incompatibility among measurements nor the assumption of freedom of choice is necessary for witnessing failures of generalized noncontextuality, and, moreover, that such failures can be witnessed even using arbitrarily inefficient detectors.}, urldate = {2021-12-10}, journal = {arXiv:2112.04521 [quant-ph]}, author = {Selby, John H. and Schmid, David and Wolfe, Elie and Sainz, Ana Belén and Kunjwal, Ravi and Spekkens, Robert W.}, month = dec, year = {2021}, note = {arXiv: 2112.04521}, keywords = {Quantum Physics}, }`

- Lorenzo Catani, Matthew Leifer, David Schmid, and Robert W. Spekkens. Why interference phenomena do not capture the essence of quantum theory.
*Arxiv:2111.13727 [quant-ph]*, 2021. arXiv: 2111.13727

[BibTeX] [Abstract] [Download PDF]

Quantum interference phenomena are widely viewed as posing a challenge to the classical worldview. Feynman even went so far as to proclaim that they are the only mystery and the basic peculiarity of quantum mechanics. Many have also argued that such phenomena force us to accept a number of radical interpretational conclusions, including: that a photon is neither a particle nor a wave but rather a schizophrenic sort of entity that toggles between the two possibilities, that reality is observer-dependent, and that systems either do not have properties prior to measurements or else have properties that are subject to nonlocal or backwards-in-time causal influences. In this work, we show that such conclusions are not, in fact, forced on us by the phenomena. We do so by describing an alternative to quantum theory, a statistical theory of a classical discrete field (the `toy field theory’) that reproduces the relevant phenomenology of quantum interference while rejecting these radical interpretational claims. It also reproduces a number of related interference experiments that are thought to support these interpretational claims, such as the Elitzur-Vaidman bomb tester, Wheeler’s delayed-choice experiment, and the quantum eraser experiment. The systems in the toy field theory are field modes, each of which possesses, at all times, both a particle-like property (a discrete occupation number) and a wave-like property (a discrete phase). Although these two properties are jointly possessed, the theory stipulates that they cannot be jointly known. The phenomenology that is generally cited in favour of nonlocal or backwards-in-time causal influences ends up being explained in terms of inferences about distant or past systems, and all that is observer-dependent is the observer’s knowledge of reality, not reality itself.

`@article{catani_why_2021, title = {Why interference phenomena do not capture the essence of quantum theory}, url = {http://arxiv.org/abs/2111.13727}, abstract = {Quantum interference phenomena are widely viewed as posing a challenge to the classical worldview. Feynman even went so far as to proclaim that they are the only mystery and the basic peculiarity of quantum mechanics. Many have also argued that such phenomena force us to accept a number of radical interpretational conclusions, including: that a photon is neither a particle nor a wave but rather a schizophrenic sort of entity that toggles between the two possibilities, that reality is observer-dependent, and that systems either do not have properties prior to measurements or else have properties that are subject to nonlocal or backwards-in-time causal influences. In this work, we show that such conclusions are not, in fact, forced on us by the phenomena. We do so by describing an alternative to quantum theory, a statistical theory of a classical discrete field (the `toy field theory') that reproduces the relevant phenomenology of quantum interference while rejecting these radical interpretational claims. It also reproduces a number of related interference experiments that are thought to support these interpretational claims, such as the Elitzur-Vaidman bomb tester, Wheeler's delayed-choice experiment, and the quantum eraser experiment. The systems in the toy field theory are field modes, each of which possesses, at all times, both a particle-like property (a discrete occupation number) and a wave-like property (a discrete phase). Although these two properties are jointly possessed, the theory stipulates that they cannot be jointly known. The phenomenology that is generally cited in favour of nonlocal or backwards-in-time causal influences ends up being explained in terms of inferences about distant or past systems, and all that is observer-dependent is the observer's knowledge of reality, not reality itself.}, urldate = {2021-11-30}, journal = {arXiv:2111.13727 [quant-ph]}, author = {Catani, Lorenzo and Leifer, Matthew and Schmid, David and Spekkens, Robert W.}, month = nov, year = {2021}, note = {arXiv: 2111.13727}, keywords = {Quantum Physics}, }`

- I. Reena, H. S. Karthik, Prabhu J. Tej, Usha A. R. Devi, Sudha, and A. K. Rajagopal. Entanglement detection in permutation symmetric states based on violation of local sum uncertainty relation.
*Arxiv:2103.15731 [quant-ph]*, 2021. arXiv: 2103.15731

[BibTeX] [Abstract] [Download PDF]

We show that violation of variance based local sum uncertainty relation (LSUR) for angular momentum operators of a bipartite system, proposed by Hofmann and Takeuchi, Phys. Rev. A 68, 032103 (2003)], is necessary and sufficient for entanglement in two-qubit permutation symmetric state. Moreover, we also establish its one-to-one connection with negativity of covariance matrix [Phys. Lett. A 364, 203 (2007)] of the two-qubit reduced system of a permutation symmetric N-qubit state. Consequently, it is seen that the violation of the angular momentum LSUR serves as a necessary condition for pairwise entanglement in \$N\$-qubit system, obeying exchange symmetry. We illustrate physical examples of entangled permutation symmetric N-qubit systems, where violation of the local sum uncertainty relation manifests itself as a signature of pairwise entanglement.

`@Article{reena_entanglement_2021, author = {Reena, I. and Karthik, H. S. and Tej, J. Prabhu and Devi, A. R. Usha and {Sudha} and Rajagopal, A. K.}, journal = {arXiv:2103.15731 [quant-ph]}, title = {Entanglement detection in permutation symmetric states based on violation of local sum uncertainty relation}, year = {2021}, month = mar, note = {arXiv: 2103.15731}, abstract = {We show that violation of variance based local sum uncertainty relation (LSUR) for angular momentum operators of a bipartite system, proposed by Hofmann and Takeuchi, Phys. Rev. A 68, 032103 (2003)], is necessary and sufficient for entanglement in two-qubit permutation symmetric state. Moreover, we also establish its one-to-one connection with negativity of covariance matrix [Phys. Lett. A 364, 203 (2007)] of the two-qubit reduced system of a permutation symmetric N-qubit state. Consequently, it is seen that the violation of the angular momentum LSUR serves as a necessary condition for pairwise entanglement in \$N\$-qubit system, obeying exchange symmetry. We illustrate physical examples of entangled permutation symmetric N-qubit systems, where violation of the local sum uncertainty relation manifests itself as a signature of pairwise entanglement.}, eprint = {arXiv:2103.15731}, keywords = {Quantum Physics}, url = {http://www.arxiv.org/abs/2103.15731}, urldate = {2021-07-28}, }`

- Robert Alicki and Alejandro Jenkins. Quantum thermodynamics of coronal heating.
*Arxiv:2103.08746 [astro-ph, physics:physics, physics:quant-ph]*, may 2021. arXiv: 2103.08746

[BibTeX] [Abstract] [Download PDF]

Using the Markovian master equation for quantum quasiparticles, we show that convection in the stellar photosphere generates plasma waves by an irreversible process akin to Zeldovich superradiance and sonic booms. In the Sun, this mechanism is most efficient in quiet regions with magnetic fields of order one gauss. Most energy is carried by Alfven waves with megahertz frequencies, which travel upwards until they reach a height at which they dissipate via mode conversion. This gives the right power flux for the observed energy transport from the colder photosphere to the hotter corona.

`@article{alicki_quantum_2021, title = {Quantum thermodynamics of coronal heating}, url = {http://arxiv.org/abs/2103.08746}, abstract = {Using the Markovian master equation for quantum quasiparticles, we show that convection in the stellar photosphere generates plasma waves by an irreversible process akin to Zeldovich superradiance and sonic booms. In the Sun, this mechanism is most efficient in quiet regions with magnetic fields of order one gauss. Most energy is carried by Alfven waves with megahertz frequencies, which travel upwards until they reach a height at which they dissipate via mode conversion. This gives the right power flux for the observed energy transport from the colder photosphere to the hotter corona.}, urldate = {2021-07-28}, journal = {arXiv:2103.08746 [astro-ph, physics:physics, physics:quant-ph]}, author = {Alicki, Robert and Jenkins, Alejandro}, month = may, year = {2021}, note = {arXiv: 2103.08746}, keywords = {Astrophysics - Solar and Stellar Astrophysics, Astrophysics - High Energy Astrophysical Phenomena, Physics - Plasma Physics, Quantum Physics}, }`

- Pawel Blasiak, Ewa Borsuk, and Marcin Markiewicz. On safe post-selection for Bell nonlocality: Causal diagram approach.
*Arxiv:2012.07285 [quant-ph]*, apr 2021. arXiv: 2012.07285

[BibTeX] [Abstract] [Download PDF]

Reasoning about Bell nonlocality from the correlations observed in post-selected data is always a matter of concern. This is because conditioning on the outcomes is a source of non-causal correlations, known as a selection bias, rising doubts whether the conclusion concerns the actual causal process or maybe it is just an effect of processing the data. Yet, even in the idealised case without detection inefficiencies, post-selection is an integral part of every experimental design, not least because it is a part of the entanglement generation process itself. In this paper we discuss a broad class of scenarios with post-selection on multiple spatially distributed outcomes. A simple criterion is worked out, called the all-but-one principle, showing when the conclusions about nonlocality from breaking Bell inequalities with post-selected data remain in force. Generality of this result, attained by adopting the high-level diagrammatic tools of causal inference, provides safe grounds for systematic reasoning based on the standard form of multipartite Bell inequalities in a wide array of entanglement generation schemes without worrying about the dangers of selection bias.

`@Article{blasiak_safe_2021, author = {Blasiak, Pawel and Borsuk, Ewa and Markiewicz, Marcin}, journal = {arXiv:2012.07285 [quant-ph]}, title = {On safe post-selection for {Bell} nonlocality: {Causal} diagram approach}, year = {2021}, month = apr, note = {arXiv: 2012.07285}, abstract = {Reasoning about Bell nonlocality from the correlations observed in post-selected data is always a matter of concern. This is because conditioning on the outcomes is a source of non-causal correlations, known as a selection bias, rising doubts whether the conclusion concerns the actual causal process or maybe it is just an effect of processing the data. Yet, even in the idealised case without detection inefficiencies, post-selection is an integral part of every experimental design, not least because it is a part of the entanglement generation process itself. In this paper we discuss a broad class of scenarios with post-selection on multiple spatially distributed outcomes. A simple criterion is worked out, called the all-but-one principle, showing when the conclusions about nonlocality from breaking Bell inequalities with post-selected data remain in force. Generality of this result, attained by adopting the high-level diagrammatic tools of causal inference, provides safe grounds for systematic reasoning based on the standard form of multipartite Bell inequalities in a wide array of entanglement generation schemes without worrying about the dangers of selection bias.}, keywords = {Quantum Physics}, shorttitle = {On safe post-selection for {Bell} nonlocality}, url = {http://arxiv.org/abs/2012.07285}, urldate = {2021-07-28}, }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. No-go for device independent protocols with Tan-Walls-Collett `nonlocality of a single photon’.
*Arxiv:2102.03254 [quant-ph]*, feb 2021. arXiv: 2102.03254

[BibTeX] [Abstract] [Download PDF]

We investigate the interferometric scheme put forward by Tan, Walls and Collett [Phys. Rev. Lett. \{{\textbackslash}bf 66\}, 256 (1991)] that aims to reveal Bell non-classicality of a single photon. By providing a local hidden variable model that reproduces their results, we decisively refute this claim. In particular, this means that the scheme cannot be used in device-independent protocols.

`@article{das_no-go_2021, title = {No-go for device independent protocols with {Tan}-{Walls}-{Collett} `nonlocality of a single photon'}, url = {http://arxiv.org/abs/2102.03254}, abstract = {We investigate the interferometric scheme put forward by Tan, Walls and Collett [Phys. Rev. Lett. \{{\textbackslash}bf 66\}, 256 (1991)] that aims to reveal Bell non-classicality of a single photon. By providing a local hidden variable model that reproduces their results, we decisively refute this claim. In particular, this means that the scheme cannot be used in device-independent protocols.}, urldate = {2021-07-28}, journal = {arXiv:2102.03254 [quant-ph]}, author = {Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Woloncewicz, Bianka and Żukowski, Marek}, month = feb, year = {2021}, note = {arXiv: 2102.03254}, keywords = {Quantum Physics}, }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. Can single photon excitation of two spatially separated modes lead to a violation of Bell inequality via homodyne measurements?.
*Arxiv:2102.06689 [quant-ph]*, feb 2021. arXiv: 2102.06689

[BibTeX] [Abstract] [Download PDF]

We reconsider the all-optical homodyne-measurement based experimental schemes that aim to reveal Bell nonclassicality of a single photon, often termed `nonlocality’. We focus on the schemes put forward by Tan, Walls and Collett (TWC, 1991) and Hardy (1994). In the light of our previous work the Tan, Walls and Collett setup can be described by a precise local hidden variable model, hence the claimed nonclassicality of this proposal is apparent, whereas the nonclassicality proof proposed by Hardy is impeccable. In this work we resolve the following problem: which feature of the Hardy’s approach is crucial for its successful confirmation of nonclassicality. The scheme of Hardy differs from the Tan, Walls and Collett setup in two aspects. (i) It introduces a superposition of a single photon excitation with vacuum as the initial state of one of the input modes of a 50-50 beamsplitter, which creates the superposition state of two separable (exit) modes under investigation. (ii) In the final measurements Hardy’s proposal utilises a varying strengths of the local oscillator fields, whereas in the TWC case they are constant. In fact the local oscillators in Hardy’s scheme are either on or off (the local setting is specified by the presence or absence of the local auxiliary field). We show that it is the varying strength of the local oscillators, from setting to setting, which is the crucial feature enabling violation of local realism in the Hardy setup, whereas it is not necessary to use initial superposition of a single photon excitation with vacuum as the initial state of the input mode. Neither one needs to operate in the fully on/off detection scheme. Despite the failure of the Tan, Walls and Collett scheme in proving Bell nonclassicality, we show that their scheme can serve as an entanglement indicator.

