## Peer-reviewed publications

### 2021

- T. Miller, M. Eckstein, P. Horodecki, and R. Horodecki, “Generally covariant N -particle dynamics,” Journal of geometry and physics, vol. 160, p. 103990, 2021. doi:10.1016/j.geomphys.2020.103990

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

- B. Ahmadi, S. Salimi, and A. S. Khorashad, “Irreversible work and Maxwell demon in terms of quantum thermodynamic force,” Scientific reports, vol. 11, iss. 1, p. 2301, 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, title = {Irreversible work and {Maxwell} demon in terms of quantum thermodynamic force}, volume = {11}, issn = {2045-2322}, url = {http://www.nature.com/articles/s41598-021-81737-z}, doi = {10.1038/s41598-021-81737-z}, 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.}, language = {en}, number = {1}, urldate = {2021-05-10}, journal = {Scientific Reports}, author = {Ahmadi, B. and Salimi, S. and Khorashad, A. S.}, month = dec, year = {2021}, pages = {2301}, }`

- Q. Guo, Y. Zhao, M. Grassl, X. Nie, G. Xiang, T. Xin, Z. Yin, and B. Zeng, “Testing a quantum error-correcting code on various platforms,” Science bulletin, vol. 66, iss. 1, p. 29–35, 2021. doi:10.1016/j.scib.2020.07.033

[BibTeX] [Download PDF]`@article{guo_testing_2021, title = {Testing a quantum error-correcting code on various platforms}, volume = {66}, issn = {20959273}, url = {https://linkinghub.elsevier.com/retrieve/pii/S2095927320305168}, doi = {10.1016/j.scib.2020.07.033}, language = {en}, number = {1}, urldate = {2021-05-10}, journal = {Science Bulletin}, 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}, month = jan, year = {2021}, pages = {29--35}, }`

- P. Mironowicz, G. Cañas, J. Cariñe, E. S. Gómez, J. F. Barra, A. Cabello, G. B. Xavier, G. Lima, and M. Pawłowski, “Quantum randomness protected against detection loophole attacks,” Quantum information processing, vol. 20, iss. 1, p. 39, 2021. doi:10.1007/s11128-020-02948-3

[BibTeX] [Download PDF]`@article{mironowicz_quantum_2021, title = {Quantum randomness protected against detection loophole attacks}, volume = {20}, issn = {1570-0755, 1573-1332}, url = {http://link.springer.com/10.1007/s11128-020-02948-3}, doi = {10.1007/s11128-020-02948-3}, language = {en}, number = {1}, urldate = {2021-05-10}, journal = {Quantum Information Processing}, 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}, month = jan, year = {2021}, pages = {39}, }`

- M. Christandl, R. Ferrara, and K. Horodecki, “Upper Bounds on Device-Independent Quantum Key Distribution,” Physical review letters, vol. 126, iss. 16, p. 160501, 2021. doi:10.1103/PhysRevLett.126.160501

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

- R. Alicki, D. Gelbwaser-Klimovsky, A. Jenkins, and E. von Hauff, “Dynamical theory for the battery’s electromotive force,” Physical chemistry chemical physics, vol. 23, iss. 15, p. 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, title = {Dynamical theory for the battery's electromotive force}, volume = {23}, issn = {1463-9076, 1463-9084}, url = {http://xlink.rsc.org/?DOI=D1CP00196E}, doi = {10.1039/D1CP00196E}, 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.}, language = {en}, number = {15}, urldate = {2021-05-10}, journal = {Physical Chemistry Chemical Physics}, author = {Alicki, Robert and Gelbwaser-Klimovsky, David and Jenkins, Alejandro and von Hauff, Elizabeth}, year = {2021}, pages = {9428--9439}, }`

- M. Markiewicz and J. Przewocki, “On construction of finite averaging sets for \textitSL (2, C) via its Cartan decomposition,” Journal of physics a: mathematical and theoretical, 2021. doi:10.1088/1751-8121/abfa44

[BibTeX] [Download PDF]`@article{markiewicz_construction_2021, title = {On construction of finite averaging sets for \textit{{SL}} (2, {C}) via its {Cartan} decomposition}, issn = {1751-8113, 1751-8121}, url = {https://iopscience.iop.org/article/10.1088/1751-8121/abfa44}, doi = {10.1088/1751-8121/abfa44}, urldate = {2021-05-10}, journal = {Journal of Physics A: Mathematical and Theoretical}, author = {Markiewicz, Marcin and Przewocki, Janusz}, month = apr, year = {2021}, }`

- M. Żukowski and M. Markiewicz, “Physics and Metaphysics of Wigner’s Friends: Even Performed Premeasurements Have No Results,” Physical review letters, vol. 126, iss. 13, p. 130402, 2021. doi:10.1103/PhysRevLett.126.130402

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

- J. H. Selby, C. M. Scandolo, and B. Coecke, “Reconstructing quantum theory from diagrammatic postulates,” Quantum, vol. 5, p. 445, 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 U. A. R. Devi, “Leggett-Garg inequalities and temporal correlations for a qubit under PT -symmetric dynamics,” Physical review a, vol. 103, iss. 3, p. 32420, 2021. doi:10.1103/PhysRevA.103.032420

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

- N. Miklin and M. Oszmaniec, “A universal scheme for robust self-testing in the prepare-and-measure scenario,” Quantum, vol. 5, p. 424, 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}, }`

- D. Schmid, J. H. Selby, E. Wolfe, R. Kunjwal, and R. W. Spekkens, “Characterization of Noncontextuality in the Framework of Generalized Probabilistic Theories,” Prx quantum, vol. 2, iss. 1, p. 10331, 2021. doi:10.1103/PRXQuantum.2.010331

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

- P. Lipka-Bartosik, P. Mazurek, and M. Horodecki, “Second law of thermodynamics for batteries with vacuum state,” Quantum, vol. 5, p. 408, 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, title = {Second law of thermodynamics for batteries with vacuum state}, volume = {5}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2021-03-10-408/}, doi = {10.22331/q-2021-03-10-408}, 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.}, language = {en}, urldate = {2021-05-10}, journal = {Quantum}, author = {Lipka-Bartosik, Patryk and Mazurek, Paweł and Horodecki, Michał}, month = mar, year = {2021}, pages = {408}, }`

- A. Z. Goldberg, P. de la Hoz, G. Björk, A. B. Klimov, M. Grassl, G. Leuchs, and L. L. Sánchez-Soto, “Quantum concepts in optical polarization,” Advances in optics and photonics, vol. 13, iss. 1, p. 1, 2021. doi:10.1364/AOP.404175

[BibTeX] [Download PDF]`@article{goldberg_quantum_2021, title = {Quantum concepts in optical polarization}, volume = {13}, issn = {1943-8206}, url = {https://www.osapublishing.org/abstract.cfm?URI=aop-13-1-1}, doi = {10.1364/AOP.404175}, language = {en}, number = {1}, urldate = {2021-05-10}, journal = {Advances in Optics and Photonics}, 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.}, month = mar, year = {2021}, pages = {1}, }`

- R. Uola, T. Kraft, S. Designolle, N. Miklin, A. Tavakoli, J. Pellonpää, O. Gühne, and N. Brunner, “Quantum measurement incompatibility in subspaces,” Physical review a, vol. 103, iss. 2, p. 22203, 2021. doi:10.1103/PhysRevA.103.022203

[BibTeX] [Download PDF]`@article{uola_quantum_2021, title = {Quantum measurement incompatibility in subspaces}, volume = {103}, issn = {2469-9926, 2469-9934}, url = {https://link.aps.org/doi/10.1103/PhysRevA.103.022203}, doi = {10.1103/PhysRevA.103.022203}, language = {en}, number = {2}, urldate = {2021-05-10}, journal = {Physical Review A}, 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}, month = feb, year = {2021}, pages = {022203}, }`

- A. Tavakoli, Máté. Farkas, D. Rosset, J. Bancal, and J. Kaniewski, “Mutually unbiased bases and symmetric informationally complete measurements in Bell experiments,” Science advances, vol. 7, iss. 7, p. eabc3847, 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 Advances}, 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, N. Guerrero, J. Cariñe, G. Cañas, and G. Lima, “Self-Testing Mutually Unbiased Bases in Higher Dimensions with Space-Division Multiplexing Optical Fiber Technology,” Physical review applied, vol. 15, iss. 1, p. 14028, 2021. doi:10.1103/PhysRevApplied.15.014028

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

- K. Schlichtholz, B. Woloncewicz, and M. Żukowski, “Nonclassicality of bright Greenberger-Horne-Zeilinger–like radiation of an optical parametric source,” Physical review a, vol. 103, iss. 4, p. 42226, 2021. doi:10.1103/PhysRevA.103.042226

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

- R. Salazar, T. Biswas, J. Czartowski, K. Życzkowski, and P. Horodecki, “Optimal allocation of quantum resources,” Quantum, vol. 5, p. 407, 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, title = {Optimal allocation of quantum resources}, volume = {5}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2021-03-10-407/}, doi = {10.22331/q-2021-03-10-407}, 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.}, language = {en}, urldate = {2021-07-28}, journal = {Quantum}, author = {Salazar, Roberto and Biswas, Tanmoy and Czartowski, Jakub and Życzkowski, Karol and Horodecki, Paweł}, month = mar, year = {2021}, pages = {407}, }`

- C. Datta, T. Biswas, D. Saha, and R. Augusiak, “Perfect discrimination of quantum measurements using entangled systems,” New journal of physics, vol. 23, iss. 4, p. 43021, 2021. doi:10.1088/1367-2630/abecaf

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

- R. Horodecki, “Quantum Information,” Acta physica polonica a, vol. 139, iss. 3, p. 197–2018, 2021. doi:10.12693/APhysPolA.139.197

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

- T. Miller, M. Eckstein, P. Horodecki, and R. Horodecki, “Generally covariant N -particle dynamics,” Journal of geometry and physics, vol. 160, p. 103990, 2021. doi:10.1016/j.geomphys.2020.103990

[BibTeX] [Download PDF]`@article{miller_generally_2021-1, title = {Generally covariant {N} -particle dynamics}, volume = {160}, issn = {03930440}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0393044020302539}, doi = {10.1016/j.geomphys.2020.103990}, language = {en}, urldate = {2021-07-28}, journal = {Journal of Geometry and Physics}, author = {Miller, Tomasz and Eckstein, Michał and Horodecki, Paweł and Horodecki, Ryszard}, month = feb, year = {2021}, pages = {103990}, }`

- M. Wieśniak, “Symmetrized persistency of Bell correlations for Dicke states and GHZ-based mixtures,” Scientific reports, vol. 11, iss. 1, p. 14333, 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, title = {Symmetrized persistency of {Bell} correlations for {Dicke} states and {GHZ}-based mixtures}, volume = {11}, issn = {2045-2322}, url = {http://www.nature.com/articles/s41598-021-93786-5}, doi = {10.1038/s41598-021-93786-5}, 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.}, language = {en}, number = {1}, urldate = {2021-07-28}, journal = {Scientific Reports}, author = {Wieśniak, Marcin}, month = dec, year = {2021}, pages = {14333}, }`

- K. Anjali, A. S. Hejamadi, H. S. Karthik, S. Sahu, Sudha, and U. A. R. Devi, “Characterizing nonlocality of pure symmetric three-qubit states,” Quantum information processing, vol. 20, iss. 5, p. 187, 2021. doi:10.1007/s11128-021-03124-x

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

- M. Banacki, R. R. Rodríguez, and P. Horodecki, “On the edge of the set of no-signaling assemblages,” Physical review a, vol. 103, iss. 5, p. 52434, 2021. 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, title = {On the edge of the set of no-signaling assemblages}, volume = {103}, issn = {2469-9926, 2469-9934}, url = {http://arxiv.org/abs/2008.12325}, doi = {10.1103/PhysRevA.103.052434}, 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.}, number = {5}, urldate = {2021-07-28}, journal = {Physical Review A}, author = {Banacki, Michał and Rodríguez, Ricard Ravell and Horodecki, Paweł}, month = may, year = {2021}, note = {arXiv: 2008.12325}, keywords = {Quantum Physics}, pages = {052434}, }`

- S. Cusumano and Ł. Rudnicki, “Comment on “Fluctuations in Extractable Work Bound the Charging Power of Quantum Batteries”,” Physical review letters, vol. 127, iss. 2, p. 28901, 2021. 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, title = {Comment on "{Fluctuations} in {Extractable} {Work} {Bound} the {Charging} {Power} of {Quantum} {Batteries}"}, volume = {127}, issn = {0031-9007, 1079-7114}, url = {http://arxiv.org/abs/2102.05627}, doi = {10.1103/PhysRevLett.127.028901}, 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.}, number = {2}, urldate = {2021-07-28}, journal = {Physical Review Letters}, author = {Cusumano, Stefano and Rudnicki, Łukasz}, month = jul, year = {2021}, note = {arXiv: 2102.05627}, keywords = {Quantum Physics}, pages = {028901}, }`

- N. Miklin and M. Pawłowski, “Information Causality without concatenation,” Physical review letters, vol. 126, iss. 22, p. 220403, 2021. 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, title = {Information {Causality} without concatenation}, volume = {126}, issn = {0031-9007, 1079-7114}, url = {http://arxiv.org/abs/2101.12710}, doi = {10.1103/PhysRevLett.126.220403}, 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.}, number = {22}, urldate = {2021-07-28}, journal = {Physical Review Letters}, author = {Miklin, Nikolai and Pawłowski, Marcin}, month = jun, year = {2021}, note = {arXiv: 2101.12710}, keywords = {Quantum Physics}, pages = {220403}, }`

- S. Chen, N. Miklin, C. Budroni, and Y. Chen, “Device-independent quantification of measurement incompatibility,” Physical review research, vol. 3, iss. 2, p. 23143, 2021. 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, title = {Device-independent quantification of measurement incompatibility}, volume = {3}, issn = {2643-1564}, url = {http://arxiv.org/abs/2010.08456}, doi = {10.1103/PhysRevResearch.3.023143}, 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.}, number = {2}, urldate = {2021-07-28}, journal = {Physical Review Research}, author = {Chen, Shin-Liang and Miklin, Nikolai and Budroni, Costantino and Chen, Yueh-Nan}, month = may, year = {2021}, note = {arXiv: 2010.08456}, keywords = {Quantum Physics}, pages = {023143}, }`

- M. Grassl, “Entanglement-Assisted Quantum Communication Beating the Quantum Singleton Bound,” Physical review a, vol. 103, iss. 2, p. L020601, 2021. 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, title = {Entanglement-{Assisted} {Quantum} {Communication} {Beating} the {Quantum} {Singleton} {Bound}}, volume = {103}, issn = {2469-9926, 2469-9934}, url = {http://arxiv.org/abs/2007.01249}, doi = {10.1103/PhysRevA.103.L020601}, 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.}, number = {2}, urldate = {2021-07-28}, journal = {Physical Review A}, author = {Grassl, Markus}, month = feb, year = {2021}, note = {arXiv: 2007.01249}, keywords = {Quantum Physics, Computer Science - Information Theory}, pages = {L020601}, }`

- W. Song, Y. Lim, H. Kwon, G. Adesso, M. Wieśniak, M. Pawłowski, J. Kim, and J. Bang, “Quantum secure learning with classical samples,” Physical review a, vol. 103, iss. 4, p. 42409, 2021. 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, title = {Quantum secure learning with classical samples}, volume = {103}, issn = {2469-9926, 2469-9934}, url = {http://arxiv.org/abs/1912.10594}, doi = {10.1103/PhysRevA.103.042409}, 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.}, number = {4}, urldate = {2021-07-28}, journal = {Physical Review A}, author = {Song, Wooyeong and Lim, Youngrong and Kwon, Hyukjoon and Adesso, Gerardo and Wieśniak, Marcin and Pawłowski, Marcin and Kim, Jaewan and Bang, Jeongho}, month = apr, year = {2021}, note = {arXiv: 1912.10594}, keywords = {Quantum Physics}, pages = {042409}, }`

- P. Lipka-Bartosik, P. Mazurek, and M. Horodecki, “Second law of thermodynamics for batteries with vacuum state,” Quantum, vol. 5, p. 408, 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_2021-1, title = {Second law of thermodynamics for batteries with vacuum state}, volume = {5}, issn = {2521-327X}, url = {http://arxiv.org/abs/1905.12072}, doi = {10.22331/q-2021-03-10-408}, 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.}, urldate = {2021-07-28}, journal = {Quantum}, author = {Lipka-Bartosik, Patryk and Mazurek, Paweł and Horodecki, Michał}, month = mar, year = {2021}, note = {arXiv: 1905.12072}, keywords = {Quantum Physics}, pages = {408}, }`

- M. Markiewicz, M. Karczewski, and P. Kurzynski, “Borromean states in discrete-time quantum walks,” Quantum, vol. 5, p. 523, 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}, }`

- R. Alicki, D. Gelbwaser-Klimovsky, and A. Jenkins, “The problem of engines in statistical physics,” Entropy, vol. 23, iss. 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}, }`

- M. Stankiewicz, K. Horodecki, O. Sakarya, and D. Makowiec, “Private weakly-random sequences from human heart rate for quantum amplification,” Entropy, vol. 23, iss. 9, p. 1182, 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 Weakly-Random Sequences from Human Heart Rate for Quantum Amplification}, 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}, }`

- M. Wie{‘s}niak, “Symmetrized persistency of bell correlations for dicke states and ghz-based mixtures,” Scientific reports, vol. 11, p. 14333, 2021. doi:10.1038/s41598-021-93786-5

[BibTeX] [Abstract] [Download PDF]

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 0.482N, significantly more than reported in previous studies.