`@article{das_can_2021, title = {Can single photon excitation of two spatially separated modes lead to a violation of {Bell} inequality via homodyne measurements?}, url = {http://arxiv.org/abs/2102.06689}, abstract = {We reconsider the all-optical homodyne-measurement based experimental schemes that aim to reveal Bell nonclassicality of a single photon, often termed `nonlocality'. We focus on the schemes put forward by Tan, Walls and Collett (TWC, 1991) and Hardy (1994). In the light of our previous work the Tan, Walls and Collett setup can be described by a precise local hidden variable model, hence the claimed nonclassicality of this proposal is apparent, whereas the nonclassicality proof proposed by Hardy is impeccable. In this work we resolve the following problem: which feature of the Hardy's approach is crucial for its successful confirmation of nonclassicality. The scheme of Hardy differs from the Tan, Walls and Collett setup in two aspects. (i) It introduces a superposition of a single photon excitation with vacuum as the initial state of one of the input modes of a 50-50 beamsplitter, which creates the superposition state of two separable (exit) modes under investigation. (ii) In the final measurements Hardy's proposal utilises a varying strengths of the local oscillator fields, whereas in the TWC case they are constant. In fact the local oscillators in Hardy's scheme are either on or off (the local setting is specified by the presence or absence of the local auxiliary field). We show that it is the varying strength of the local oscillators, from setting to setting, which is the crucial feature enabling violation of local realism in the Hardy setup, whereas it is not necessary to use initial superposition of a single photon excitation with vacuum as the initial state of the input mode. Neither one needs to operate in the fully on/off detection scheme. Despite the failure of the Tan, Walls and Collett scheme in proving Bell nonclassicality, we show that their scheme can serve as an entanglement indicator.}, urldate = {2021-07-28}, journal = {arXiv:2102.06689 [quant-ph]}, author = {Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Woloncewicz, Bianka and Żukowski, Marek}, month = feb, year = {2021}, note = {arXiv: 2102.06689}, keywords = {Quantum Physics}, }`

- Pawel Blasiak, Ewa Borsuk, Marcin Markiewicz, and Yong-Su Kim. Efficient linear optical generation of a multipartite W state.
*Arxiv:2103.02206 [quant-ph]*, mar 2021. arXiv: 2103.02206

[BibTeX] [Abstract] [Download PDF]

A novel scheme is presented for generation of a multipartite W state for arbitrary number of qubits. Based on a recent proposal of entanglement without touching, it serves to demonstrate the potential of particle indistinguishability as a useful resource of entanglement for practical applications. The devised scheme is efficient in design, meaning that it is built with linear optics without the need for auxiliary particles nor measurements. Yet, the success probability is shown to be highly competitive compared with the existing proposals (i.e. decreases polynomially with the number of qubits) and remains insensitive to particle statistics (i.e. has the same efficiency for bosons and fermions).

`@Article{blasiak_efficient_2021, author = {Blasiak, Pawel and Borsuk, Ewa and Markiewicz, Marcin and Kim, Yong-Su}, journal = {arXiv:2103.02206 [quant-ph]}, title = {Efficient linear optical generation of a multipartite {W} state}, year = {2021}, month = mar, note = {arXiv: 2103.02206}, abstract = {A novel scheme is presented for generation of a multipartite W state for arbitrary number of qubits. Based on a recent proposal of entanglement without touching, it serves to demonstrate the potential of particle indistinguishability as a useful resource of entanglement for practical applications. The devised scheme is efficient in design, meaning that it is built with linear optics without the need for auxiliary particles nor measurements. Yet, the success probability is shown to be highly competitive compared with the existing proposals (i.e. decreases polynomially with the number of qubits) and remains insensitive to particle statistics (i.e. has the same efficiency for bosons and fermions).}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2103.02206}, urldate = {2021-07-28}, }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. On detecting violation of local realism with photon-number resolving weak-field homodyne measurements.
*Arxiv:2104.10703 [quant-ph]*, apr 2021. arXiv: 2104.10703

[BibTeX] [Abstract] [Download PDF]

Non-existence of a local hidden variables (LHV) model for a phenomenon benchmarks its use in device-independent quantum protocols. Nowadays photon-number resolving weak-field homodyne measurements allow realization of emblematic gedanken experiments. Alas, claims that we can have no LHV models for such experiments on (a) excitation of a pair of spatial modes by a single photon, and (b) two spatial modes in a weakly squeezed vacuum state, involving constant local oscillator strengths, are unfounded. For (a) an exact LHV model resolves the dispute on the “non-locality of a single photon” in its original formulation. It is measurements with local oscillators on or off that do not have LHV models.

`@article{das_detecting_2021, title = {On detecting violation of local realism with photon-number resolving weak-field homodyne measurements}, url = {http://arxiv.org/abs/2104.10703}, abstract = {Non-existence of a local hidden variables (LHV) model for a phenomenon benchmarks its use in device-independent quantum protocols. Nowadays photon-number resolving weak-field homodyne measurements allow realization of emblematic gedanken experiments. Alas, claims that we can have no LHV models for such experiments on (a) excitation of a pair of spatial modes by a single photon, and (b) two spatial modes in a weakly squeezed vacuum state, involving constant local oscillator strengths, are unfounded. For (a) an exact LHV model resolves the dispute on the "non-locality of a single photon" in its original formulation. It is measurements with local oscillators on or off that do not have LHV models.}, urldate = {2021-07-28}, journal = {arXiv:2104.10703 [quant-ph]}, author = {Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Woloncewicz, Bianka and Żukowski, Marek}, month = apr, year = {2021}, note = {arXiv: 2104.10703}, keywords = {Quantum Physics}, }`

- Giovanni Scala, Karolina Słowik, Paolo Facchi, Saverio Pascazio, and Francesco Pepe. Beyond the Rabi model: light interactions with polar atomic systems in a cavity.
*Arxiv:2103.11232 [quant-ph]*, mar 2021. arXiv: 2103.11232

[BibTeX] [Abstract] [Download PDF]

The Rabi Hamiltonian, describing the interaction between a two-level atomic system and a single cavity mode of the electromagnetic field, is one of the fundamental models in quantum optics. The model becomes exactly solvable by considering an atom without permanent dipole moments, whose excitation energy is quasi-resonant with the cavity photon energy, and by neglecting the non resonant (counter-rotating) terms. In this case, after including the decay of either the atom or the cavity mode to a continuum, one is able to derive the well-known phenomenology of quasi-resonant transitions, including the fluorescence triplets. In this work we consider the most general Rabi model, incorporating the effects of permanent atomic electric dipole moments, and, based on a perturbative analysis, we compare the intensities of emission lines induced by rotating terms, counter-rotating terms and parity-symmetry-breaking terms. The analysis reveals that the emission strength related to the existence of permanent dipoles may surpass the one due to the counter-rotating interaction terms, but is usually much weaker than the emission due to the main, resonant coupling. This ratio can be modified in systems with a reduced dimensionality or by engineering the energy spectral density of the continuum.

`@article{scala_beyond_2021, title = {Beyond the {Rabi} model: light interactions with polar atomic systems in a cavity}, shorttitle = {Beyond the {Rabi} model}, url = {http://arxiv.org/abs/2103.11232}, abstract = {The Rabi Hamiltonian, describing the interaction between a two-level atomic system and a single cavity mode of the electromagnetic field, is one of the fundamental models in quantum optics. The model becomes exactly solvable by considering an atom without permanent dipole moments, whose excitation energy is quasi-resonant with the cavity photon energy, and by neglecting the non resonant (counter-rotating) terms. In this case, after including the decay of either the atom or the cavity mode to a continuum, one is able to derive the well-known phenomenology of quasi-resonant transitions, including the fluorescence triplets. In this work we consider the most general Rabi model, incorporating the effects of permanent atomic electric dipole moments, and, based on a perturbative analysis, we compare the intensities of emission lines induced by rotating terms, counter-rotating terms and parity-symmetry-breaking terms. The analysis reveals that the emission strength related to the existence of permanent dipoles may surpass the one due to the counter-rotating interaction terms, but is usually much weaker than the emission due to the main, resonant coupling. This ratio can be modified in systems with a reduced dimensionality or by engineering the energy spectral density of the continuum.}, urldate = {2021-07-28}, journal = {arXiv:2103.11232 [quant-ph]}, author = {Scala, Giovanni and Słowik, Karolina and Facchi, Paolo and Pascazio, Saverio and Pepe, Francesco}, month = mar, year = {2021}, note = {arXiv: 2103.11232}, keywords = {Quantum Physics}, }`

- Mariami Gachechiladze, Bartłomiej Bąk, Marcin Pawłowski, and Nikolai Miklin. Quantum Bell inequalities from Information Causality – tight for Macroscopic Locality.
*Arxiv:2103.05029 [quant-ph]*, mar 2021. arXiv: 2103.05029

[BibTeX] [Abstract] [Download PDF]

Quantum generalizations of Bell inequalities are analytical expressions of correlations observed in the Bell experiment that are used to explain or estimate the set of correlations that quantum theory allows. Unlike standard Bell inequalities, their quantum analogs are rare in the literature, as no known algorithm can be used to find them systematically. In this work, we present a family of quantum Bell inequalities in scenarios where the number of settings or outcomes can be arbitrarily high. We derive these inequalities from the principle of Information Causality, and thus, we do not assume the formalism of quantum mechanics. Considering the symmetries of the derived inequalities, we show that the latter give the necessary and sufficient condition for the correlations to comply with Macroscopic Locality. As a result, we conclude that the principle of Information Causality is strictly stronger than the principle of Macroscopic Locality in the subspace defined by these symmetries.

`@article{gachechiladze_quantum_2021, title = {Quantum {Bell} inequalities from {Information} {Causality} -- tight for {Macroscopic} {Locality}}, url = {http://arxiv.org/abs/2103.05029}, abstract = {Quantum generalizations of Bell inequalities are analytical expressions of correlations observed in the Bell experiment that are used to explain or estimate the set of correlations that quantum theory allows. Unlike standard Bell inequalities, their quantum analogs are rare in the literature, as no known algorithm can be used to find them systematically. In this work, we present a family of quantum Bell inequalities in scenarios where the number of settings or outcomes can be arbitrarily high. We derive these inequalities from the principle of Information Causality, and thus, we do not assume the formalism of quantum mechanics. Considering the symmetries of the derived inequalities, we show that the latter give the necessary and sufficient condition for the correlations to comply with Macroscopic Locality. As a result, we conclude that the principle of Information Causality is strictly stronger than the principle of Macroscopic Locality in the subspace defined by these symmetries.}, urldate = {2021-07-28}, journal = {arXiv:2103.05029 [quant-ph]}, author = {Gachechiladze, Mariami and Bąk, Bartłomiej and Pawłowski, Marcin and Miklin, Nikolai}, month = mar, year = {2021}, note = {arXiv: 2103.05029}, keywords = {Quantum Physics}, }`

- David Schmid, Haoxing Du, John H. Selby, and Matthew F. Pusey. The only noncontextual model of the stabilizer subtheory is Gross’s.
*Arxiv:2101.06263 [quant-ph]*, feb 2021. arXiv: 2101.06263

[BibTeX] [Abstract] [Download PDF]

We prove that there is a unique nonnegative and diagram-preserving quasiprobability representation of the stabilizer subtheory in all odd dimensions, namely Gross’s discrete Wigner function. This representation is equivalent to Spekkens’ epistemically restricted toy theory, which is consequently singled out as the unique noncontextual ontological model for the stabilizer subtheory. Strikingly, the principle of noncontextuality is powerful enough (at least in this setting) to single out one particular classical realist interpretation. Our result explains the practical utility of Gross’s representation, e.g. why (in the setting of the stabilizer subtheory) negativity in this particular representation implies generalized contextuality, and hence sheds light on why negativity of this particular representation is a resource for quantum computational speedup. It also allows us to prove that generalized contextuality is a necessary resource for universal quantum computation in the state injection model. In all even dimensions, we prove that there does not exist any nonnegative and diagram-preserving quasiprobability representation of the stabilizer subtheory, and, hence, that the stabilizer subtheory is contextual in all even dimensions. Together, these results constitute a complete characterization of the (non)classicality of all stabilizer subtheories.

`@article{schmid_only_2021, title = {The only noncontextual model of the stabilizer subtheory is {Gross}'s}, url = {http://arxiv.org/abs/2101.06263}, abstract = {We prove that there is a unique nonnegative and diagram-preserving quasiprobability representation of the stabilizer subtheory in all odd dimensions, namely Gross's discrete Wigner function. This representation is equivalent to Spekkens' epistemically restricted toy theory, which is consequently singled out as the unique noncontextual ontological model for the stabilizer subtheory. Strikingly, the principle of noncontextuality is powerful enough (at least in this setting) to single out one particular classical realist interpretation. Our result explains the practical utility of Gross's representation, e.g. why (in the setting of the stabilizer subtheory) negativity in this particular representation implies generalized contextuality, and hence sheds light on why negativity of this particular representation is a resource for quantum computational speedup. It also allows us to prove that generalized contextuality is a necessary resource for universal quantum computation in the state injection model. In all even dimensions, we prove that there does not exist any nonnegative and diagram-preserving quasiprobability representation of the stabilizer subtheory, and, hence, that the stabilizer subtheory is contextual in all even dimensions. Together, these results constitute a complete characterization of the (non)classicality of all stabilizer subtheories.}, urldate = {2021-07-28}, journal = {arXiv:2101.06263 [quant-ph]}, author = {Schmid, David and Du, Haoxing and Selby, John H. and Pusey, Matthew F.}, month = feb, year = {2021}, note = {arXiv: 2101.06263}, keywords = {Quantum Physics}, }`

- Thomas D. Galley, Flaminia Giacomini, and John H. Selby. A no-go theorem on the nature of the gravitational field beyond quantum theory.
*Arxiv:2012.01441 [gr-qc, physics:quant-ph]*, 2021. arXiv: 2012.01441

[BibTeX] [Abstract] [Download PDF]

Recently, table-top experiments involving massive quantum systems have been proposed to test the interface of quantum theory and gravity. In particular, the crucial point of the debate is whether it is possible to conclude anything on the quantum nature of the gravitational field, provided that two quantum systems become entangled due to solely the gravitational interaction. Typically, this question has been addressed by assuming an underlying physical theory to describe the gravitational interaction, but no systematic approach to characterise the set of possible gravitational theories which are compatible with the observation of entanglement has been proposed. Here, we introduce the framework of Generalised Probabilistic Theories (GPTs) to the study of the nature of the gravitational field. This framework has the advantage that it only relies on the set of operationally accessible states, transformations, and measurements, without presupposing an underlying theory. Hence, it provides a framework to systematically study all theories compatible with the detection of entanglement generated via the gravitational interaction between two non-classical systems. Assuming that such entanglement is observed we prove a no-go theorem stating that the following statements are incompatible: i) the two non-classical systems are independent subsystems, ii) the gravitational field is a physical degree of freedom which mediates the interaction and iii) the gravitational field is classical. Moreover we argue that conditions i) and ii) should be met, and hence that the gravitational field is non-classical. Non-classicality does not imply that the gravitational field is quantum, and to illustrate this we provide examples of non-classical and non-quantum theories which are logically consistent with the other conditions.

`@article{galley_no-go_2021, title = {A no-go theorem on the nature of the gravitational field beyond quantum theory}, url = {http://arxiv.org/abs/2012.01441}, abstract = {Recently, table-top experiments involving massive quantum systems have been proposed to test the interface of quantum theory and gravity. In particular, the crucial point of the debate is whether it is possible to conclude anything on the quantum nature of the gravitational field, provided that two quantum systems become entangled due to solely the gravitational interaction. Typically, this question has been addressed by assuming an underlying physical theory to describe the gravitational interaction, but no systematic approach to characterise the set of possible gravitational theories which are compatible with the observation of entanglement has been proposed. Here, we introduce the framework of Generalised Probabilistic Theories (GPTs) to the study of the nature of the gravitational field. This framework has the advantage that it only relies on the set of operationally accessible states, transformations, and measurements, without presupposing an underlying theory. Hence, it provides a framework to systematically study all theories compatible with the detection of entanglement generated via the gravitational interaction between two non-classical systems. Assuming that such entanglement is observed we prove a no-go theorem stating that the following statements are incompatible: i) the two non-classical systems are independent subsystems, ii) the gravitational field is a physical degree of freedom which mediates the interaction and iii) the gravitational field is classical. Moreover we argue that conditions i) and ii) should be met, and hence that the gravitational field is non-classical. Non-classicality does not imply that the gravitational field is quantum, and to illustrate this we provide examples of non-classical and non-quantum theories which are logically consistent with the other conditions.}, urldate = {2021-07-28}, journal = {arXiv:2012.01441 [gr-qc, physics:quant-ph]}, author = {Galley, Thomas D. and Giacomini, Flaminia and Selby, John H.}, month = jun, year = {2021}, note = {arXiv: 2012.01441}, keywords = {Quantum Physics, General Relativity and Quantum Cosmology}, }`

- David Schmid, John H. Selby, and Robert W. Spekkens. Unscrambling the omelette of causation and inference: The framework of causal-inferential theories.
*Arxiv:2009.03297 [quant-ph]*, may 2021. arXiv: 2009.03297

[BibTeX] [Abstract] [Download PDF]

Using a process-theoretic formalism, we introduce the notion of a causal-inferential theory: a triple consisting of a theory of causal influences, a theory of inferences (of both the Boolean and Bayesian varieties), and a specification of how these interact. Recasting the notions of operational and realist theories in this mold clarifies what a realist account of an experiment offers beyond an operational account. It also yields a novel characterization of the assumptions and implications of standard no-go theorems for realist representations of operational quantum theory, namely, those based on Bell’s notion of locality and those based on generalized noncontextuality. Moreover, our process-theoretic characterization of generalised noncontextuality is shown to be implied by an even more natural principle which we term Leibnizianity. Most strikingly, our framework offers a way forward in a research program that seeks to circumvent these no-go results. Specifically, we argue that if one can identify axioms for a realist causal-inferential theory such that the notions of causation and inference can differ from their conventional (classical) interpretations, then one has the means of defining an intrinsically quantum notion of realism, and thereby a realist representation of operational quantum theory that salvages the spirit of locality and of noncontextuality.

`@article{schmid_unscrambling_2021, title = {Unscrambling the omelette of causation and inference: {The} framework of causal-inferential theories}, shorttitle = {Unscrambling the omelette of causation and inference}, url = {http://arxiv.org/abs/2009.03297}, abstract = {Using a process-theoretic formalism, we introduce the notion of a causal-inferential theory: a triple consisting of a theory of causal influences, a theory of inferences (of both the Boolean and Bayesian varieties), and a specification of how these interact. Recasting the notions of operational and realist theories in this mold clarifies what a realist account of an experiment offers beyond an operational account. It also yields a novel characterization of the assumptions and implications of standard no-go theorems for realist representations of operational quantum theory, namely, those based on Bell's notion of locality and those based on generalized noncontextuality. Moreover, our process-theoretic characterization of generalised noncontextuality is shown to be implied by an even more natural principle which we term Leibnizianity. Most strikingly, our framework offers a way forward in a research program that seeks to circumvent these no-go results. Specifically, we argue that if one can identify axioms for a realist causal-inferential theory such that the notions of causation and inference can differ from their conventional (classical) interpretations, then one has the means of defining an intrinsically quantum notion of realism, and thereby a realist representation of operational quantum theory that salvages the spirit of locality and of noncontextuality.}, urldate = {2021-07-28}, journal = {arXiv:2009.03297 [quant-ph]}, author = {Schmid, David and Selby, John H. and Spekkens, Robert W.}, month = may, year = {2021}, note = {arXiv: 2009.03297}, keywords = {Quantum Physics}, }`

- John H. Selby, Ana Belén Sainz, and Paweł Horodecki. Revisiting dynamics of quantum causal structures – when can causal order evolve?.
*Arxiv:2008.12757 [quant-ph]*, mar 2021. arXiv: 2008.12757

[BibTeX] [Abstract] [Download PDF]

Recently, there has been substantial interest in studying the dynamics of quantum theory beyond that of states, in particular, the dynamics of channels, measurements, and higher-order transformations. Ref. [Phys. Rev. X 8(1), 011047 (2018)] pursues this using the process matrix formalism, together with a definition of the possible dynamics of such process matrices, and focusing especially on the question of evolution of causal structures. One of its major conclusions is a strong theorem saying that, within the formalism, under continuous and reversible transformations, the causal order between operations must be preserved. Here we find a surprising result: if one is to take into account a full picture of the physical evolution of operations within the standard quantum-mechanical formalism, then one can actually draw the opposite conclusion. That is, we show that under certain continuous and reversible dynamics the causal order between operations is not necessarily preserved. We moreover identify and analyse the root of this apparent contradiction, specifically, that the commonly accepted and widely applied framework of higher-order processes, whilst mathematically sound, is not always appropriate for drawing conclusions on the fundamentals of physical dynamics. Finally we show how to reconcile the elements of the whole picture following the intuition based on entanglement processing by local operations and classical communication.

`@Article{selby_revisiting_2021, author = {Selby, John H. and Sainz, Ana Belén and Horodecki, Paweł}, journal = {arXiv:2008.12757 [quant-ph]}, title = {Revisiting dynamics of quantum causal structures -- when can causal order evolve?}, year = {2021}, month = mar, note = {arXiv: 2008.12757}, abstract = {Recently, there has been substantial interest in studying the dynamics of quantum theory beyond that of states, in particular, the dynamics of channels, measurements, and higher-order transformations. Ref. [Phys. Rev. X 8(1), 011047 (2018)] pursues this using the process matrix formalism, together with a definition of the possible dynamics of such process matrices, and focusing especially on the question of evolution of causal structures. One of its major conclusions is a strong theorem saying that, within the formalism, under continuous and reversible transformations, the causal order between operations must be preserved. Here we find a surprising result: if one is to take into account a full picture of the physical evolution of operations within the standard quantum-mechanical formalism, then one can actually draw the opposite conclusion. That is, we show that under certain continuous and reversible dynamics the causal order between operations is not necessarily preserved. We moreover identify and analyse the root of this apparent contradiction, specifically, that the commonly accepted and widely applied framework of higher-order processes, whilst mathematically sound, is not always appropriate for drawing conclusions on the fundamentals of physical dynamics. Finally we show how to reconcile the elements of the whole picture following the intuition based on entanglement processing by local operations and classical communication.}, groups = {Pawel_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2008.12757}, urldate = {2021-07-28}, }`

- David Schmid, Thomas C. Fraser, Ravi Kunjwal, Ana Belen Sainz, Elie Wolfe, and Robert W. Spekkens. Understanding the interplay of entanglement and nonlocality: motivating and developing a new branch of entanglement theory.
*Arxiv:2004.09194 [quant-ph]*, may 2021. arXiv: 2004.09194

[BibTeX] [Abstract] [Download PDF]

A standard approach to quantifying resources is to determine which operations on the resources are freely available, and to deduce the partial order over resources that is induced by the relation of convertibility under the free operations. If the resource of interest is the nonclassicality of the correlations embodied in a quantum state, i.e., entanglement, then the common assumption is that the appropriate choice of free operations is Local Operations and Classical Communication (LOCC). We here advocate for the study of a different choice of free operations, namely, Local Operations and Shared Randomness (LOSR), and demonstrate its utility in understanding the interplay between the entanglement of states and the nonlocality of the correlations in Bell experiments. Specifically, we show that the LOSR paradigm (i) provides a resolution of the anomalies of nonlocality, wherein partially entangled states exhibit more nonlocality than maximally entangled states, (ii) entails new notions of genuine multipartite entanglement and nonlocality that are free of the pathological features of the conventional notions, and (iii) makes possible a resource-theoretic account of the self-testing of entangled states which generalizes and simplifies prior results. Along the way, we derive some fundamental results concerning the necessary and sufficient conditions for convertibility between pure entangled states under LOSR and highlight some of their consequences, such as the impossibility of catalysis for bipartite pure states. The resource-theoretic perspective also clarifies why it is neither surprising nor problematic that there are mixed entangled states which do not violate any Bell inequality. Our results motivate the study of LOSR-entanglement as a new branch of entanglement theory.

`@article{schmid_understanding_2021, title = {Understanding the interplay of entanglement and nonlocality: motivating and developing a new branch of entanglement theory}, shorttitle = {Understanding the interplay of entanglement and nonlocality}, url = {http://arxiv.org/abs/2004.09194}, abstract = {A standard approach to quantifying resources is to determine which operations on the resources are freely available, and to deduce the partial order over resources that is induced by the relation of convertibility under the free operations. If the resource of interest is the nonclassicality of the correlations embodied in a quantum state, i.e., entanglement, then the common assumption is that the appropriate choice of free operations is Local Operations and Classical Communication (LOCC). We here advocate for the study of a different choice of free operations, namely, Local Operations and Shared Randomness (LOSR), and demonstrate its utility in understanding the interplay between the entanglement of states and the nonlocality of the correlations in Bell experiments. Specifically, we show that the LOSR paradigm (i) provides a resolution of the anomalies of nonlocality, wherein partially entangled states exhibit more nonlocality than maximally entangled states, (ii) entails new notions of genuine multipartite entanglement and nonlocality that are free of the pathological features of the conventional notions, and (iii) makes possible a resource-theoretic account of the self-testing of entangled states which generalizes and simplifies prior results. Along the way, we derive some fundamental results concerning the necessary and sufficient conditions for convertibility between pure entangled states under LOSR and highlight some of their consequences, such as the impossibility of catalysis for bipartite pure states. The resource-theoretic perspective also clarifies why it is neither surprising nor problematic that there are mixed entangled states which do not violate any Bell inequality. Our results motivate the study of LOSR-entanglement as a new branch of entanglement theory.}, urldate = {2021-07-28}, journal = {arXiv:2004.09194 [quant-ph]}, author = {Schmid, David and Fraser, Thomas C. and Kunjwal, Ravi and Sainz, Ana Belen and Wolfe, Elie and Spekkens, Robert W.}, month = may, year = {2021}, note = {arXiv: 2004.09194}, keywords = {Quantum Physics}, }`

- Paulo J. Cavalcanti, John H. Selby, Jamie Sikora, Thomas D. Galley, and Ana Belén Sainz. Witworld: A generalised probabilistic theory featuring post-quantum steering.
*Arxiv:2102.06581 [quant-ph]*, feb 2021. arXiv: 2102.06581

[BibTeX] [Abstract] [Download PDF]

We introduce Witworld: a generalised probabilistic theory with strong post-quantum features, which subsumes Boxworld. Witworld is the first theory that features post-quantum steering, and also the first that outperforms quantum theory at the task of remote state preparation. We further show post-quantum steering to be the source of this advantage, and hence present the first instance where post-quantum steering is a stronger-than-quantum resource for information processing.

`@article{cavalcanti_witworld:_2021, title = {Witworld: {A} generalised probabilistic theory featuring post-quantum steering}, shorttitle = {Witworld}, url = {http://arxiv.org/abs/2102.06581}, abstract = {We introduce Witworld: a generalised probabilistic theory with strong post-quantum features, which subsumes Boxworld. Witworld is the first theory that features post-quantum steering, and also the first that outperforms quantum theory at the task of remote state preparation. We further show post-quantum steering to be the source of this advantage, and hence present the first instance where post-quantum steering is a stronger-than-quantum resource for information processing.}, urldate = {2021-07-28}, journal = {arXiv:2102.06581 [quant-ph]}, author = {Cavalcanti, Paulo J. and Selby, John H. and Sikora, Jamie and Galley, Thomas D. and Sainz, Ana Belén}, month = feb, year = {2021}, note = {arXiv: 2102.06581}, keywords = {Quantum Physics}, }`

- Markus Grassl, Felix Huber, and Andreas Winter. Entropic proofs of Singleton bounds for quantum error-correcting codes.
*Arxiv:2010.07902 [quant-ph]*, feb 2021. arXiv: 2010.07902

[BibTeX] [Abstract] [Download PDF]

We show that a relatively simple reasoning using von Neumann entropy inequalities yields a robust proof of the quantum Singleton bound for quantum error-correcting codes (QECC). For entanglement-assisted quantum error-correcting codes (EAQECC) and catalytic codes (CQECC), the generalised quantum Singleton bound was believed to hold for many years until recently one of us found a counterexample [MG, arXiv:2007.01249]. Here, we rectify this state of affairs by proving the correct generalised quantum Singleton bound for CQECC, extending the above-mentioned proof method for QECC; we also prove information-theoretically tight bounds on the entanglement-communication tradeoff for EAQECC. All of the bounds relate block length \$n\$ and code length \$k\$ for given minimum distance \$d\$ and we show that they are robust, in the sense that they hold with small perturbations for codes which only correct most of the erasure errors of less than \$d\$ letters. In contrast to the classical case, the bounds take on qualitatively different forms depending on whether the minimum distance is smaller or larger than half the block length. We also provide a propagation rule, where any pure QECC yields an EAQECC with the same distance and dimension but of shorter block length.