`@Article{Wiesniak2021, author = {Wie{\'s}niak, Marcin}, journal = {Scientific Reports}, title = {Symmetrized persistency of Bell correlations for Dicke states and GHZ-based mixtures}, year = {2021}, month = jan, pages = {14333}, volume = {11}, 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 0.482N, significantly more than reported in previous studies.}, archiveprefix = {arXiv}, doi = {10.1038/s41598-021-93786-5}, eid = {14333}, eprint = {2102.08141}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021NatSR..1114333W}, }`

- A. Chaturvedi, Máté. Farkas, and V. J. Wright, “Characterising and bounding the set of quantum behaviours in contextuality scenarios,” Quantum, vol. 5, p. 484, 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}, publisher = {Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften}, url = {https://quantum-journal.org/papers/q-2021-06-29-484/pdf/}, }`

- S. Cusumano and Ł. Rudnicki, “Thermodynamics of reduced state of the field,” , vol. 23, p. 1198, 2021. doi:10.3390/e23091198

[BibTeX]`@Article{Cusumano, author = {Stefano Cusumano and Łukasz Rudnicki}, title = {Thermodynamics of Reduced State of the Field}, year = {2021}, issn = {1099-4300}, pages = {1198}, volume = {23}, doi = {10.3390/e23091198}, }`

- Ł. Rudnicki, “Quantum speed limit and geometric measure of entanglement,” , vol. 104, 2021. doi:10.1103/physreva.104.032417

[BibTeX]`@Article{Rudnicki, author = {Łukasz Rudnicki}, title = {Quantum speed limit and geometric measure of entanglement}, year = {2021}, issn = {2469-9926}, volume = {104}, doi = {10.1103/physreva.104.032417}, }`

- A. Barasiński, A. Černoch, W. Laskowski, K. Lemr, T. Vértesi, and J. Soubusta, “Experimentally friendly approach towards nonlocal correlations in multisetting N-partite Bell scenarios,” Quantum, vol. 5, p. 430, 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}, }`

- P. Blasiak, E. Borsuk, M. Markiewicz, and Y. Kim, “Efficient linear-optical generation of a multipartite W state,” Physical review a, vol. 104, iss. 2, p. 23701, 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 Review 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}, }`

- E. Aurell, M. Eckstein, and P. Horodecki, “Quantum Black Holes as Solvents,” Foundations of physics, vol. 51, iss. 2, p. 54, 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 Physics}, 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}, language = {en}, url = {https://link.springer.com/10.1007/s10701-021-00456-7}, urldate = {2021-10-11}, }`

- C. M. Scandolo, R. Salazar, J. K. Korbicz, and P. Horodecki, “Universal structure of objective states in all fundamental causal theories,” , vol. 3, 2021. doi:10.1103/physrevresearch.3.033148

[BibTeX]`@Article{Scandolo, author = {Carlo Maria Scandolo and Roberto Salazar and Jarosław K. Korbicz and Paweł Horodecki}, title = {Universal structure of objective states in all fundamental causal theories}, year = {2021}, issn = {2643-1564}, volume = {3}, doi = {10.1103/physrevresearch.3.033148}, }`

- M. Markiewicz and J. Przewocki, “On construction of finite averaging sets for sl(2,c) via its cartan decomposition,” , vol. 54, p. 235302, 2021. doi:10.1088/1751-8121/abfa44

[BibTeX]`@Article{Markiewicz2021, author = {Marcin Markiewicz and Janusz Przewocki}, title = {On construction of finite averaging sets for SL(2,C) via its Cartan decomposition}, year = {2021}, issn = {1751-8113}, pages = {235302}, volume = {54}, doi = {10.1088/1751-8121/abfa44}, }`

### 2020

- P. Mazurek, Máté. Farkas, A. Grudka, M. Horodecki, and M. Studziński, “Quantum error-correction codes and absolutely maximally entangled states,” Physical review a, vol. 101, iss. 4, p. 42305, 2020. doi:10.1103/PhysRevA.101.042305

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

- K. Horodecki and M. Stankiewicz, “Semi-device-independent quantum money,” New journal of physics, vol. 22, iss. 2, p. 23007, 2020. doi:10.1088/1367-2630/ab6872

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

- T. Linowski, G. Rajchel-Mieldzioć, and K. Życzkowski, “Entangling power of multipartite unitary gates,” Journal of physics a: mathematical and theoretical, vol. 53, iss. 12, p. 125303, 2020. doi:10.1088/1751-8121/ab749a

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

- M. Pawłowski, “Entropy in Foundations of Quantum Physics,” Entropy, vol. 22, iss. 3, p. 371, 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}, }`

- M. Smania, P. Mironowicz, M. Nawareg, M. Pawłowski, A. Cabello, and M. Bourennane, “Experimental certification of an informationally complete quantum measurement in a device-independent protocol,” Optica, vol. 7, iss. 2, p. 123, 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}, }`

- S. Milz, F. Sakuldee, F. A. Pollock, and K. Modi, “Kolmogorov extension theorem for (quantum) causal modelling and general probabilistic theories,” Quantum, vol. 4, p. 255, 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}, }`

- K. Szczygielski and R. Alicki, “On Howland time-independent formulation of CP-divisible quantum evolutions,” Reviews in mathematical physics, p. 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, title = {On {Howland} time-independent formulation of {CP}-divisible quantum evolutions}, issn = {0129-055X, 1793-6659}, url = {https://www.worldscientific.com/doi/abs/10.1142/S0129055X2050021X}, doi = {10.1142/S0129055X2050021X}, 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.}, language = {en}, urldate = {2020-05-13}, journal = {Reviews in Mathematical Physics}, author = {Szczygielski, Krzysztof and Alicki, Robert}, month = jan, year = {2020}, pages = {2050021}, }`

- M. Rosicka, P. Mazurek, A. Grudka, and M. Horodecki, “Generalized XOR non-locality games with graph description on a square lattice,” Journal of physics a: mathematical and theoretical, vol. 53, iss. 26, p. 265302, 2020. doi:10.1088/1751-8121/ab8f3e

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

- F. Huber and M. Grassl, “Quantum Codes of Maximal Distance and Highly Entangled Subspaces,” Quantum, vol. 4, p. 284, 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}, }`

- P. Skrzypczyk, M. J. Hoban, A. B. Sainz, and N. Linden, “Complexity of compatible measurements,” Physical review research, vol. 2, iss. 2, p. 23292, 2020. doi:10.1103/PhysRevResearch.2.023292

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

- A. Tavakoli, M. Żukowski, and Č. Brukner, “Does violation of a Bell inequality always imply quantum advantage in a communication complexity problem?,” Quantum, vol. 4, p. 316, 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, vol. 45, iss. 16, p. 4607, 2020. doi:10.1364/OL.392955

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

- A. B. Sainz, M. J. Hoban, P. Skrzypczyk, and L. Aolita, “Bipartite Postquantum Steering in Generalized Scenarios,” Physical review letters, vol. 125, iss. 5, p. 50404, 2020. doi:10.1103/PhysRevLett.125.050404

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

- S. Popescu, A. B. Sainz, A. J. Short, and A. Winter, “Reference Frames Which Separately Store Noncommuting Conserved Quantities,” Physical review letters, vol. 125, iss. 9, p. 90601, 2020. doi:10.1103/PhysRevLett.125.090601

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

- D. Saha, M. Oszmaniec, L. Czekaj, M. Horodecki, and R. Horodecki, “Operational foundations for complementarity and uncertainty relations,” Physical review a, vol. 101, iss. 5, p. 52104, 2020. doi:10.1103/PhysRevA.101.052104

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

- G. Tóth, T. Vértesi, P. Horodecki, and R. Horodecki, “Activating Hidden Metrological Usefulness,” Physical review letters, vol. 125, iss. 2, p. 20402, 2020. doi:10.1103/PhysRevLett.125.020402

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

- K. Horodecki, R. P. Kostecki, R. Salazar, and M. Studziński, “Limitations for private randomness repeaters,” Physical review a, vol. 102, iss. 1, p. 12615, 2020. doi:10.1103/PhysRevA.102.012615

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

- A. de Rosier, J. Gruca, F. Parisio, T. Vértesi, and W. Laskowski, “Strength and typicality of nonlocality in multisetting and multipartite Bell scenarios,” Physical review a, vol. 101, iss. 1, p. 12116, 2020. doi:10.1103/PhysRevA.101.012116

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

- L. Knips, J. Dziewior, W. Kłobus, W. Laskowski, T. Paterek, P. J. Shadbolt, H. Weinfurter, and J. D. A. Meinecke, “Multipartite entanglement analysis from random correlations,” Npj quantum information, vol. 6, iss. 1, p. 51, 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, title = {Multipartite entanglement analysis from random correlations}, volume = {6}, issn = {2056-6387}, url = {http://www.nature.com/articles/s41534-020-0281-5}, doi = {10.1038/s41534-020-0281-5}, 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.}, language = {en}, number = {1}, urldate = {2021-05-10}, journal = {npj Quantum Information}, 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.}, month = dec, year = {2020}, pages = {51}, }`

- S. Roy, T. Das, and A. Sen(De), “Computable genuine multimode entanglement measure: Gaussian versus non-Gaussian,” Physical review a, vol. 102, iss. 1, p. 12421, 2020. doi:10.1103/PhysRevA.102.012421

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

- E. Wolfe, D. Schmid, A. B. Sainz, R. Kunjwal, and R. W. Spekkens, “Quantifying Bell: the Resource Theory of Nonclassicality of Common-Cause Boxes,” Quantum, vol. 4, p. 280, 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}, }`

- R. Ramanathan, M. Rosicka, K. Horodecki, S. Pironio, M. Horodecki, and P. Horodecki, “Gadget structures in proofs of the Kochen-Specker theorem,” Quantum, vol. 4, p. 308, 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, title = {Gadget structures in proofs of the {Kochen}-{Specker} theorem}, volume = {4}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2020-08-14-308/}, doi = {10.22331/q-2020-08-14-308}, 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\}.}, language = {en}, urldate = {2021-05-10}, journal = {Quantum}, author = {Ramanathan, Ravishankar and Rosicka, Monika and Horodecki, Karol and Pironio, Stefano and Horodecki, Michał and Horodecki, Paweł}, month = aug, year = {2020}, pages = {308}, }`

- F. B. Maciejewski, Z. Zimborás, and M. Oszmaniec, “Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography,” Quantum, vol. 4, p. 257, 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, vol. 137, iss. 3, p. 374–378, 2020. doi:10.12693/APhysPolA.137.374

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

- M. Wieśniak, P. Pandya, O. Sakarya, and B. Woloncewicz, “Distance between Bound Entangled States from Unextendible Product Bases and Separable States,” Quantum reports, vol. 2, iss. 1, p. 49–56, 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, title = {Distance between {Bound} {Entangled} {States} from {Unextendible} {Product} {Bases} and {Separable} {States}}, volume = {2}, issn = {2624-960X}, url = {https://www.mdpi.com/2624-960X/2/1/4}, doi = {10.3390/quantum2010004}, 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.}, language = {en}, number = {1}, urldate = {2021-05-10}, journal = {Quantum Reports}, author = {Wieśniak, Marcin and Pandya, Palash and Sakarya, Omer and Woloncewicz, Bianka}, month = jan, year = {2020}, pages = {49--56}, }`

- T. Linowski, C. Gneiting, and Ł. Rudnicki, “Stabilizing entanglement in two-mode Gaussian states,” Physical review a, vol. 102, iss. 4, p. 42405, 2020. doi:10.1103/PhysRevA.102.042405

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

- M. Eckstein and P. Horodecki, “The Experiment Paradox in Physics,” Foundations of science, 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, title = {The {Experiment} {Paradox} in {Physics}}, issn = {1233-1821, 1572-8471}, url = {http://link.springer.com/10.1007/s10699-020-09711-y}, doi = {10.1007/s10699-020-09711-y}, 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.}, language = {en}, urldate = {2021-05-10}, journal = {Foundations of Science}, author = {Eckstein, Michał and Horodecki, Paweł}, month = oct, year = {2020}, }`

- A. Z. Goldberg, A. B. Klimov, M. Grassl, G. Leuchs, and L. L. Sánchez-Soto, “Extremal quantum states,” Avs quantum science, vol. 2, iss. 4, p. 44701, 2020. doi:10.1116/5.0025819

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

- B. Groisman, M. Mc Gettrick, M. Mhalla, and M. Pawlowski, “How Quantum Information Can Improve Social Welfare,” Ieee journal on selected areas in information theory, vol. 1, iss. 2, p. 445–453, 2020. doi:10.1109/JSAIT.2020.3012922

[BibTeX] [Download PDF]`@article{groisman_how_2020, title = {How {Quantum} {Information} {Can} {Improve} {Social} {Welfare}}, volume = {1}, issn = {2641-8770}, url = {https://ieeexplore.ieee.org/document/9173538/}, doi = {10.1109/JSAIT.2020.3012922}, number = {2}, urldate = {2021-05-10}, journal = {IEEE Journal on Selected Areas in Information Theory}, author = {Groisman, Berry and Mc Gettrick, Michael and Mhalla, Mehdi and Pawlowski, Marcin}, month = aug, year = {2020}, pages = {445--453}, }`

- O. Sakarya, M. Winczewski, A. Rutkowski, and K. Horodecki, “Hybrid quantum network design against unauthorized secret-key generation, and its memory cost,” Physical review research, vol. 2, iss. 4, p. 43022, 2020. doi:10.1103/PhysRevResearch.2.043022

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

- J. H. Selby and C. M. Lee, “Compositional resource theories of coherence,” Quantum, vol. 4, p. 319, 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}, }`

- P. Pandya, O. Sakarya, and M. Wieśniak, “Hilbert-Schmidt distance and entanglement witnessing,” Physical review a, vol. 102, iss. 1, p. 12409, 2020. doi:10.1103/PhysRevA.102.012409

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

- J. Sikora and J. H. Selby, “Impossibility of coin flipping in generalized probabilistic theories via discretizations of semi-infinite programs,” Physical review research, vol. 2, iss. 4, p. 43128, 2020. doi:10.1103/PhysRevResearch.2.043128

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

- A. Hameedi, B. Marques, P. Mironowicz, D. Saha, M. Pawłowski, and M. Bourennane, “Experimental test of nonclassicality with arbitrarily low detection efficiency,” Physical review a, vol. 102, iss. 3, p. 32621, 2020. doi:10.1103/PhysRevA.102.032621

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

- T. P. Le, P. Mironowicz, and P. Horodecki, “Blurred quantum Darwinism across quantum reference frames,” Physical review a, vol. 102, iss. 6, p. 62420, 2020. doi:10.1103/PhysRevA.102.062420

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

- Sudha, H. S. Karthik, R. Pal, K. S. Akhilesh, S. Ghosh, K. S. Mallesh, and A. R. Usha Devi, “Canonical forms of two-qubit states under local operations,” Physical review a, vol. 102, iss. 5, p. 52419, 2020. doi:10.1103/PhysRevA.102.052419

[BibTeX] [Download PDF]`@article{sudha_canonical_2020, title = {Canonical forms of two-qubit states under local operations}, volume = {102}, issn = {2469-9926, 2469-9934}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.052419}, doi = {10.1103/PhysRevA.102.052419}, language = {en}, number = {5}, urldate = {2021-05-10}, journal = {Physical Review A}, 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.}, month = nov, year = {2020}, pages = {052419}, }`

- A. Chaturvedi and D. Saha, “Quantum prescriptions are more ontologically distinct than they are operationally distinguishable,” Quantum, vol. 4, p. 345, 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}, }`

- M. Grassl, “Algebraic quantum codes: linking quantum mechanics and discrete mathematics,” International journal of computer mathematics: computer systems theory, p. 1–17, 2020. doi:10.1080/23799927.2020.1850530

[BibTeX] [Download PDF]`@article{grassl_algebraic_2020, title = {Algebraic quantum codes: linking quantum mechanics and discrete mathematics}, issn = {2379-9927, 2379-9935}, shorttitle = {Algebraic quantum codes}, url = {https://www.tandfonline.com/doi/full/10.1080/23799927.2020.1850530}, doi = {10.1080/23799927.2020.1850530}, language = {en}, urldate = {2021-05-10}, journal = {International Journal of Computer Mathematics: Computer Systems Theory}, author = {Grassl, Markus}, month = dec, year = {2020}, pages = {1--17}, }`