`@article{grassl_entropic_2021, title = {Entropic proofs of {Singleton} bounds for quantum error-correcting codes}, url = {http://arxiv.org/abs/2010.07902}, abstract = {We show that a relatively simple reasoning using von Neumann entropy inequalities yields a robust proof of the quantum Singleton bound for quantum error-correcting codes (QECC). For entanglement-assisted quantum error-correcting codes (EAQECC) and catalytic codes (CQECC), the generalised quantum Singleton bound was believed to hold for many years until recently one of us found a counterexample [MG, arXiv:2007.01249]. Here, we rectify this state of affairs by proving the correct generalised quantum Singleton bound for CQECC, extending the above-mentioned proof method for QECC; we also prove information-theoretically tight bounds on the entanglement-communication tradeoff for EAQECC. All of the bounds relate block length \$n\$ and code length \$k\$ for given minimum distance \$d\$ and we show that they are robust, in the sense that they hold with small perturbations for codes which only correct most of the erasure errors of less than \$d\$ letters. In contrast to the classical case, the bounds take on qualitatively different forms depending on whether the minimum distance is smaller or larger than half the block length. We also provide a propagation rule, where any pure QECC yields an EAQECC with the same distance and dimension but of shorter block length.}, urldate = {2021-07-28}, journal = {arXiv:2010.07902 [quant-ph]}, author = {Grassl, Markus and Huber, Felix and Winter, Andreas}, month = feb, year = {2021}, note = {arXiv: 2010.07902}, keywords = {Quantum Physics, Computer Science - Information Theory}, }`

- B. Ahmadi, S. Salimi, and A. S. Khorashad. Refined Definitions of Heat and Work in Quantum Thermodynamics.
*Arxiv:1912.01983 [quant-ph]*, jul 2021. arXiv: 1912.01983

[BibTeX] [Abstract] [Download PDF]

In this paper, unambiguous redefinitions of heat and work are presented for quantum thermodynamic systems. We will use genuine reasoning based on which Clausius originally defined work and heat in establishing thermodynamics. The change in the energy which is accompanied by a change in the entropy is identified as heat, while any change in the energy which does not lead to a change in the entropy is known as work. It will be seen that quantum coherence does not allow all the energy exchanged between two quantum systems to be only of the heat form. Several examples will also be discussed. Finally, it will be shown that these refined definitions will strongly affect the entropy production of quantum thermodynamic processes giving new insight into the irreversibility of quantum processes.

`@Article{ahmadi_refined_2021, author = {Ahmadi, B. and Salimi, S. and Khorashad, A. S.}, journal = {arXiv:1912.01983 [quant-ph]}, title = {Refined {Definitions} of {Heat} and {Work} in {Quantum} {Thermodynamics}}, year = {2021}, month = jul, note = {arXiv: 1912.01983}, abstract = {In this paper, unambiguous redefinitions of heat and work are presented for quantum thermodynamic systems. We will use genuine reasoning based on which Clausius originally defined work and heat in establishing thermodynamics. The change in the energy which is accompanied by a change in the entropy is identified as heat, while any change in the energy which does not lead to a change in the entropy is known as work. It will be seen that quantum coherence does not allow all the energy exchanged between two quantum systems to be only of the heat form. Several examples will also be discussed. Finally, it will be shown that these refined definitions will strongly affect the entropy production of quantum thermodynamic processes giving new insight into the irreversibility of quantum processes.}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/1912.01983}, urldate = {2021-07-28}, }`

- Aaron Z. Goldberg, Markus Grassl, Gerd Leuchs, and Luis L. Sánchez-Soto. Quantumness Beyond Entanglement: The Case of Symmetric States.
*Arxiv:2110.11361 [quant-ph]*, 2021.

[BibTeX] [Abstract] [Download PDF]

It is nowadays accepted that truly quantum correlations can exist even in the absence of entanglement. For the case of symmetric states, a physically trivial unitary transformation can alter a quantum state from entangled to separable and vice versa. We propose to certify the presence of quantumness via an average over all physically relevant modal decompositions. We investigate extremal states for such a measure: SU(2)-coherent states possess the least quantumness whereas the opposite extreme is inhabited by states with maximally spread Majorana constellations.

`@Article{Goldberg2021, author = {Aaron Z. Goldberg and Markus Grassl and Gerd Leuchs and Luis L. Sánchez-Soto}, journal = {arXiv:2110.11361 [quant-ph]}, title = {Quantumness {B}eyond {E}ntanglement: {T}he {C}ase of {S}ymmetric {S}tates}, year = {2021}, month = oct, abstract = {It is nowadays accepted that truly quantum correlations can exist even in the absence of entanglement. For the case of symmetric states, a physically trivial unitary transformation can alter a quantum state from entangled to separable and vice versa. We propose to certify the presence of quantumness via an average over all physically relevant modal decompositions. We investigate extremal states for such a measure: SU(2)-coherent states possess the least quantumness whereas the opposite extreme is inhabited by states with maximally spread Majorana constellations.}, archiveprefix = {arXiv}, eprint = {2110.11361}, keywords = {quant-ph}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv211011361G}, }`

- Lucas Chibebe Céleri and Łukasz Rudnicki. Gauge invariant quantum thermodynamics: consequences for the first law.
*Arxiv e-prints*, pages arXiv:2104.10153, apr 2021.

[BibTeX] [Abstract] [Download PDF]

Universality of classical thermodynamics rests on the central limit theorem, due to which, measurements of thermal fluctuations are unable to reveal detailed information regarding the microscopic structure of a macroscopic body. When small systems are considered and fluctuations become important, thermodynamic quantities can be understood in the context of classical stochastic mechanics. A fundamental assumption behind thermodynamics is therefore that of coarse-graning, which stems from a substantial lack of control over all degrees of freedom. However, when quantum systems are concerned, one claims a high level of control. As a consequence, information theory plays a major role in the identification of thermodynamic functions. Here, drawing from the concept of gauge symmetry, essential in all modern physical theories, we put forward a new possible, intermediate route. Working within the realm of quantum thermodynamics we explicitly construct physically motivated gauge transformations which encode a gentle variant of coarse- graining behind thermodynamics. As a consequence, we reinterpret quantum work and heat, as well as the role of quantum coherence.

`@Article{ChibebeCeleri2021, author = {Chibebe Céleri, Lucas and Rudnicki, Łukasz}, journal = {arXiv e-prints}, title = {Gauge invariant quantum thermodynamics: consequences for the first law}, year = {2021}, month = apr, pages = {arXiv:2104.10153}, abstract = {Universality of classical thermodynamics rests on the central limit theorem, due to which, measurements of thermal fluctuations are unable to reveal detailed information regarding the microscopic structure of a macroscopic body. When small systems are considered and fluctuations become important, thermodynamic quantities can be understood in the context of classical stochastic mechanics. A fundamental assumption behind thermodynamics is therefore that of coarse-graning, which stems from a substantial lack of control over all degrees of freedom. However, when quantum systems are concerned, one claims a high level of control. As a consequence, information theory plays a major role in the identification of thermodynamic functions. Here, drawing from the concept of gauge symmetry, essential in all modern physical theories, we put forward a new possible, intermediate route. Working within the realm of quantum thermodynamics we explicitly construct physically motivated gauge transformations which encode a gentle variant of coarse- graining behind thermodynamics. As a consequence, we reinterpret quantum work and heat, as well as the role of quantum coherence.}, archiveprefix = {arXiv}, eid = {arXiv:2104.10153}, eprint = {2104.10153}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv210410153C}, }`

- Tomasz Linowski and Łukasz Rudnicki. Dissipative evolution of covariance matrix beyond quadratic order.
*Arxiv e-prints*, pages arXiv:2105.12644, may 2021.

[BibTeX] [Abstract] [Download PDF]

The covariance matrix contains the complete information about the second-order expectation values of the mode quadratures (position and momentum operators) of the system. Due to its prominence in studies of continuous variable systems, most significantly Gaussian states, special emphasis is put on time evolution models that result in self-contained equations for the covariance matrix. So far, despite not being explicitly implied by this requirement, virtually all such models assume a so- called quadratic, or second-order case, in which the generator of the evolution is at most second-order in the mode quadratures. Here, we provide an explicit model of covariance matrix evolution of infinite order. Furthermore, we derive the solution, including stationary states, for a large subclass of proposed evolutions. Our findings challenge the contemporary understanding of covariance matrix dynamics and may give rise to new methods and improvements in quantum technologies employing continuous variable systems.

`@Article{Linowski2021a, author = {Linowski, Tomasz and Rudnicki, Łukasz}, journal = {arXiv e-prints}, title = {Dissipative evolution of covariance matrix beyond quadratic order}, year = {2021}, month = may, pages = {arXiv:2105.12644}, abstract = {The covariance matrix contains the complete information about the second-order expectation values of the mode quadratures (position and momentum operators) of the system. Due to its prominence in studies of continuous variable systems, most significantly Gaussian states, special emphasis is put on time evolution models that result in self-contained equations for the covariance matrix. So far, despite not being explicitly implied by this requirement, virtually all such models assume a so- called quadratic, or second-order case, in which the generator of the evolution is at most second-order in the mode quadratures. Here, we provide an explicit model of covariance matrix evolution of infinite order. Furthermore, we derive the solution, including stationary states, for a large subclass of proposed evolutions. Our findings challenge the contemporary understanding of covariance matrix dynamics and may give rise to new methods and improvements in quantum technologies employing continuous variable systems.}, archiveprefix = {arXiv}, eid = {arXiv:2105.12644}, eprint = {2105.12644}, keywords = {Quantum {P}hysics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv210512644L}, }`

- Tomasz Linowski and Łukasz Rudnicki. Classical description of bosonic quantum fields in terms of the reduced-state-of-the-field framework.
*Arxiv e-prints*, pages arXiv:2107.03196, jul 2021.

[BibTeX] [Abstract] [Download PDF]

We discuss compatibility between various quantum aspects of bosonic fields, relevant for quantum optics and quantum thermodynamics, and the mesoscopic formalism of reduced state of the field (RSF). In particular, we derive exact conditions under which Gaussian and Bogoliubov-type evolutions can be cast into the RSF framework. In that regard, special emphasis is put on Gaussian thermal operations. To strengthen the link between the RSF formalism and the notion of classicality for bosonic quantum fields, we prove that RSF contains no information about entanglement in two-mode Gaussian states. For the same purpose, we show that the entropic characterisation of RSF by means of the von Neumann entropy is qualitatively the same as its description based on the Wehrl entropy. Our findings help bridge the conceptual gap between quantum and classical mechanics.

`@Article{Linowski2021, author = {Linowski, Tomasz and Rudnicki, Łukasz}, journal = {arXiv e-prints}, title = {Classical description of bosonic quantum fields in terms of the reduced-state-of-the-field framework}, year = {2021}, month = jul, pages = {arXiv:2107.03196}, abstract = {We discuss compatibility between various quantum aspects of bosonic fields, relevant for quantum optics and quantum thermodynamics, and the mesoscopic formalism of reduced state of the field (RSF). In particular, we derive exact conditions under which Gaussian and Bogoliubov-type evolutions can be cast into the RSF framework. In that regard, special emphasis is put on Gaussian thermal operations. To strengthen the link between the RSF formalism and the notion of classicality for bosonic quantum fields, we prove that RSF contains no information about entanglement in two-mode Gaussian states. For the same purpose, we show that the entropic characterisation of RSF by means of the von Neumann entropy is qualitatively the same as its description based on the Wehrl entropy. Our findings help bridge the conceptual gap between quantum and classical mechanics.}, archiveprefix = {arXiv}, eid = {arXiv:2107.03196}, eprint = {2107.03196}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv210703196L}, }`

- Beata Zjawin, David Schmid, Matty J. Hoban, and Ana Belén Sainz. Quantifying EPR: the resource theory of nonclassicality of common-cause assemblages.
*Arxiv e-prints*, pages arXiv:2111.10244, nov 2021.

[BibTeX] [Abstract] [Download PDF]

Einstein-Podolsky-Rosen (EPR) steering is often (implicitly or explicitly) taken to be evidence for spooky action-at-a-distance. An alternative perspective on steering – endorsed by EPR themselves – is that Alice has no causal influence on the physical state of Bob’s system; rather, Alice merely updates her knowledge of the state of Bob’s system by performing a measurement on a system correlated with his. In this work, we elaborate on this perspective (from which the very term `steering’ is seen to be inappropriate), and we are led to a resource-theoretic treatment of correlations in EPR scenarios. For both bipartite and multipartite scenarios, we develop the resulting resource theory, wherein the free operations are local operations and shared randomness (LOSR). We show that resource conversion under free operations in this paradigm can be evaluated with a single instance of a semidefinite program, making the problem numerically tractable. Moreover, we find that the structure of the pre-order of resources features interesting properties, such as infinite families of incomparable resources. In showing this, we derive new EPR resource monotones. We also discuss advantages of our approach over a pre-existing proposal for a resource theory of `steering’, and discuss how our approach sheds light on basic questions, such as which multipartite assemblages are classically explainable.

`@Article{Zjawin2021, author = {Zjawin, Beata and Schmid, David and Hoban, Matty J. and Sainz, Ana Belén}, journal = {arXiv e-prints}, title = {Quantifying {EPR}: the resource theory of nonclassicality of common-cause assemblages}, year = {2021}, month = nov, pages = {arXiv:2111.10244}, abstract = {Einstein-Podolsky-Rosen (EPR) steering is often (implicitly or explicitly) taken to be evidence for spooky action-at-a-distance. An alternative perspective on steering - endorsed by EPR themselves - is that Alice has no causal influence on the physical state of Bob's system; rather, Alice merely updates her knowledge of the state of Bob's system by performing a measurement on a system correlated with his. In this work, we elaborate on this perspective (from which the very term `steering' is seen to be inappropriate), and we are led to a resource-theoretic treatment of correlations in EPR scenarios. For both bipartite and multipartite scenarios, we develop the resulting resource theory, wherein the free operations are local operations and shared randomness (LOSR). We show that resource conversion under free operations in this paradigm can be evaluated with a single instance of a semidefinite program, making the problem numerically tractable. Moreover, we find that the structure of the pre-order of resources features interesting properties, such as infinite families of incomparable resources. In showing this, we derive new EPR resource monotones. We also discuss advantages of our approach over a pre-existing proposal for a resource theory of `steering', and discuss how our approach sheds light on basic questions, such as which multipartite assemblages are classically explainable.}, archiveprefix = {arXiv}, eid = {arXiv:2111.10244}, eprint = {2111.10244}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv211110244Z}, }`

- Massimiliano Incudini, Fabio Tarocco, Riccardo Mengoni, Alessandra Di Pierro, and Antonio Mandarino. Benchmarking Small-Scale Quantum Devices on Computing Graph Edit Distance.
*Arxiv e-prints*, pages arXiv:2111.10183, nov 2021.