- M. Eckstein, P. Horodecki, R. Horodecki, and T. Miller, “Operational causality in spacetime,” Physical review a, vol. 101, iss. 4, p. 42128, 2020. 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, title = {Operational causality in spacetime}, volume = {101}, issn = {2469-9926, 2469-9934}, url = {http://arxiv.org/abs/1902.05002}, doi = {10.1103/PhysRevA.101.042128}, 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.}, number = {4}, urldate = {2021-05-11}, journal = {Physical Review A}, author = {Eckstein, Michał and Horodecki, Paweł and Horodecki, Ryszard and Miller, Tomasz}, month = apr, year = {2020}, note = {arXiv: 1902.05002}, keywords = {Quantum Physics, General Relativity and Quantum Cosmology, Mathematical Physics, 81P16 (Primary), 81P15, 28E99, 60B05 (Secondary)}, pages = {042128}, }`

- G. Tóth, T. Vértesi, P. Horodecki, and R. Horodecki, “Activating Hidden Metrological Usefulness,” Physical review letters, vol. 125, iss. 2, p. 20402, 2020. doi:10.1103/PhysRevLett.125.020402

[BibTeX] [Download PDF]`@article{toth_activating_2020-1, title = {Activating {Hidden} {Metrological} {Usefulness}}, volume = {125}, issn = {0031-9007, 1079-7114}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.125.020402}, doi = {10.1103/PhysRevLett.125.020402}, language = {en}, number = {2}, urldate = {2021-07-28}, journal = {Physical Review Letters}, author = {Tóth, Géza and Vértesi, Tamás and Horodecki, Paweł and Horodecki, Ryszard}, month = jul, year = {2020}, pages = {020402}, }`

- M. Łobejko, P. Mazurek, and M. Horodecki, “Thermodynamics of Minimal Coupling Quantum Heat Engines,” Quantum, vol. 4, p. 375, 2020. 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, title = {Thermodynamics of {Minimal} {Coupling} {Quantum} {Heat} {Engines}}, volume = {4}, issn = {2521-327X}, url = {http://arxiv.org/abs/2003.05788}, doi = {10.22331/q-2020-12-23-375}, 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.}, urldate = {2021-07-28}, journal = {Quantum}, author = {Łobejko, Marcin and Mazurek, Paweł and Horodecki, Michał}, month = dec, year = {2020}, note = {arXiv: 2003.05788}, keywords = {Quantum Physics}, pages = {375}, }`

- P. Mazurek, Máté. Farkas, A. Grudka, M. Horodecki, and M. Studziński, “Quantum error correction codes and absolutely maximally entangled states,” Physical review a, vol. 101, iss. 4, p. 42305, 2020. doi:10.1103/PhysRevA.101.042305

[BibTeX] [Abstract] [Download PDF]

For every stabiliser \$N\$-qudit absolutely maximally entangled state, we present a method for determining the stabiliser generators and logical operators of a corresponding quantum error correction code. These codes encode \$k\$ qudits into \$N-k\$ qudits, with \$k{\textbackslash}leq {\textbackslash}left {\textbackslash}lfloor\{N/2\} {\textbackslash}right {\textbackslash}rfloor\$, where the local dimension \$d\$ is prime. We use these methods to analyse the concatenation of such quantum codes and link this procedure to entanglement swapping. Using our techniques, we investigate the spread of quantum information on a tensor network code formerly used as a toy model for the AdS/CFT correspondence. In this network, we show how corrections arise to the Ryu-Takayanagi formula in the case of entangled input state, and that the bound on the entanglement entropy of the boundary state is saturated for absolutely maximally entangled input states.

`@article{mazurek_quantum_2020-1, title = {Quantum error correction codes and absolutely maximally entangled states}, volume = {101}, issn = {2469-9926, 2469-9934}, url = {http://arxiv.org/abs/1910.07427}, doi = {10.1103/PhysRevA.101.042305}, abstract = {For every stabiliser \$N\$-qudit absolutely maximally entangled state, we present a method for determining the stabiliser generators and logical operators of a corresponding quantum error correction code. These codes encode \$k\$ qudits into \$N-k\$ qudits, with \$k{\textbackslash}leq {\textbackslash}left {\textbackslash}lfloor\{N/2\} {\textbackslash}right {\textbackslash}rfloor\$, where the local dimension \$d\$ is prime. We use these methods to analyse the concatenation of such quantum codes and link this procedure to entanglement swapping. Using our techniques, we investigate the spread of quantum information on a tensor network code formerly used as a toy model for the AdS/CFT correspondence. In this network, we show how corrections arise to the Ryu-Takayanagi formula in the case of entangled input state, and that the bound on the entanglement entropy of the boundary state is saturated for absolutely maximally entangled input states.}, number = {4}, urldate = {2021-07-28}, journal = {Physical Review A}, author = {Mazurek, Paweł and Farkas, Máté and Grudka, Andrzej and Horodecki, Michał and Studziński, Michał}, month = apr, year = {2020}, note = {arXiv: 1910.07427}, keywords = {Quantum Physics}, pages = {042305}, }`

- C. Cirstoiu, K. Korzekwa, and D. Jennings, “Robustness of Noether’s principle: Maximal disconnects between conservation laws and symmetries in quantum theory,” Physical review x, vol. 10, iss. 4, p. 41035, 2020. 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, title = {Robustness of {Noether}'s principle: {Maximal} disconnects between conservation laws and symmetries in quantum theory}, volume = {10}, issn = {2160-3308}, shorttitle = {Robustness of {Noether}'s principle}, url = {http://arxiv.org/abs/1908.04254}, doi = {10.1103/PhysRevX.10.041035}, 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.}, number = {4}, urldate = {2021-07-28}, journal = {Physical Review X}, author = {Cirstoiu, Cristina and Korzekwa, Kamil and Jennings, David}, month = nov, year = {2020}, note = {arXiv: 1908.04254}, keywords = {Quantum Physics}, pages = {041035}, }`

- A. Tavakoli, M. Żukowski, and Č. Brukner, “Does violation of a Bell inequality always imply quantum advantage in a communication complexity problem?,” Quantum, vol. 4, p. 316, 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, [Brukner et. al., Phys Rev. Lett. 89, 197901 (2002)]; 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_2020-1, title = {Does violation of a {Bell} inequality always imply quantum advantage in a communication complexity problem?}, volume = {4}, issn = {2521-327X}, url = {http://arxiv.org/abs/1907.01322}, doi = {10.22331/q-2020-09-07-316}, 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, [Brukner et. al., Phys Rev. Lett. 89, 197901 (2002)]; 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}, urldate = {2021-07-28}, journal = {Quantum}, author = {Tavakoli, Armin and Żukowski, Marek and Brukner, Časlav}, month = sep, year = {2020}, note = {arXiv: 1907.01322}, keywords = {Quantum Physics}, pages = {316}, }`

- F. B. Maciejewski, Z. Zimborás, and M. Oszmaniec, “Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography,” Quantum, vol. 4, p. 257, 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_2020-1, title = {Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography}, volume = {4}, issn = {2521-327X}, url = {http://arxiv.org/abs/1907.08518}, 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.}, urldate = {2021-07-28}, journal = {Quantum}, author = {Maciejewski, Filip B. and Zimborás, Zoltán and Oszmaniec, Michał}, month = apr, year = {2020}, note = {arXiv: 1907.08518}, keywords = {Quantum Physics}, pages = {257}, }`

- D. J. Brod and M. Oszmaniec, “Classical simulation of linear optics subject to nonuniform losses,” Quantum, vol. 4, p. 267, 2020. 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, title = {Classical simulation of linear optics subject to nonuniform losses}, volume = {4}, issn = {2521-327X}, url = {http://arxiv.org/abs/1906.06696}, doi = {10.22331/q-2020-05-14-267}, 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.}, urldate = {2021-07-28}, journal = {Quantum}, author = {Brod, Daniel Jost and Oszmaniec, Michał{\textbackslash}}, month = may, year = {2020}, note = {arXiv: 1906.06696}, keywords = {Quantum Physics, Mathematical Physics}, pages = {267}, }`

- A. de Rosier, J. Gruca, F. Parisio, T. Vertesi, and W. Laskowski, “Strength and typicality of nonlocality in multisetting and multipartite Bell scenarios,” Physical review a, vol. 101, iss. 1, p. 12116, 2020. doi:10.1103/PhysRevA.101.012116

[BibTeX] [Abstract] [Download PDF]

In this work we investigate the probability of violation of local realism under random measurements in parallel with the strength of these violations as described by resistance to white noise admixture. We address multisetting Bell scenarios involving up to 7 qubits. As a result, in the first part of this manuscript we report statistical distributions of a quantity reciprocal to the critical visibility for various multipartite quantum states subjected to random measurements. The statistical relevance of different classes of multipartite tight Bell inequalities violated with random measurements is investigated. We also introduce the concept of typicality of quantum correlations for pure states as the probability to generate a nonlocal behaviour with both random state and measurement. Although this typicality is slightly above 5.3{\textbackslash}\% for the CHSH scenario, for a modest increase in the number of involved qubits it quickly surpasses 99.99{\textbackslash}\%.