[BibTeX] [Abstract] [Download PDF]

Distance measures provide the foundation for many popular algorithms in Machine Learning and Pattern Recognition. Different notions of distance can be used depending on the types of the data the algorithm is working on. For graph-shaped data, an important notion is the Graph Edit Distance (GED) that measures the degree of (dis)similarity between two graphs in terms of the operations needed to make them identical. As the complexity of computing GED is the same as NP-hard problems, it is reasonable to consider approximate solutions. In this paper we present a comparative study of two quantum approaches to computing GED: quantum annealing and variational quantum algorithms, which refer to the two types of quantum hardware currently available, namely quantum annealer and gate-based quantum computer, respectively. Considering the current state of noisy intermediate-scale quantum computers, we base our study on proof-of-principle tests of the performance of these quantum algorithms.

`@Article{Incudini2021a, author = {Incudini, Massimiliano and Tarocco, Fabio and Mengoni, Riccardo and Di Pierro, Alessandra and Mandarino, Antonio}, journal = {arXiv e-prints}, title = {Benchmarking {S}mall-{S}cale {Q}uantum {D}evices on {C}omputing {G}raph {E}dit {D}istance}, year = {2021}, month = nov, pages = {arXiv:2111.10183}, abstract = {Distance measures provide the foundation for many popular algorithms in Machine Learning and Pattern Recognition. Different notions of distance can be used depending on the types of the data the algorithm is working on. For graph-shaped data, an important notion is the Graph Edit Distance (GED) that measures the degree of (dis)similarity between two graphs in terms of the operations needed to make them identical. As the complexity of computing GED is the same as NP-hard problems, it is reasonable to consider approximate solutions. In this paper we present a comparative study of two quantum approaches to computing GED: quantum annealing and variational quantum algorithms, which refer to the two types of quantum hardware currently available, namely quantum annealer and gate-based quantum computer, respectively. Considering the current state of noisy intermediate-scale quantum computers, we base our study on proof-of-principle tests of the performance of these quantum algorithms.}, archiveprefix = {arXiv}, eid = {arXiv:2111.10183}, eprint = {2111.10183}, keywords = {Quantum Physics, Computer Science - Machine Learning}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv211110183I}, }`

- Tanmoy Biswas, Oliveira A. de Junior, Michał Horodecki, and Kamil Korzekwa. Fluctuation-dissipation relations for thermodynamic distillation processes.
*Arxiv:2105.11759 [quant-ph]*, 5 2021.

[BibTeX] [Abstract] [Download PDF]

The fluctuation-dissipation theorem is a fundamental result in statistical physics that establishes a connection between the response of a system subject to a perturbation and the fluctuations associated with observables in equilibrium. Here we derive its version within a resource-theoretic framework, where one investigates optimal quantum state transitions under thermodynamic constraints. More precisely, we first characterise optimal thermodynamic distillation processes, and then prove a relation between the amount of free energy dissipated in such processes and the free energy fluctuations of the initial state of the system. Our results apply to initial states given by either asymptotically many identical pure systems or arbitrary number of independent energy-incoherent systems, and allow not only for a state transformation, but also for the change of Hamiltonian. The fluctuation-dissipation relations we derive enable us to find the optimal performance of thermodynamic protocols such as work extraction, information erasure and thermodynamically-free communication, up to second-order asymptotics in the number $N$ of processed systems. We thus provide a first rigorous analysis of these thermodynamic protocols for quantum states with coherence between different energy eigenstates in the intermediate regime of large but finite $N$.

`@Article{Biswas2021a, author = {Biswas, Tanmoy and Junior, A. de Oliveira and Horodecki, Michał and Korzekwa, Kamil}, journal = {arXiv:2105.11759 [quant-ph]}, title = {Fluctuation-dissipation relations for thermodynamic distillation processes}, year = {2021}, month = {5}, abstract = {The fluctuation-dissipation theorem is a fundamental result in statistical physics that establishes a connection between the response of a system subject to a perturbation and the fluctuations associated with observables in equilibrium. Here we derive its version within a resource-theoretic framework, where one investigates optimal quantum state transitions under thermodynamic constraints. More precisely, we first characterise optimal thermodynamic distillation processes, and then prove a relation between the amount of free energy dissipated in such processes and the free energy fluctuations of the initial state of the system. Our results apply to initial states given by either asymptotically many identical pure systems or arbitrary number of independent energy-incoherent systems, and allow not only for a state transformation, but also for the change of Hamiltonian. The fluctuation-dissipation relations we derive enable us to find the optimal performance of thermodynamic protocols such as work extraction, information erasure and thermodynamically-free communication, up to second-order asymptotics in the number $N$ of processed systems. We thus provide a first rigorous analysis of these thermodynamic protocols for quantum states with coherence between different energy eigenstates in the intermediate regime of large but finite $N$.}, archiveprefix = {arXiv}, eprint = {2105.11759}, groups = {Michal_H}, keywords = {Quantum Physics, Condensed Matter - Statistical Mechanics}, primaryclass = {quant-ph}, url = {https://arxiv.org/pdf/2105.11759}, }`

- Rivu Gupta, Saptarshi Roy, Tamoghna Das, and Aditi Sen De. Quantum illumination with a light absorbing target.
*Arxiv e-prints*, pages arXiv:2111.01069, nov 2021.

[BibTeX] [Abstract] [Download PDF]

In a quantum illumination (QI) protocol, the task is to detect the presence of the target which is typically modelled by a partially reflecting beam splitter. We analyze the performance of QI when the target absorbs part of the light that falls on it, thereby making the scenario more realistic. We present an optical setup that models a target with these characteristics and explore its detectability in the quantum domain in terms of the Chernoff bound (CB). For an idler-free setup, we use the coherent state for QI while the two mode squeezed vacuum (TMSV) state is employed in the signal-idler scheme. In both the cases, we report an absorption-induced enhancement of the detection efficiency indicated by a lowering of CB with increasing amounts of absorption. Interestingly, we show that in the presence of absorption, a more intense thermal background can lead to target detection with enhanced efficiency. Moreover, we observe that the quantum advantage persists even for finite amounts of absorption. However, we find that the quantum advantage offered by TMSV decreases monotonically with absorption, and becomes vanishingly small in the high absorption regime. We also demonstrate the optimality of both the coherent and the TMSV states in their respective setups (idler-free and signal-idler) in the limit of low reflectivity and absorption.

`@Article{Gupta2021, author = {Gupta, Rivu and Roy, Saptarshi and Das, Tamoghna and De, Aditi Sen}, journal = {arXiv e-prints}, title = {Quantum illumination with a light absorbing target}, year = {2021}, month = nov, pages = {arXiv:2111.01069}, abstract = {In a quantum illumination (QI) protocol, the task is to detect the presence of the target which is typically modelled by a partially reflecting beam splitter. We analyze the performance of QI when the target absorbs part of the light that falls on it, thereby making the scenario more realistic. We present an optical setup that models a target with these characteristics and explore its detectability in the quantum domain in terms of the Chernoff bound (CB). For an idler-free setup, we use the coherent state for QI while the two mode squeezed vacuum (TMSV) state is employed in the signal-idler scheme. In both the cases, we report an absorption-induced enhancement of the detection efficiency indicated by a lowering of CB with increasing amounts of absorption. Interestingly, we show that in the presence of absorption, a more intense thermal background can lead to target detection with enhanced efficiency. Moreover, we observe that the quantum advantage persists even for finite amounts of absorption. However, we find that the quantum advantage offered by TMSV decreases monotonically with absorption, and becomes vanishingly small in the high absorption regime. We also demonstrate the optimality of both the coherent and the TMSV states in their respective setups (idler-free and signal-idler) in the limit of low reflectivity and absorption.}, archiveprefix = {arXiv}, eid = {arXiv:2111.01069}, eprint = {2111.01069}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv211101069G}, }`

- Anubhav Chaturvedi, Marcin Paw{l}owski, and Debashis Saha. Quantum description of reality is empirically incomplete.
*Arxiv e-prints*, pages arXiv:2110.13124, oct 2021.

[BibTeX] [Abstract] [Download PDF]

Empirical falsifiability of the predictions of physical theories is the cornerstone of the scientific method. Physical theories attribute empirically falsifiable operational properties to sets of physical preparations. A theory is said to be empirically complete if such properties allow for a not fine-tuned realist explanation, as properties of underlying probability distributions over states of reality. Such theories satisfy a family of equalities among fundamental operational properties, characterized exclusively by the number of preparations. Quantum preparations deviate from these equalities, and the maximal quantum deviation increases with the number of preparations. These deviations not only signify the incompleteness of the operational quantum formalism, but they simultaneously imply quantum over classical advantage in suitably constrained one-way communication tasks, highlighting the delicate interplay between the two.

`@Article{Chaturvedi2021, author = {Chaturvedi, Anubhav and Paw{\l}owski, Marcin and Saha, Debashis}, journal = {arXiv e-prints}, title = {Quantum description of reality is empirically incomplete}, year = {2021}, month = oct, pages = {arXiv:2110.13124}, abstract = {Empirical falsifiability of the predictions of physical theories is the cornerstone of the scientific method. Physical theories attribute empirically falsifiable operational properties to sets of physical preparations. A theory is said to be empirically complete if such properties allow for a not fine-tuned realist explanation, as properties of underlying probability distributions over states of reality. Such theories satisfy a family of equalities among fundamental operational properties, characterized exclusively by the number of preparations. Quantum preparations deviate from these equalities, and the maximal quantum deviation increases with the number of preparations. These deviations not only signify the incompleteness of the operational quantum formalism, but they simultaneously imply quantum over classical advantage in suitably constrained one-way communication tasks, highlighting the delicate interplay between the two.}, archiveprefix = {arXiv}, eid = {arXiv:2110.13124}, eprint = {2110.13124}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv211013124C}, }`

- Nikolai Miklin, Mariami Gachechiladze, George Moreno, and Rafael Chaves. Causal inference with imperfect instrumental variables.
*Arxiv e-prints*, pages arXiv:2111.03029, nov 2021.

[BibTeX] [Abstract] [Download PDF]

Instrumental variables allow for quantification of cause and effect relationships even in the absence of interventions. To achieve this, a number of causal assumptions must be met, the most important of which is the independence assumption, which states that the instrument and any confounding factor must be independent. However, if this independence condition is not met, can we still work with imperfect instrumental variables? Imperfect instruments can manifest themselves by violations of the instrumental inequalities that constrain the set of correlations in the scenario. In this paper, we establish a quantitative relationship between such violations of instrumental inequalities and the minimal amount of measurement dependence required to explain them. As a result, we provide adapted inequalities that are valid in the presence of a relaxed measurement dependence assumption in the instrumental scenario. This allows for the adaptation of existing and new lower bounds on the average causal effect for instrumental scenarios with binary outcomes. Finally, we discuss our findings in the context of quantum mechanics.

`@Article{Miklin2021, author = {Miklin, Nikolai and Gachechiladze, Mariami and Moreno, George and Chaves, Rafael}, journal = {arXiv e-prints}, title = {Causal inference with imperfect instrumental variables}, year = {2021}, month = nov, pages = {arXiv:2111.03029}, abstract = {Instrumental variables allow for quantification of cause and effect relationships even in the absence of interventions. To achieve this, a number of causal assumptions must be met, the most important of which is the independence assumption, which states that the instrument and any confounding factor must be independent. However, if this independence condition is not met, can we still work with imperfect instrumental variables? Imperfect instruments can manifest themselves by violations of the instrumental inequalities that constrain the set of correlations in the scenario. In this paper, we establish a quantitative relationship between such violations of instrumental inequalities and the minimal amount of measurement dependence required to explain them. As a result, we provide adapted inequalities that are valid in the presence of a relaxed measurement dependence assumption in the instrumental scenario. This allows for the adaptation of existing and new lower bounds on the average causal effect for instrumental scenarios with binary outcomes. Finally, we discuss our findings in the context of quantum mechanics.}, archiveprefix = {arXiv}, eid = {arXiv:2111.03029}, eprint = {2111.03029}, keywords = {Statistics - Machine Learning, Computer Science - Machine Learning, Quantum Physics}, primaryclass = {stat.ML}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv211103029M}, }`

- Iris Agresti, Davide Poderini, Beatrice Polacchi, Nikolai Miklin, Mariami Gachechiladze, Alessia Suprano, Emanuele Polino, Giorgio Milani, Gonzalo Carvacho, Rafael Chaves, and Fabio Sciarrino. Experimental test of quantum causal influences.
*Arxiv e-prints*, pages arXiv:2108.08926, aug 2021.

[BibTeX] [Abstract] [Download PDF]

Since Bell’s theorem, it is known that the concept of local realism fails to explain quantum phenomena. Indeed, the violation of a Bell inequality has become a synonym of the incompatibility of quantum theory with our classical notion of cause and effect. As recently discovered, however, the instrumental scenario – a tool of central importance in causal inference – allows for signatures of nonclassicality that do not hinge on this paradigm. If, instead of relying on observational data only, we can also intervene in our experimental setup, quantum correlations can violate classical bounds on the causal influence even in scenarios where no violation of a Bell inequality is ever possible. That is, through interventions, we can witness the quantum behaviour of a system that would look classical otherwise. Using a photonic setup – faithfully implementing the instrumental causal structure and allowing to switch between the observational and interventional modes in a run to run basis – we experimentally observe this new witness of nonclassicality for the first time. In parallel, we also test quantum bounds for the causal influence, showing that they provide a reliable tool for quantum causal modelling.