`@article{de_rosier_strength_2020-1, title = {Strength and typicality of nonlocality in multisetting and multipartite {Bell} scenarios}, volume = {101}, issn = {2469-9926, 2469-9934}, url = {http://arxiv.org/abs/1906.03235}, doi = {10.1103/PhysRevA.101.012116}, abstract = {In this work we investigate the probability of violation of local realism under random measurements in parallel with the strength of these violations as described by resistance to white noise admixture. We address multisetting Bell scenarios involving up to 7 qubits. As a result, in the first part of this manuscript we report statistical distributions of a quantity reciprocal to the critical visibility for various multipartite quantum states subjected to random measurements. The statistical relevance of different classes of multipartite tight Bell inequalities violated with random measurements is investigated. We also introduce the concept of typicality of quantum correlations for pure states as the probability to generate a nonlocal behaviour with both random state and measurement. Although this typicality is slightly above 5.3{\textbackslash}\% for the CHSH scenario, for a modest increase in the number of involved qubits it quickly surpasses 99.99{\textbackslash}\%.}, number = {1}, urldate = {2021-07-28}, journal = {Physical Review A}, author = {de Rosier, Anna and Gruca, Jacek and Parisio, Fernando and Vertesi, Tamas and Laskowski, Wieslaw}, month = jan, year = {2020}, note = {arXiv: 1906.03235}, keywords = {Quantum Physics}, pages = {012116}, }`

- M. Rosicka, P. Mazurek, A. Grudka, and M. Horodecki, “Generalized XOR non-locality games with graph description on a square lattice,” Journal of physics a: mathematical and theoretical, vol. 53, iss. 26, p. 265302, 2020. doi:10.1088/1751-8121/ab8f3e

[BibTeX] [Abstract] [Download PDF]

We propose a family of non-locality unique games for 2 parties based on a square lattice on an arbitrary surface. We show that, due to structural similarities with error correction codes of Kitaev for fault tolerant quantum computation, the games have classical values computable in polynomial time for \$d=2\$ measurement outcomes. By representing games in their graph form, for arbitrary \$d\$ and underlying surface we provide their classification into equivalence classes with respect to relabeling of measurement outcomes, for a selected set of permutations which define the winning conditions. A case study of games with periodic boundary conditions is presented in order to verify their impact on classical and quantum values of the family of games. It suggests that quantum values suffer independently from presence of different winning conditions that can be imposed due to periodicity, as long as no local restrictions are in place.

`@article{rosicka_generalized_2020-1, title = {Generalized {XOR} non-locality games with graph description on a square lattice}, volume = {53}, issn = {1751-8113, 1751-8121}, url = {http://arxiv.org/abs/1902.11053}, doi = {10.1088/1751-8121/ab8f3e}, abstract = {We propose a family of non-locality unique games for 2 parties based on a square lattice on an arbitrary surface. We show that, due to structural similarities with error correction codes of Kitaev for fault tolerant quantum computation, the games have classical values computable in polynomial time for \$d=2\$ measurement outcomes. By representing games in their graph form, for arbitrary \$d\$ and underlying surface we provide their classification into equivalence classes with respect to relabeling of measurement outcomes, for a selected set of permutations which define the winning conditions. A case study of games with periodic boundary conditions is presented in order to verify their impact on classical and quantum values of the family of games. It suggests that quantum values suffer independently from presence of different winning conditions that can be imposed due to periodicity, as long as no local restrictions are in place.}, number = {26}, urldate = {2021-07-28}, journal = {Journal of Physics A: Mathematical and Theoretical}, author = {Rosicka, Monika and Mazurek, Paweł and Grudka, Andrzej and Horodecki, Michał}, month = jul, year = {2020}, note = {arXiv: 1902.11053}, keywords = {Quantum Physics, Mathematics - Combinatorics}, pages = {265302}, }`

- M. Eckstein, P. Horodecki, R. Horodecki, and T. Miller, “Operational causality in spacetime,” Physical review a, vol. 101, iss. 4, p. 42128, 2020. 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, title = {Operational causality in spacetime}, volume = {101}, issn = {2469-9926, 2469-9934}, url = {http://arxiv.org/abs/1902.05002}, doi = {10.1103/PhysRevA.101.042128}, 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.}, number = {4}, urldate = {2021-07-28}, journal = {Physical Review A}, author = {Eckstein, Michał and Horodecki, Paweł and Horodecki, Ryszard and Miller, Tomasz}, month = apr, year = {2020}, note = {arXiv: 1902.05002}, keywords = {Quantum Physics, General Relativity and Quantum Cosmology, Mathematical Physics, 81P16 (Primary), 81P15, 28E99, 60B05 (Secondary)}, pages = {042128}, }`

- J. Czartowski, D. Goyeneche, M. Grassl, and K. Życzkowski, “Isoentangled Mutually Unbiased Bases, Symmetric Quantum Measurements, and Mixed-State Designs,” Physical review letters, vol. 124, iss. 9, p. 90503, 2020. doi:10.1103/PhysRevLett.124.090503

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

- R. Alicki and A. Jenkins, “Quantum theory of triboelectricity,” , vol. 125, 2020. doi:10.1103/physrevlett.125.186101

[BibTeX] [Download PDF]`@Article{Alicki, author = {Robert Alicki and Alejandro Jenkins}, title = {Quantum Theory of Triboelectricity}, year = {2020}, issn = {0031-9007}, volume = {125}, doi = {10.1103/physrevlett.125.186101}, url = {https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.125.186101}, }`

### 2019

- A. Pozas-Kerstjens, R. Rabelo, Ł. Rudnicki, R. Chaves, D. Cavalcanti, M. Navascués, and A. Acín, “Bounding the sets of classical and quantum correlations in networks,” Physical review letters, vol. 123, iss. 14, p. 140503, 2019. doi:10.1103/PhysRevLett.123.140503

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

- D. Yang, K. Horodecki, and A. Winter, “Distributed private randomness distillation,” Physical review letters, vol. 123, iss. 17, p. 170501, 2019. doi:10.1103/PhysRevLett.123.170501

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

- T. Van Himbeeck, J. Bohr Brask, S. Pironio, R. Ramanathan, A. B. Sainz, and E. Wolfe, “Quantum violations in the Instrumental scenario and their relations to the Bell scenario,” Quantum, vol. 3, p. 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, title = {Quantum violations in the {Instrumental} scenario and their relations to the {Bell} scenario}, volume = {3}, issn = {2521-327X}, url = {https://quantum-journal.org/papers/q-2019-09-16-186/}, doi = {10.22331/q-2019-09-16-186}, 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.}, language = {en}, urldate = {2020-04-22}, journal = {Quantum}, author = {Van Himbeeck, Thomas and Bohr Brask, Jonatan and Pironio, Stefano and Ramanathan, Ravishankar and Sainz, Ana Belén and Wolfe, Elie}, month = sep, year = {2019}, pages = {186}, }`

- P. Mironowicz and M. Pawłowski, “Experimentally feasible semi-device-independent certification of four-outcome positive-operator-valued measurements,” Physical review a, vol. 100, iss. 3, p. 30301, 2019. doi:10.1103/PhysRevA.100.030301

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

- R. Alicki, “A quantum open system model of molecular battery charged by excitons,” The journal of chemical physics, vol. 150, iss. 21, p. 214110, 2019. doi:10.1063/1.5096772

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

- G. Baio, D. Chruściński, P. Horodecki, A. Messina, and G. Sarbicki, “Bounds on the entanglement of two-qutrit systems from fixed marginals,” Physical review a, vol. 99, iss. 6, p. 62312, 2019. doi:10.1103/PhysRevA.99.062312

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

- Robert Alicki, “Quantum Features of Macroscopic Fields: Entropy and Dynamics,” Entropy, vol. 21, iss. 7, p. 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}, }`

- P. Horodecki and R. Ramanathan, “The relativistic causality versus no-signaling paradigm for multi-party correlations,” Nature communications, vol. 10, iss. 1, p. 1701, 2019. doi:10.1038/s41467-019-09505-2

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

- J. Ryu, B. Woloncewicz, M. Marciniak, M. Wieśniak, and M. Żukowski, “General mapping of multiqudit entanglement conditions to nonseparability indicators for quantum-optical fields,” Physical review research, vol. 1, iss. 3, p. 32041, 2019. doi:10.1103/PhysRevResearch.1.032041