`@Article{Agresti2021, author = {Agresti, Iris and Poderini, Davide and Polacchi, Beatrice and Miklin, Nikolai and Gachechiladze, Mariami and Suprano, Alessia and Polino, Emanuele and Milani, Giorgio and Carvacho, Gonzalo and Chaves, Rafael and Sciarrino, Fabio}, journal = {arXiv e-prints}, title = {Experimental test of quantum causal influences}, year = {2021}, month = aug, pages = {arXiv:2108.08926}, abstract = {Since Bell's theorem, it is known that the concept of local realism fails to explain quantum phenomena. Indeed, the violation of a Bell inequality has become a synonym of the incompatibility of quantum theory with our classical notion of cause and effect. As recently discovered, however, the instrumental scenario -- a tool of central importance in causal inference -- allows for signatures of nonclassicality that do not hinge on this paradigm. If, instead of relying on observational data only, we can also intervene in our experimental setup, quantum correlations can violate classical bounds on the causal influence even in scenarios where no violation of a Bell inequality is ever possible. That is, through interventions, we can witness the quantum behaviour of a system that would look classical otherwise. Using a photonic setup -- faithfully implementing the instrumental causal structure and allowing to switch between the observational and interventional modes in a run to run basis -- we experimentally observe this new witness of nonclassicality for the first time. In parallel, we also test quantum bounds for the causal influence, showing that they provide a reliable tool for quantum causal modelling.}, archiveprefix = {arXiv}, eid = {arXiv:2108.08926}, eprint = {2108.08926}, keywords = {Quantum Physics, Physics - Optics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv210808926A}, }`

- Ray Ganardi, Marek Miller, Tomasz Paterek, and Marek Żukowski. Hierarchy of correlation quantifiers comparable to negativity.
*Arxiv e-prints*, pages arXiv:2111.11887, nov 2021.

[BibTeX] [Abstract] [Download PDF]

Quantum systems generally exhibit different kinds of correlations. In order to compare them on equal footing, one uses the so-called distance-based approach where different types of correlations are captured by the distance to different set of states. However, these quantifiers are usually hard to compute as their definition involves optimization aiming to find the closest states within the set. On the other hand, negativity is one of the few computable entanglement monotones, but its comparison with other correlations required further justification. Here we place negativity as part of a family of correlation measures that has a distance-based construction. We introduce a suitable distance, discuss the emerging measures and their applications, and compare them to relative entropy-based correlation quantifiers. This work is a step towards correlation measures that are simultaneously comparable and computable.

`@Article{Ganardi2021, author = {Ganardi, Ray and Miller, Marek and Paterek, Tomasz and {\.Z}ukowski, Marek}, journal = {arXiv e-prints}, title = {Hierarchy of correlation quantifiers comparable to negativity}, year = {2021}, month = nov, pages = {arXiv:2111.11887}, abstract = {Quantum systems generally exhibit different kinds of correlations. In order to compare them on equal footing, one uses the so-called distance-based approach where different types of correlations are captured by the distance to different set of states. However, these quantifiers are usually hard to compute as their definition involves optimization aiming to find the closest states within the set. On the other hand, negativity is one of the few computable entanglement monotones, but its comparison with other correlations required further justification. Here we place negativity as part of a family of correlation measures that has a distance-based construction. We introduce a suitable distance, discuss the emerging measures and their applications, and compare them to relative entropy-based correlation quantifiers. This work is a step towards correlation measures that are simultaneously comparable and computable.}, archiveprefix = {arXiv}, eid = {arXiv:2111.11887}, eprint = {2111.11887}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv211111887G}, }`

- Karol Horodecki, Marek Winczewski, and Siddhartha Das. Fundamental limitations on device-independent quantum conference key agreement.
*Arxiv e-prints*, pages arXiv:2111.02467, nov 2021.

[BibTeX] [Abstract] [Download PDF]

We provide several general upper bounds on device-independent conference key agreement (DI-CKA) against the quantum adversary. They include bounds by reduced entanglement measures and those based on multipartite secrecy monotones such as reduced cc-squashed entanglement. We compare the latter bound with the known lower bound for the protocol of conference key distillation based on the parity-CHSH game. We also show that the gap between DI-CKA rate and the rate of device-dependent is inherited from the bipartite gap between device-independent and device-dependent key rates, giving examples that exhibit the strict gap.

`@Article{Horodecki2021, author = {Horodecki, Karol and Winczewski, Marek and Das, Siddhartha}, journal = {arXiv e-prints}, title = {Fundamental limitations on device-independent quantum conference key agreement}, year = {2021}, month = nov, pages = {arXiv:2111.02467}, abstract = {We provide several general upper bounds on device-independent conference key agreement (DI-CKA) against the quantum adversary. They include bounds by reduced entanglement measures and those based on multipartite secrecy monotones such as reduced cc-squashed entanglement. We compare the latter bound with the known lower bound for the protocol of conference key distillation based on the parity-CHSH game. We also show that the gap between DI-CKA rate and the rate of device-dependent is inherited from the bipartite gap between device-independent and device-dependent key rates, giving examples that exhibit the strict gap.}, archiveprefix = {arXiv}, eid = {arXiv:2111.02467}, eprint = {2111.02467}, keywords = {Quantum Physics, Computer Science - Information Theory, Mathematical Physics}, primaryclass = {quant-ph}, url = {https://arxiv.org/pdf/2111.02467}, }`

### 2020

- David Schmid, John H. Selby, Matthew F. Pusey, and Robert W. Spekkens. A structure theorem for generalized-noncontextual ontological models.
*Arxiv:2005.07161 [quant-ph]*, 2020. arXiv: 2005.07161

[BibTeX] [Abstract] [Download PDF]

It is useful to have a criterion for when the predictions of an operational theory should be considered classically explainable. Here we take the criterion to be that the theory admits of a generalized-noncontextual ontological model. Existing works on generalized noncontextuality have focused on experimental scenarios having a simple structure, typically, prepare-measure scenarios. Here, we formally extend the framework of ontological models as well as the principle of generalized noncontextuality to arbitrary compositional scenarios. We leverage this process-theoretic framework to prove that, under some reasonable assumptions, every generalized-noncontextual ontological model of a tomographically local operational theory has a surprisingly rigid and simple mathematical structure; in short, it corresponds to a frame representation which is not overcomplete. One consequence of this theorem is that the largest number of ontic states possible in any such model is given by the dimension of the associated generalized probabilistic theory. This constraint is useful for generating noncontextuality no-go theorems as well as techniques for experimentally certifying contextuality. Along the way, we extend known results concerning the equivalence of different notions of classicality from prepare-measure scenarios to arbitrary compositional scenarios. Specifically, we prove a correspondence between the following three notions of classical explainability of an operational theory: (i) admitting a noncontextual ontological model, (ii) admitting of a positive quasiprobability representation, and (iii) being simplex-embeddable.

`@article{schmid_structure_2020, title = {A structure theorem for generalized-noncontextual ontological models}, url = {http://arxiv.org/abs/2005.07161}, abstract = {It is useful to have a criterion for when the predictions of an operational theory should be considered classically explainable. Here we take the criterion to be that the theory admits of a generalized-noncontextual ontological model. Existing works on generalized noncontextuality have focused on experimental scenarios having a simple structure, typically, prepare-measure scenarios. Here, we formally extend the framework of ontological models as well as the principle of generalized noncontextuality to arbitrary compositional scenarios. We leverage this process-theoretic framework to prove that, under some reasonable assumptions, every generalized-noncontextual ontological model of a tomographically local operational theory has a surprisingly rigid and simple mathematical structure; in short, it corresponds to a frame representation which is not overcomplete. One consequence of this theorem is that the largest number of ontic states possible in any such model is given by the dimension of the associated generalized probabilistic theory. This constraint is useful for generating noncontextuality no-go theorems as well as techniques for experimentally certifying contextuality. Along the way, we extend known results concerning the equivalence of different notions of classicality from prepare-measure scenarios to arbitrary compositional scenarios. Specifically, we prove a correspondence between the following three notions of classical explainability of an operational theory: (i) admitting a noncontextual ontological model, (ii) admitting of a positive quasiprobability representation, and (iii) being simplex-embeddable.}, urldate = {2020-09-04}, journal = {arXiv:2005.07161 [quant-ph]}, author = {Schmid, David and Selby, John H. and Pusey, Matthew F. and Spekkens, Robert W.}, month = may, year = {2020}, note = {arXiv: 2005.07161}, keywords = {Quantum Physics}, }`

- Mariami Gachechiladze, Nikolai Miklin, and Rafael Chaves. Quantifying causal influences in the presence of a quantum common cause.
*Arxiv:2007.01221 [quant-ph, stat]*, 2020. arXiv: 2007.01221

[BibTeX] [Abstract] [Download PDF]

Quantum mechanics challenges our intuition on the cause-effect relations in nature. Some fundamental concepts, including Reichenbach’s common cause principle or the notion of local realism, have to be reconsidered. Traditionally, this is witnessed by the violation of a Bell inequality. But are Bell inequalities the only signature of the incompatibility between quantum correlations and causality theory? Motivated by this question we introduce a general framework able to estimate causal influences between two variables, without the need of interventions and irrespectively of the classical, quantum, or even post-quantum nature of a common cause. In particular, by considering the simplest instrumental scenario – for which violation of Bell inequalities is not possible – we show that every pure bipartite entangled state violates the classical bounds on causal influence, thus answering in negative to the posed question and opening a new venue to explore the role of causality within quantum theory.

`@article{gachechiladze_quantifying_2020, title = {Quantifying causal influences in the presence of a quantum common cause}, url = {http://arxiv.org/abs/2007.01221}, abstract = {Quantum mechanics challenges our intuition on the cause-effect relations in nature. Some fundamental concepts, including Reichenbach's common cause principle or the notion of local realism, have to be reconsidered. Traditionally, this is witnessed by the violation of a Bell inequality. But are Bell inequalities the only signature of the incompatibility between quantum correlations and causality theory? Motivated by this question we introduce a general framework able to estimate causal influences between two variables, without the need of interventions and irrespectively of the classical, quantum, or even post-quantum nature of a common cause. In particular, by considering the simplest instrumental scenario -- for which violation of Bell inequalities is not possible -- we show that every pure bipartite entangled state violates the classical bounds on causal influence, thus answering in negative to the posed question and opening a new venue to explore the role of causality within quantum theory.}, urldate = {2020-09-04}, journal = {arXiv:2007.01221 [quant-ph, stat]}, author = {Gachechiladze, Mariami and Miklin, Nikolai and Chaves, Rafael}, month = jul, year = {2020}, note = {arXiv: 2007.01221}, keywords = {Quantum Physics, Statistics - Machine Learning}, }`

- Lukas Knips, Jan Dziewior, Waldemar Kłobus, Wiesław Laskowski, Tomasz Paterek, Peter J. Shadbolt, Harald Weinfurter, and Jasmin D. A. Meinecke. Multipartite entanglement analysis from random correlations.
*Npj quantum information*, 6(1):51, dec 2020. doi:10.1038/s41534-020-0281-5

[BibTeX] [Abstract] [Download PDF]

Abstract Quantum entanglement is usually revealed via a well aligned, carefully chosen set of measurements. Yet, under a number of experimental conditions, for example in communication within multiparty quantum networks, noise along the channels or fluctuating orientations of reference frames may ruin the quality of the distributed states. Here, we show that even for strong fluctuations one can still gain detailed information about the state and its entanglement using random measurements. Correlations between all or subsets of the measurement outcomes and especially their distributions provide information about the entanglement structure of a state. We analytically derive an entanglement criterion for two-qubit states and provide strong numerical evidence for witnessing genuine multipartite entanglement of three and four qubits. Our methods take the purity of the states into account and are based on only the second moments of measured correlations. Extended features of this theory are demonstrated experimentally with four photonic qubits. As long as the rate of entanglement generation is sufficiently high compared to the speed of the fluctuations, this method overcomes any type and strength of localized unitary noise.

`@Article{knips_multipartite_2020, author = {Knips, Lukas and Dziewior, Jan and Kłobus, Waldemar and Laskowski, Wiesław and Paterek, Tomasz and Shadbolt, Peter J. and Weinfurter, Harald and Meinecke, Jasmin D. A.}, journal = {npj Quantum Information}, title = {Multipartite entanglement analysis from random correlations}, year = {2020}, issn = {2056-6387}, month = dec, number = {1}, pages = {51}, volume = {6}, abstract = {Abstract Quantum entanglement is usually revealed via a well aligned, carefully chosen set of measurements. Yet, under a number of experimental conditions, for example in communication within multiparty quantum networks, noise along the channels or fluctuating orientations of reference frames may ruin the quality of the distributed states. Here, we show that even for strong fluctuations one can still gain detailed information about the state and its entanglement using random measurements. Correlations between all or subsets of the measurement outcomes and especially their distributions provide information about the entanglement structure of a state. We analytically derive an entanglement criterion for two-qubit states and provide strong numerical evidence for witnessing genuine multipartite entanglement of three and four qubits. Our methods take the purity of the states into account and are based on only the second moments of measured correlations. Extended features of this theory are demonstrated experimentally with four photonic qubits. As long as the rate of entanglement generation is sufficiently high compared to the speed of the fluctuations, this method overcomes any type and strength of localized unitary noise.}, doi = {10.1038/s41534-020-0281-5}, language = {en}, url = {http://www.nature.com/articles/s41534-020-0281-5}, urldate = {2021-05-10}, }`

- Ravishankar Ramanathan, Michał Horodecki, Hammad Anwer, Stefano Pironio, Karol Horodecki, Marcus Grünfeld, Sadiq Muhammad, Mohamed Bourennane, and Paweł Horodecki. Practical No-Signalling proof Randomness Amplification using Hardy paradoxes and its experimental implementation.
*Arxiv:1810.11648 [quant-ph]*, 2020. arXiv: 1810.11648

[BibTeX] [Abstract] [Download PDF]

Device-Independent (DI) security is the best form of quantum cryptography, providing information-theoretic security based on the very laws of nature. In its highest form, security is guaranteed against adversaries limited only by the no-superluminal signalling rule of relativity. The task of randomness amplification, to generate secure fully uniform bits starting from weakly random seeds, is of both cryptographic and foundational interest, being important for the generation of cryptographically secure random numbers as well as bringing deep connections to the existence of free-will. DI no-signalling proof protocols for this fundamental task have thus far relied on esoteric proofs of non-locality termed pseudo-telepathy games, complicated multi-party setups or high-dimensional quantum systems, and have remained out of reach of experimental implementation. In this paper, we construct the first practically relevant no-signalling proof DI protocols for randomness amplification based on the simplest proofs of Bell non-locality and illustrate them with an experimental implementation in a quantum optical setup using polarised photons. Technically, we relate the problem to the vast field of Hardy paradoxes, without which it would be impossible to achieve amplification of arbitrarily weak sources in the simplest Bell non-locality scenario consisting of two parties choosing between two binary inputs. Furthermore, we identify a deep connection between proofs of the celebrated Kochen-Specker theorem and Hardy paradoxes that enables us to construct Hardy paradoxes with the non-zero probability taking any value in \$(0,1]\$. Our methods enable us, under the fair-sampling assumption of the experiment, to realize up to \$25\$ bits of randomness in \$20\$ hours of experimental data collection from an initial private source of randomness \$0.1\$ away from uniform.