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

- W. Kłobus, A. Burchardt, A. Kołodziejski, M. Pandit, T. Vértesi, K. Życzkowski, and W. Laskowski, “K -uniform mixed states,” Physical review a, vol. 100, iss. 3, p. 32112, 2019. doi:10.1103/PhysRevA.100.032112

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

- Máté. Farkas and J. Kaniewski, “Self-testing mutually unbiased bases in the prepare-and-measure scenario,” Physical review a, vol. 99, iss. 3, p. 32316, 2019. doi:10.1103/PhysRevA.99.032316

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

- W. Klobus, A. Burchardt, A. Kolodziejski, M. Pandit, T. Vertesi, K. Zyczkowski, and W. Laskowski, “\$k\$-uniform mixed states,” Physical review a, vol. 100, iss. 3, p. 32112, 2019. 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, title = {\$k\$-uniform mixed states}, volume = {100}, issn = {2469-9926, 2469-9934}, url = {http://arxiv.org/abs/1906.01311}, doi = {10.1103/PhysRevA.100.032112}, 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.}, number = {3}, urldate = {2021-07-28}, journal = {Physical Review A}, author = {Klobus, Waldemar and Burchardt, Adam and Kolodziejski, Adrian and Pandit, Mahasweta and Vertesi, Tamas and Zyczkowski, Karol and Laskowski, Wieslaw}, month = sep, year = {2019}, note = {arXiv: 1906.01311}, keywords = {Quantum Physics}, pages = {032112}, }`

- J. Ryu, B. Woloncewicz, M. Marciniak, M. Wieśniak, and M. Żukowski, “General mapping of multi-qu\$d\$it entanglement conditions to non-separability indicators for quantum optical fields,” Physical review research, vol. 1, iss. 3, p. 32041, 2019. doi:10.1103/PhysRevResearch.1.032041

[BibTeX] [Abstract] [Download PDF]

We show that any multi-qudit entanglement witness leads to a non-separability indicator for quantum optical fields, which involves intensity correlations. We get, e.g., necessary and sufficient conditions for intensity or intensity-rate correlations to reveal polarization entanglement. We also derive separability conditions for experiments involving multiport interferometers, now feasible with integrated optics. We show advantages of using intensity rates rather than intensities, e.g., a mapping of Bell inequalities to ones for optical fields. The results have implication for studies of non-classicality of “macroscopic” systems of undefined or uncontrollable number of “particles”.

`@article{ryu_general_2019-1, title = {General mapping of multi-qu\$d\$it entanglement conditions to non-separability indicators for quantum optical fields}, volume = {1}, issn = {2643-1564}, url = {http://arxiv.org/abs/1903.03526}, doi = {10.1103/PhysRevResearch.1.032041}, abstract = {We show that any multi-qudit entanglement witness leads to a non-separability indicator for quantum optical fields, which involves intensity correlations. We get, e.g., necessary and sufficient conditions for intensity or intensity-rate correlations to reveal polarization entanglement. We also derive separability conditions for experiments involving multiport interferometers, now feasible with integrated optics. We show advantages of using intensity rates rather than intensities, e.g., a mapping of Bell inequalities to ones for optical fields. The results have implication for studies of non-classicality of "macroscopic" systems of undefined or uncontrollable number of "particles".}, number = {3}, urldate = {2021-07-28}, journal = {Physical Review Research}, author = {Ryu, Junghee and Woloncewicz, Bianka and Marciniak, Marcin and Wieśniak, Marcin and Żukowski, Marek}, month = dec, year = {2019}, note = {arXiv: 1903.03526}, keywords = {Quantum Physics}, pages = {032041}, }`

### 2018

- A. Dutta, T. Nahm, J. Lee, and M. Żukowski, “Geometric extension of Clauser–Horne inequality to more qubits,” New journal of physics, vol. 20, iss. 9, p. 93006, 2018. doi:10.1088/1367-2630/aadc78

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

## arXiv preprints

### 2021

- I. Reena, H. S. Karthik, P. J. Tej, U. 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.

[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}, }`

- R. Alicki and A. Jenkins, “Quantum thermodynamics of coronal heating,” Arxiv:2103.08746 [astro-ph, physics:physics, physics:quant-ph], 2021.

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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}, }`

- M. Markiewicz, M. Karczewski, and P. Kurzynski, “Borromean states in discrete-time quantum walks,” Arxiv:2005.13588 [quant-ph], 2021.

[BibTeX] [Abstract] [Download PDF]

In the right conditions, removing one particle from a multipartite bound state can make it fall apart. This feature, known as the “Borromean property”, has been recently demonstrated experimentally in Efimov states. One could expect that such peculiar behavior should be linked with the presence of strong inter-particle correlations. However, any exploration of this connection is hindered by the complexity of the physical systems exhibiting the Borromean property. To overcome this problem, we introduce a simple dynamical toy model based on a discrete-time quantum walk of many interacting particles. We show that the particles described by it need to exhibit the Greenberger-Horne-Zeillinger (GHZ) entanglement to form Borromean bound states. As this type of entanglement is very prone to particle losses, our work demonstrates an intuitive link between correlations and Borromean properties of the system. Moreover, we discuss our findings in the context of the formation of composite particles.

`@article{markiewicz_borromean_2021, title = {Borromean states in discrete-time quantum walks}, url = {http://arxiv.org/abs/2005.13588}, abstract = {In the right conditions, removing one particle from a multipartite bound state can make it fall apart. This feature, known as the "Borromean property", has been recently demonstrated experimentally in Efimov states. One could expect that such peculiar behavior should be linked with the presence of strong inter-particle correlations. However, any exploration of this connection is hindered by the complexity of the physical systems exhibiting the Borromean property. To overcome this problem, we introduce a simple dynamical toy model based on a discrete-time quantum walk of many interacting particles. We show that the particles described by it need to exhibit the Greenberger-Horne-Zeillinger (GHZ) entanglement to form Borromean bound states. As this type of entanglement is very prone to particle losses, our work demonstrates an intuitive link between correlations and Borromean properties of the system. Moreover, we discuss our findings in the context of the formation of composite particles.}, urldate = {2021-07-28}, journal = {arXiv:2005.13588 [quant-ph]}, author = {Markiewicz, Marcin and Karczewski, Marcin and Kurzynski, Pawel}, month = mar, year = {2021}, note = {arXiv: 2005.13588}, keywords = {Quantum Physics}, }`

- P. Blasiak, E. Borsuk, and M. Markiewicz, “On safe post-selection for Bell nonlocality: Causal diagram approach,” Arxiv:2012.07285 [quant-ph], 2021.

[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, title = {On safe post-selection for {Bell} nonlocality: {Causal} diagram approach}, shorttitle = {On safe post-selection for {Bell} nonlocality}, url = {http://arxiv.org/abs/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.}, urldate = {2021-07-28}, journal = {arXiv:2012.07285 [quant-ph]}, author = {Blasiak, Pawel and Borsuk, Ewa and Markiewicz, Marcin}, month = apr, year = {2021}, note = {arXiv: 2012.07285}, keywords = {Quantum Physics}, }`

- T. Das, M. Karczewski, A. Mandarino, M. Markiewicz, B. Woloncewicz, and M. Żukowski, “No-go for device independent protocols with Tan-Walls-Collett `nonlocality of a single photon’,” Arxiv:2102.03254 [quant-ph], 2021.

[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}, }`

- T. Das, M. Karczewski, A. Mandarino, M. Markiewicz, B. Woloncewicz, and M. 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], 2021.

[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}, }`

- P. Blasiak, E. Borsuk, M. Markiewicz, and Y. Kim, “Efficient linear optical generation of a multipartite W state,” Arxiv:2103.02206 [quant-ph], 2021.

[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, title = {Efficient linear optical generation of a multipartite {W} state}, url = {http://arxiv.org/abs/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).}, urldate = {2021-07-28}, journal = {arXiv:2103.02206 [quant-ph]}, author = {Blasiak, Pawel and Borsuk, Ewa and Markiewicz, Marcin and Kim, Yong-Su}, month = mar, year = {2021}, note = {arXiv: 2103.02206}, keywords = {Quantum Physics}, }`

- T. Das, M. Karczewski, A. Mandarino, M. Markiewicz, B. Woloncewicz, and M. Żukowski, “On detecting violation of local realism with photon-number resolving weak-field homodyne measurements,” Arxiv:2104.10703 [quant-ph], 2021.

[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}, }`

- G. Scala, K. Słowik, P. Facchi, S. Pascazio, and F. Pepe, “Beyond the Rabi model: light interactions with polar atomic systems in a cavity,” Arxiv:2103.11232 [quant-ph], 2021.

[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}, }`

- M. Gachechiladze, B. Bąk, M. Pawłowski, and N. Miklin, “Quantum Bell inequalities from Information Causality – tight for Macroscopic Locality,” Arxiv:2103.05029 [quant-ph], 2021.

[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}, }`

- D. Schmid, H. Du, J. H. Selby, and M. F. Pusey, “The only noncontextual model of the stabilizer subtheory is Gross’s,” Arxiv:2101.06263 [quant-ph], 2021.

[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}, }`

- T. D. Galley, F. Giacomini, and J. 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.

[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}, }`

- D. Schmid, J. H. Selby, and R. W. Spekkens, “Unscrambling the omelette of causation and inference: The framework of causal-inferential theories,” Arxiv:2009.03297 [quant-ph], 2021.

[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}, }`

- J. H. Selby, A. B. Sainz, and P. Horodecki, “Revisiting dynamics of quantum causal structures – when can causal order evolve?,” Arxiv:2008.12757 [quant-ph], 2021.

[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, title = {Revisiting dynamics of quantum causal structures -- when can causal order evolve?}, url = {http://arxiv.org/abs/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.}, urldate = {2021-07-28}, journal = {arXiv:2008.12757 [quant-ph]}, author = {Selby, John H. and Sainz, Ana Belén and Horodecki, Paweł}, month = mar, year = {2021}, note = {arXiv: 2008.12757}, keywords = {Quantum Physics}, }`

- D. Schmid, T. C. Fraser, R. Kunjwal, A. B. Sainz, E. Wolfe, and R. W. Spekkens, “Understanding the interplay of entanglement and nonlocality: motivating and developing a new branch of entanglement theory,” Arxiv:2004.09194 [quant-ph], 2021.

[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}, }`

- P. J. Cavalcanti, J. H. Selby, J. Sikora, T. D. Galley, and A. B. Sainz, “Witworld: A generalised probabilistic theory featuring post-quantum steering,” Arxiv:2102.06581 [quant-ph], 2021.

[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}, }`

- M. Grassl, F. Huber, and A. Winter, “Entropic proofs of Singleton bounds for quantum error-correcting codes,” Arxiv:2010.07902 [quant-ph], 2021.

[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], 2021.

[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, title = {Refined {Definitions} of {Heat} and {Work} in {Quantum} {Thermodynamics}}, url = {http://arxiv.org/abs/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.}, urldate = {2021-07-28}, journal = {arXiv:1912.01983 [quant-ph]}, author = {Ahmadi, B. and Salimi, S. and Khorashad, A. S.}, month = jul, year = {2021}, note = {arXiv: 1912.01983}, keywords = {Quantum Physics}, }`

### 2020

- D. Schmid, J. H. Selby, M. F. Pusey, and R. W. Spekkens, “A structure theorem for generalized-noncontextual ontological models,” Arxiv:2005.07161 [quant-ph], 2020.

[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}, }`

- M. Gachechiladze, N. Miklin, and R. Chaves, “Quantifying causal influences in the presence of a quantum common cause,” Arxiv:2007.01221 [quant-ph, stat], 2020.

[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}, }`

- L. Knips, J. Dziewior, W. Kłobus, W. Laskowski, T. Paterek, P. J. Shadbolt, H. Weinfurter, and J. D. A. Meinecke, “Multipartite entanglement analysis from random correlations,” Npj quantum information, vol. 6, iss. 1, p. 51, 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, title = {Multipartite entanglement analysis from random correlations}, volume = {6}, issn = {2056-6387}, url = {http://www.nature.com/articles/s41534-020-0281-5}, doi = {10.1038/s41534-020-0281-5}, 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.}, language = {en}, number = {1}, urldate = {2021-05-10}, journal = {npj Quantum Information}, 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.}, month = dec, year = {2020}, pages = {51}, }`

- R. Ramanathan, M. Horodecki, H. Anwer, S. Pironio, K. Horodecki, M. Grünfeld, S. Muhammad, M. Bourennane, and P. Horodecki, “Practical No-Signalling proof Randomness Amplification using Hardy paradoxes and its experimental implementation,” Arxiv:1810.11648 [quant-ph], 2020.

[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, title = {Practical {No}-{Signalling} proof {Randomness} {Amplification} using {Hardy} paradoxes and its experimental implementation}, url = {http://arxiv.org/abs/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.}, urldate = {2021-05-11}, journal = {arXiv:1810.11648 [quant-ph]}, 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}, month = sep, year = {2020}, note = {arXiv: 1810.11648}, keywords = {Quantum Physics}, }`

- M. Banacki, M. Marciniak, K. Horodecki, and P. Horodecki, “Information backflow may not indicate quantum memory,” Arxiv:2008.12638 [quant-ph], 2020.

[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, title = {Information backflow may not indicate quantum memory}, url = {http://arxiv.org/abs/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.}, urldate = {2021-07-28}, journal = {arXiv:2008.12638 [quant-ph]}, author = {Banacki, Michal and Marciniak, Marcin and Horodecki, Karol and Horodecki, Pawel}, month = aug, year = {2020}, note = {arXiv: 2008.12638}, keywords = {Quantum Physics}, }`

- R. Ramanathan, M. Banacki, R. R. Rodríguez, and P. Horodecki, “Single trusted qubit is necessary and sufficient for quantum realisation of extremal no-signaling correlations,” Arxiv:2004.14782 [quant-ph], 2020.

[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, title = {Single trusted qubit is necessary and sufficient for quantum realisation of extremal no-signaling correlations}, url = {http://arxiv.org/abs/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.}, urldate = {2021-07-28}, journal = {arXiv:2004.14782 [quant-ph]}, author = {Ramanathan, Ravishankar and Banacki, Michał and Rodríguez, Ricard Ravell and Horodecki, Paweł}, month = apr, year = {2020}, note = {arXiv: 2004.14782}, keywords = {Quantum Physics}, }`

- M. Markiewicz, M. Pandit, and W. Laskowski, “Multiparameter estimation in generalized Mach-Zehnder interferometer,” Arxiv:2012.07645 [quant-ph], 2020.

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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, title = {Multiparameter estimation in generalized {Mach}-{Zehnder} interferometer}, url = {http://arxiv.org/abs/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.}, urldate = {2021-07-28}, journal = {arXiv:2012.07645 [quant-ph]}, author = {Markiewicz, Marcin and Pandit, Mahasweta and Laskowski, Wieslaw}, month = dec, year = {2020}, note = {arXiv: 2012.07645}, keywords = {Quantum Physics}, }`

- Ł. Czekaj, A. B. Sainz, J. Selby, and M. Horodecki, “Correlations constrained by composite measurements,” Arxiv:2009.04994 [quant-ph], 2020.

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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, title = {Correlations constrained by composite measurements}, url = {http://arxiv.org/abs/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.}, urldate = {2021-07-28}, journal = {arXiv:2009.04994 [quant-ph]}, author = {Czekaj, Łukasz and Sainz, Ana Belén and Selby, John and Horodecki, Michał}, month = sep, year = {2020}, note = {arXiv: 2009.04994}, keywords = {Quantum Physics}, }`

- A. Z. Goldberg, P. de la Hoz, G. Bjork, A. B. Klimov, M. Grassl, G. Leuchs, and L. L. Sanchez-Soto, “Quantum concepts in optical polarization,” Arxiv:2011.03979 [quant-ph], 2020.

[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], 2020.

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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}, }`

- S. Das, S. Bäuml, M. Winczewski, and K. Horodecki, “Universal limitations on quantum key distribution over a network,” Arxiv:1912.03646 [quant-ph], 2020.

[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}, }`

- W. Song, M. Wieśniak, N. Liu, M. Pawłowski, J. Lee, J. Kim, and J. Bang, “Tangible Reduction of Sample Complexity with Large Classical Samples and Small Quantum System,” Arxiv:1905.05751 [quant-ph], 2020.

[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}, }`

### 2019

- M. Eckstein and P. Horodecki, “The experiment paradox in physics,” Arxiv:1904.04117 [gr-qc, physics:hep-th, physics:physics, physics:quant-ph], 2019.

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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, title = {The experiment paradox in physics}, url = {http://arxiv.org/abs/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.}, urldate = {2021-07-28}, journal = {arXiv:1904.04117 [gr-qc, physics:hep-th, physics:physics, physics:quant-ph]}, author = {Eckstein, Michał and Horodecki, Paweł}, month = apr, year = {2019}, note = {arXiv: 1904.04117}, keywords = {Physics - History and Philosophy of Physics, General Relativity and Quantum Cosmology, High Energy Physics - Theory, Physics - Classical Physics, Quantum Physics}, }`