`@Article{ramanathan_practical_2020, author = {Ramanathan, Ravishankar and Horodecki, Michał and Anwer, Hammad and Pironio, Stefano and Horodecki, Karol and Grünfeld, Marcus and Muhammad, Sadiq and Bourennane, Mohamed and Horodecki, Paweł}, journal = {arXiv:1810.11648 [quant-ph]}, title = {Practical {No}-{Signalling} proof {Randomness} {Amplification} using {Hardy} paradoxes and its experimental implementation}, year = {2020}, month = sep, note = {arXiv: 1810.11648}, abstract = {Device-Independent (DI) security is the best form of quantum cryptography, providing information-theoretic security based on the very laws of nature. In its highest form, security is guaranteed against adversaries limited only by the no-superluminal signalling rule of relativity. The task of randomness amplification, to generate secure fully uniform bits starting from weakly random seeds, is of both cryptographic and foundational interest, being important for the generation of cryptographically secure random numbers as well as bringing deep connections to the existence of free-will. DI no-signalling proof protocols for this fundamental task have thus far relied on esoteric proofs of non-locality termed pseudo-telepathy games, complicated multi-party setups or high-dimensional quantum systems, and have remained out of reach of experimental implementation. In this paper, we construct the first practically relevant no-signalling proof DI protocols for randomness amplification based on the simplest proofs of Bell non-locality and illustrate them with an experimental implementation in a quantum optical setup using polarised photons. Technically, we relate the problem to the vast field of Hardy paradoxes, without which it would be impossible to achieve amplification of arbitrarily weak sources in the simplest Bell non-locality scenario consisting of two parties choosing between two binary inputs. Furthermore, we identify a deep connection between proofs of the celebrated Kochen-Specker theorem and Hardy paradoxes that enables us to construct Hardy paradoxes with the non-zero probability taking any value in \$(0,1]\$. Our methods enable us, under the fair-sampling assumption of the experiment, to realize up to \$25\$ bits of randomness in \$20\$ hours of experimental data collection from an initial private source of randomness \$0.1\$ away from uniform.}, groups = {Michal_H, Pawel_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/1810.11648}, urldate = {2021-05-11}, }`

- Michał Banacki, Marcin Marciniak, Karol Horodecki, and Paweł Horodecki. Information backflow may not indicate quantum memory.
*Arxiv:2008.12638 [quant-ph]*, 2020. arXiv: 2008.12638

[BibTeX] [Abstract] [Download PDF]

We analyze recent approaches to quantum Markovianity and how they relate to the proper definition of quantum memory. We point out that the well-known criterion of information backflow may not correctly report character of the memory falsely signaling its quantumness. Therefore, as a complement to the well-known criteria, we propose several concepts of elementary dynamical maps. Maps of this type do not increase distinguishability of states which are indistinguishable by von Neumann measurements in a given basis. Those notions and convexity allows us to define general classes of processes without quantum memory in a weak and strong sense. Finally, we provide a practical characterization of the most intuitive class in terms of the new concept of witness of quantum information backflow.

`@Article{banacki_information_2020, author = {Banacki, Michał and Marciniak, Marcin and Horodecki, Karol and Horodecki, Paweł}, journal = {arXiv:2008.12638 [quant-ph]}, title = {Information backflow may not indicate quantum memory}, year = {2020}, month = aug, note = {arXiv: 2008.12638}, abstract = {We analyze recent approaches to quantum Markovianity and how they relate to the proper definition of quantum memory. We point out that the well-known criterion of information backflow may not correctly report character of the memory falsely signaling its quantumness. Therefore, as a complement to the well-known criteria, we propose several concepts of elementary dynamical maps. Maps of this type do not increase distinguishability of states which are indistinguishable by von Neumann measurements in a given basis. Those notions and convexity allows us to define general classes of processes without quantum memory in a weak and strong sense. Finally, we provide a practical characterization of the most intuitive class in terms of the new concept of witness of quantum information backflow.}, groups = {Pawel_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2008.12638}, urldate = {2021-07-28}, }`

- Ravishankar Ramanathan, Michał Banacki, Ricard Ravell Rodríguez, and Paweł Horodecki. Single trusted qubit is necessary and sufficient for quantum realisation of extremal no-signaling correlations.
*Arxiv:2004.14782 [quant-ph]*, apr 2020. arXiv: 2004.14782

[BibTeX] [Abstract] [Download PDF]

Quantum statistics can be considered from the perspective of postquantum no-signaling theories in which either none or only a certain number of quantum systems are trusted. In these scenarios, the role of states is played by the so-called no-signaling boxes or no-signaling assemblages respectively. It has been shown so far that in the usual Bell non-locality scenario with a single measurement run, quantum statistics can never reproduce an extremal non-local point within the set of no-signaling boxes. We provide here a general no-go rule showing that the latter stays true even if arbitrary sequential measurements are allowed. On the other hand, we prove a positive result showing that already a single trusted qubit is enough for quantum theory to produce a self-testable extremal point within the corresponding set of no-signaling assemblages. This result opens up the possibility for security proofs of cryptographic protocols against general no-signaling adversaries.

`@Article{ramanathan_single_2020, author = {Ramanathan, Ravishankar and Banacki, Michał and Rodríguez, Ricard Ravell and Horodecki, Paweł}, journal = {arXiv:2004.14782 [quant-ph]}, title = {Single trusted qubit is necessary and sufficient for quantum realisation of extremal no-signaling correlations}, year = {2020}, month = apr, note = {arXiv: 2004.14782}, abstract = {Quantum statistics can be considered from the perspective of postquantum no-signaling theories in which either none or only a certain number of quantum systems are trusted. In these scenarios, the role of states is played by the so-called no-signaling boxes or no-signaling assemblages respectively. It has been shown so far that in the usual Bell non-locality scenario with a single measurement run, quantum statistics can never reproduce an extremal non-local point within the set of no-signaling boxes. We provide here a general no-go rule showing that the latter stays true even if arbitrary sequential measurements are allowed. On the other hand, we prove a positive result showing that already a single trusted qubit is enough for quantum theory to produce a self-testable extremal point within the corresponding set of no-signaling assemblages. This result opens up the possibility for security proofs of cryptographic protocols against general no-signaling adversaries.}, groups = {Pawel_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2004.14782}, urldate = {2021-07-28}, }`

- Marcin Markiewicz, Mahasweta Pandit, and Wiesław Laskowski. Multiparameter estimation in generalized Mach-Zehnder interferometer.
*Arxiv:2012.07645 [quant-ph]*, dec 2020. arXiv: 2012.07645

[BibTeX] [Abstract] [Download PDF]

In this work, we investigate the problem of multiphase estimation using generalized \$3\$- and \$4\$-mode Mach-Zehnder interferometer. In our setup, we assume that the number of unknown phases is the same as the number of modes in the interferometer, which introduces strong correlations between estimators of the phases. We show that despite these correlations and despite the lack of optimisation of a measurement strategy (a fixed interferometer is used) we can still obtain the Heisenberg-like scaling of precision of estimation of all the parameters. Our estimation scheme can be applied to the task of quantum-enhanced sensing in 3-dimensional interferometric configurations.

`@Article{markiewicz_multiparameter_2020, author = {Markiewicz, Marcin and Pandit, Mahasweta and Laskowski, Wiesław}, journal = {arXiv:2012.07645 [quant-ph]}, title = {Multiparameter estimation in generalized {Mach}-{Zehnder} interferometer}, year = {2020}, month = dec, note = {arXiv: 2012.07645}, abstract = {In this work, we investigate the problem of multiphase estimation using generalized \$3\$- and \$4\$-mode Mach-Zehnder interferometer. In our setup, we assume that the number of unknown phases is the same as the number of modes in the interferometer, which introduces strong correlations between estimators of the phases. We show that despite these correlations and despite the lack of optimisation of a measurement strategy (a fixed interferometer is used) we can still obtain the Heisenberg-like scaling of precision of estimation of all the parameters. Our estimation scheme can be applied to the task of quantum-enhanced sensing in 3-dimensional interferometric configurations.}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2012.07645}, urldate = {2021-07-28}, }`

- Łukasz Czekaj, Ana Belén Sainz, John Selby, and Michał Horodecki. Correlations constrained by composite measurements.
*Arxiv:2009.04994 [quant-ph]*, sep 2020. arXiv: 2009.04994

[BibTeX] [Abstract] [Download PDF]

How to understand the set of correlations admissible in nature is one outstanding open problem in the core of the foundations of quantum theory. Here we take a complementary viewpoint to the device-independent approach, and explore the correlations that physical theories may feature when restricted by some particular constraints on their measurements. We show that demanding that a theory exhibits a composite measurement imposes a hierarchy of constraints on the structure of its sets of states and effects, which translate to a hierarchy of constraints on the allowed correlations themselves. We moreover focus on the particular case where one demands the existence of an entangled measurement that reads out the parity of local fiducial measurements. By formulating a non-linear Optimisation Problem, and semidefinite relaxations of it, we explore the consequences of the existence of such a parity reading measurement for violations of Bell inequalities. In particular, we show that in certain situations this assumption has surprisingly strong consequences, namely, that Tsirelson’s bound can be recovered.

`@Article{czekaj_correlations_2020, author = {Czekaj, Łukasz and Sainz, Ana Belén and Selby, John and Horodecki, Michał}, journal = {arXiv:2009.04994 [quant-ph]}, title = {Correlations constrained by composite measurements}, year = {2020}, month = sep, note = {arXiv: 2009.04994}, abstract = {How to understand the set of correlations admissible in nature is one outstanding open problem in the core of the foundations of quantum theory. Here we take a complementary viewpoint to the device-independent approach, and explore the correlations that physical theories may feature when restricted by some particular constraints on their measurements. We show that demanding that a theory exhibits a composite measurement imposes a hierarchy of constraints on the structure of its sets of states and effects, which translate to a hierarchy of constraints on the allowed correlations themselves. We moreover focus on the particular case where one demands the existence of an entangled measurement that reads out the parity of local fiducial measurements. By formulating a non-linear Optimisation Problem, and semidefinite relaxations of it, we explore the consequences of the existence of such a parity reading measurement for violations of Bell inequalities. In particular, we show that in certain situations this assumption has surprisingly strong consequences, namely, that Tsirelson's bound can be recovered.}, groups = {Michal_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2009.04994}, urldate = {2021-07-28}, }`

- Aaron Z. Goldberg, Pablo de la Hoz, Gunnar Bjork, Andrei B. Klimov, Markus Grassl, Gerd Leuchs, and Luis L. Sanchez-Soto. Quantum concepts in optical polarization.
*Arxiv:2011.03979 [quant-ph]*, nov 2020. arXiv: 2011.03979

[BibTeX] [Abstract] [Download PDF]

We comprehensively review the quantum theory of the polarization properties of light. In classical optics, these traits are characterized by the Stokes parameters, which can be geometrically interpreted using the Poincar{\textbackslash}’e sphere. Remarkably, these Stokes parameters can also be applied to the quantum world, but then important differences emerge: now, because fluctuations in the number of photons are unavoidable, one is forced to work in the three-dimensional Poincar{\textbackslash}’e space that can be regarded as a set of nested spheres. Additionally, higher-order moments of the Stokes variables might play a substantial role for quantum states, which is not the case for most classical Gaussian states. This brings about important differences between these two worlds that we review in detail. In particular, the classical degree of polarization produces unsatisfactory results in the quantum domain. We compare alternative quantum degrees and put forth that they order various states differently. Finally, intrinsically nonclassical states are explored and their potential applications in quantum technologies are discussed.

`@article{goldberg_quantum_2020, title = {Quantum concepts in optical polarization}, url = {http://arxiv.org/abs/2011.03979}, abstract = {We comprehensively review the quantum theory of the polarization properties of light. In classical optics, these traits are characterized by the Stokes parameters, which can be geometrically interpreted using the Poincar{\textbackslash}'e sphere. Remarkably, these Stokes parameters can also be applied to the quantum world, but then important differences emerge: now, because fluctuations in the number of photons are unavoidable, one is forced to work in the three-dimensional Poincar{\textbackslash}'e space that can be regarded as a set of nested spheres. Additionally, higher-order moments of the Stokes variables might play a substantial role for quantum states, which is not the case for most classical Gaussian states. This brings about important differences between these two worlds that we review in detail. In particular, the classical degree of polarization produces unsatisfactory results in the quantum domain. We compare alternative quantum degrees and put forth that they order various states differently. Finally, intrinsically nonclassical states are explored and their potential applications in quantum technologies are discussed.}, urldate = {2021-07-28}, journal = {arXiv:2011.03979 [quant-ph]}, author = {Goldberg, Aaron Z. and de la Hoz, Pablo and Bjork, Gunnar and Klimov, Andrei B. and Grassl, Markus and Leuchs, Gerd and Sanchez-Soto, Luis L.}, month = nov, year = {2020}, note = {arXiv: 2011.03979}, keywords = {Quantum Physics}, }`

- B. Ahmadi, S. Salimi, and A. S. Khorashad. No Entropy Production in Quantum Thermodynamics.
*Arxiv:2002.10747 [quant-ph]*, feb 2020. arXiv: 2002.10747

[BibTeX] [Abstract] [Download PDF]

In this work we will show that there exists a fundamental difference between microscopic quantum thermodynamics and macroscopic classical thermodynamics. It will be proved that the entropy production in quantum thermodynamics always vanishes for both closed and open quantum thermodynamic systems. This novel and very surprising result is derived based on the genuine reasoning Clausius used to establish the science of thermodynamics in the first place. This result will interestingly lead to define the generalized temperature for any non-equilibrium quantum system.

`@article{ahmadi_no_2020, title = {No {Entropy} {Production} in {Quantum} {Thermodynamics}}, url = {http://arxiv.org/abs/2002.10747}, abstract = {In this work we will show that there exists a fundamental difference between microscopic quantum thermodynamics and macroscopic classical thermodynamics. It will be proved that the entropy production in quantum thermodynamics always vanishes for both closed and open quantum thermodynamic systems. This novel and very surprising result is derived based on the genuine reasoning Clausius used to establish the science of thermodynamics in the first place. This result will interestingly lead to define the generalized temperature for any non-equilibrium quantum system.}, urldate = {2021-07-28}, journal = {arXiv:2002.10747 [quant-ph]}, author = {Ahmadi, B. and Salimi, S. and Khorashad, A. S.}, month = feb, year = {2020}, note = {arXiv: 2002.10747}, keywords = {Quantum Physics}, }`

- Siddhartha Das, Stefan Bäuml, Marek Winczewski, and Karol Horodecki. Universal limitations on quantum key distribution over a network.
*Arxiv:1912.03646 [quant-ph]*, sep 2020. arXiv: 1912.03646

[BibTeX] [Abstract] [Download PDF]

The possibility to achieve secure communication among trusted parties by means of the quantum entanglement is intriguing both from a fundamental and an application purpose. In this work, we show that any state (after distillation) from which a quantum secret key can be obtained by local measurements has to be genuinely multipartite entangled. We introduce the most general form of memoryless network quantum channel: quantum multiplex channels. We define and determine asymptotic and non-asymptotic LOCC assisted conference key agreement capacities for quantum multiplex channels and provide various strong and weak converse bounds in terms of the divergence based entanglement measures of the quantum multiplex channels. The structure of our protocol manifested by an adaptive strategy of secret key and entanglement (GHZ state) distillation over an arbitrary multiplex quantum channel is generic. In particular, it provides a universal framework to study the performance of quantum key repeaters and – for the first time – of the MDI-QKD setups of channels. For teleportation-covariant multiplex quantum channels, which are channels with certain symmetries, we get upper bounds on the secret key agreement capacities in terms of the entanglement measures of their Choi states. For some network prototypes of practical relevance, we evaluate upper bounds on the conference key agreement capacities and MDI-QKD capacities. Upper bounds on the LOCC-assisted conference key agreement rates are also upper bounds on the distillation rates of GHZ states, a class of genuinely entangled pure states. We also obtain bounds on the rates at which conference key and GHZ states can be distilled from a finite number of copies of an arbitrary multipartite quantum state. Using our bounds, in particular cases, we are able to determine the capacities for quantum key distribution channels and rates of GHZ-state distillation.

`@article{das_universal_2020, title = {Universal limitations on quantum key distribution over a network}, url = {http://arxiv.org/abs/1912.03646}, abstract = {The possibility to achieve secure communication among trusted parties by means of the quantum entanglement is intriguing both from a fundamental and an application purpose. In this work, we show that any state (after distillation) from which a quantum secret key can be obtained by local measurements has to be genuinely multipartite entangled. We introduce the most general form of memoryless network quantum channel: quantum multiplex channels. We define and determine asymptotic and non-asymptotic LOCC assisted conference key agreement capacities for quantum multiplex channels and provide various strong and weak converse bounds in terms of the divergence based entanglement measures of the quantum multiplex channels. The structure of our protocol manifested by an adaptive strategy of secret key and entanglement (GHZ state) distillation over an arbitrary multiplex quantum channel is generic. In particular, it provides a universal framework to study the performance of quantum key repeaters and - for the first time - of the MDI-QKD setups of channels. For teleportation-covariant multiplex quantum channels, which are channels with certain symmetries, we get upper bounds on the secret key agreement capacities in terms of the entanglement measures of their Choi states. For some network prototypes of practical relevance, we evaluate upper bounds on the conference key agreement capacities and MDI-QKD capacities. Upper bounds on the LOCC-assisted conference key agreement rates are also upper bounds on the distillation rates of GHZ states, a class of genuinely entangled pure states. We also obtain bounds on the rates at which conference key and GHZ states can be distilled from a finite number of copies of an arbitrary multipartite quantum state. Using our bounds, in particular cases, we are able to determine the capacities for quantum key distribution channels and rates of GHZ-state distillation.}, urldate = {2021-07-28}, journal = {arXiv:1912.03646 [quant-ph]}, author = {Das, Siddhartha and Bäuml, Stefan and Winczewski, Marek and Horodecki, Karol}, month = sep, year = {2020}, note = {arXiv: 1912.03646}, keywords = {Quantum Physics, Computer Science - Information Theory}, }`

- Wooyeong Song, Marcin Wieśniak, Nana Liu, Marcin Pawłowski, Jinhyoung Lee, Jaewan Kim, and Jeongho Bang. Tangible Reduction of Sample Complexity with Large Classical Samples and Small Quantum System.
*Arxiv:1905.05751 [quant-ph]*, jun 2020. arXiv: 1905.05751

[BibTeX] [Abstract] [Download PDF]

Quantum computation requires large classical datasets to be embedded into quantum states in order to exploit quantum parallelism. However, this embedding requires considerable resources. It would therefore be desirable to avoid it, if possible, for noisy intermediate-scale quantum (NISQ) implementation. Accordingly, we consider a classical-quantum hybrid architecture, which allows large classical input data, with a relatively small-scale quantum system. This hybrid architecture is used to implement an oracle. It is shown that in the presence of noise in the hybrid oracle, the effects of internal noise can cancel each other out and thereby improve the query success rate. It is also shown that such an immunity of the hybrid oracle to noise directly and tangibly reduces the sample complexity in the probably-approximately-correct learning framework. This NISQ-compatible learning advantage is attributed to the oracle’s ability to handle large input features.

`@article{song_tangible_2020, title = {Tangible {Reduction} of {Sample} {Complexity} with {Large} {Classical} {Samples} and {Small} {Quantum} {System}}, url = {http://arxiv.org/abs/1905.05751}, abstract = {Quantum computation requires large classical datasets to be embedded into quantum states in order to exploit quantum parallelism. However, this embedding requires considerable resources. It would therefore be desirable to avoid it, if possible, for noisy intermediate-scale quantum (NISQ) implementation. Accordingly, we consider a classical-quantum hybrid architecture, which allows large classical input data, with a relatively small-scale quantum system. This hybrid architecture is used to implement an oracle. It is shown that in the presence of noise in the hybrid oracle, the effects of internal noise can cancel each other out and thereby improve the query success rate. It is also shown that such an immunity of the hybrid oracle to noise directly and tangibly reduces the sample complexity in the probably-approximately-correct learning framework. This NISQ-compatible learning advantage is attributed to the oracle's ability to handle large input features.}, urldate = {2021-07-28}, journal = {arXiv:1905.05751 [quant-ph]}, author = {Song, Wooyeong and Wieśniak, Marcin and Liu, Nana and Pawłowski, Marcin and Lee, Jinhyoung and Kim, Jaewan and Bang, Jeongho}, month = jun, year = {2020}, note = {arXiv: 1905.05751}, keywords = {Quantum Physics}, }`

- Thais L. Silva, Łukasz Rudnicki, Daniel S. Tasca, and Stephen P. Walborn. Periodic discretized continuous observables are neither continuous nor discrete.
*Arxiv:2009.05062 [quant-ph]*, sep 2020.

[BibTeX] [Abstract] [Download PDF]

Most of the fundamental characteristics of quantum mechanics, such as non-locality and contextuality, are manifest in discrete, finite-dimensional systems. However, many quantum information tasks that exploit these properties cannot be directly adapted to continuous-variable systems. To access these quantum features, continuous quantum variables can be made discrete by binning together their different values, resulting in observables with a finite number “$d$” of outcomes. While direct measurement indeed confirms their manifestly discrete character, here we employ a salient feature of quantum physics known as mutual unbiasedness to show that such coarse-grained observables are in a sense neither continuous nor discrete. Depending on $d$, the observables can reproduce either the discrete or the continuous behavior, or neither. To illustrate these results, we present an example for the construction of such measurements and employ it in an optical experiment confirming the existence of four mutually unbiased measurements with $d = 3$ outcomes in a continuous variable system, surpassing the number of mutually unbiased continuous variable observables.

`@Article{Silva2020, author = {Thais L. Silva and Łukasz Rudnicki and Daniel S. Tasca and Stephen P. Walborn}, journal = {arXiv:2009.05062 [quant-ph]}, title = {Periodic discretized continuous observables are neither continuous nor discrete}, year = {2020}, month = sep, abstract = {Most of the fundamental characteristics of quantum mechanics, such as non-locality and contextuality, are manifest in discrete, finite-dimensional systems. However, many quantum information tasks that exploit these properties cannot be directly adapted to continuous-variable systems. To access these quantum features, continuous quantum variables can be made discrete by binning together their different values, resulting in observables with a finite number "$d$" of outcomes. While direct measurement indeed confirms their manifestly discrete character, here we employ a salient feature of quantum physics known as mutual unbiasedness to show that such coarse-grained observables are in a sense neither continuous nor discrete. Depending on $d$, the observables can reproduce either the discrete or the continuous behavior, or neither. To illustrate these results, we present an example for the construction of such measurements and employ it in an optical experiment confirming the existence of four mutually unbiased measurements with $d = 3$ outcomes in a continuous variable system, surpassing the number of mutually unbiased continuous variable observables.}, archiveprefix = {arXiv}, eprint = {2009.05062}, file = {:Silva2020 - Periodic Discretized Continuous Observables Are Neither Continuous nor Discrete.pdf:PDF}, keywords = {quant-ph}, primaryclass = {quant-ph}, url = {https://arxiv.org/pdf/2009.05062.pdf}, }`

- Gniewomir Sarbicki, Giovanni Scala, and Dariusz Chru{‘s}ciński. Detection power of separability criteria based on a correlation tensor: a case study.
*Arxiv e-prints*, pages arXiv:2012.04359, dec 2020.

[BibTeX] [Abstract] [Download PDF]

Detection power of separability criteria based on a correlation tensor is tested within a family of generalized isotropic state in $d_1 \otimes d_2$. For $d_1 \neq d_2$ all these criteria are weaker than positive partial transposition (PPT) criterion. Interestingly, our analysis supports the recent conjecture that a criterion based on symmetrically informationaly complete positive operator-valued measure (SIC-POVMs) is stronger than realignment criterion.

`@Article{Sarbicki2020, author = {Sarbicki, Gniewomir and Scala, Giovanni and Chru{\'s}ci{\'n}ski, Dariusz}, journal = {arXiv e-prints}, title = {Detection power of separability criteria based on a correlation tensor: a case study}, year = {2020}, month = dec, pages = {arXiv:2012.04359}, abstract = {Detection power of separability criteria based on a correlation tensor is tested within a family of generalized isotropic state in $d_1 \otimes d_2$. For $d_1 \neq d_2$ all these criteria are weaker than positive partial transposition (PPT) criterion. Interestingly, our analysis supports the recent conjecture that a criterion based on symmetrically informationaly complete positive operator-valued measure (SIC-POVMs) is stronger than realignment criterion.}, archiveprefix = {arXiv}, eid = {arXiv:2012.04359}, eprint = {2012.04359}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2020arXiv201204359S}, }`

### 2019

- Michał Eckstein and Paweł Horodecki. The experiment paradox in physics.
*Arxiv:1904.04117 [gr-qc, physics:hep-th, physics:physics, physics:quant-ph]*, apr 2019. arXiv: 1904.04117

[BibTeX] [Abstract] [Download PDF]

Modern physics is founded on two mainstays: mathematical modelling and empirical verification. These two assumptions are prerequisite for the objectivity of scientific discourse. Here we show, however, that they are contradictory, leading to the `experiment paradox’. We reveal that any experiment performed on a physical system is – by necessity – invasive and thus establishes inevitable limits to the accuracy of any mathematical model. We track its manifestations in both classical and quantum physics and show how it is overcome `in practice’ via the concept of environment. We argue that the scientific pragmatism ordains two methodological principles of compressibility and stability.

`@Article{eckstein_experiment_2019, author = {Eckstein, Michał and Horodecki, Paweł}, journal = {arXiv:1904.04117 [gr-qc, physics:hep-th, physics:physics, physics:quant-ph]}, title = {The experiment paradox in physics}, year = {2019}, month = apr, note = {arXiv: 1904.04117}, abstract = {Modern physics is founded on two mainstays: mathematical modelling and empirical verification. These two assumptions are prerequisite for the objectivity of scientific discourse. Here we show, however, that they are contradictory, leading to the `experiment paradox'. We reveal that any experiment performed on a physical system is - by necessity - invasive and thus establishes inevitable limits to the accuracy of any mathematical model. We track its manifestations in both classical and quantum physics and show how it is overcome `in practice' via the concept of environment. We argue that the scientific pragmatism ordains two methodological principles of compressibility and stability.}, groups = {Pawel_H}, keywords = {Physics - History and Philosophy of Physics, General Relativity and Quantum Cosmology, High Energy Physics - Theory, Physics - Classical Physics, Quantum Physics}, url = {http://arxiv.org/abs/1904.04117}, urldate = {2021-07-28}, }`

- Sumit Rout, Ananda G. Maity, Amit Mukherjee, Saronath Halder, and Manik Banik. Multiparty orthogonal product states with minimal genuine nonlocality.
*Arxiv:1910.14308 [quant-ph]*, oct 2019.

[BibTeX] [Abstract] [Download PDF]

Nonlocality without entanglement and its subsequent generalizations offer deep information-theoretic insights and subsequently find several useful applications. Concept of genuinely nonlocal set of product states emerges as a natural multipartite generalization of this phenomenon. Existence of such sets eventually motivates the problem concerning their entanglement-assisted discrimination. Here, we construct examples of genuinely nonlocal product states for arbitrary number of parties. Strength of genuine nonlocality of these sets can be considered minimal as their perfect discrimination is possible with entangled resources residing in Hilbert spaces having the smallest possible dimensions. Our constructions lead to fully separable measurements that are impossible to implement even if all but one party come together. Furthermore, they also provide the opportunity to compare different multipartite states that otherwise are incomparable under single copy local manipulation.

`@Article{Rout2019, author = {Sumit Rout and Ananda G. Maity and Amit Mukherjee and Saronath Halder and Manik Banik}, journal = {arXiv:1910.14308 [quant-ph]}, title = {Multiparty orthogonal product states with minimal genuine nonlocality}, year = {2019}, month = oct, abstract = {Nonlocality without entanglement and its subsequent generalizations offer deep information-theoretic insights and subsequently find several useful applications. Concept of genuinely nonlocal set of product states emerges as a natural multipartite generalization of this phenomenon. Existence of such sets eventually motivates the problem concerning their entanglement-assisted discrimination. Here, we construct examples of genuinely nonlocal product states for arbitrary number of parties. Strength of genuine nonlocality of these sets can be considered minimal as their perfect discrimination is possible with entangled resources residing in Hilbert spaces having the smallest possible dimensions. Our constructions lead to fully separable measurements that are impossible to implement even if all but one party come together. Furthermore, they also provide the opportunity to compare different multipartite states that otherwise are incomparable under single copy local manipulation.}, archiveprefix = {arXiv}, eprint = {1910.14308}, keywords = {quant-ph}, primaryclass = {quant-ph}, url = {https://arxiv.org/pdf/1910.14308}, }`