Speaker: Piotr Kopszak
Abstract
Port-based teleportation (PBT) is a quantum teleportation protocol, in which the parties exploit joint measurements performed on $N$ shared $d$-dimensional maximally entangled pairs (the resource) and the state to be teleported, with the addition of the one-way classical communication. The lack of correction in the last step is an essential feature distinguishing PBT from standard quantum teleportation. In my talk I shall consider the idea of entanglement recycling, i.e. the repeated use of the same resource for multiple rounds of PBT. The question is how the resource degrades after one or multiple uses. To answer it, we analyse the structure of the measurement employed in the protocol (the square-root-measurement, to be precise), depending greatly on the symmetries present in the system. In particular, as the result we evaluate its roots and compositions. These findings allow us to present the explicit formula for the recycling fidelity involving only group-theoretic parameters describing irreducible representations of the symmetric group $S(n)$. Additionally, I shall present the analysis of the resource degradation in the optimal PBT.
Zoom link : https://zoom.us/j/94361414308?pwd=cC8xaTNBd290Rzk1TUgrZzZhcWczUT09
Speaker: Piotr Kopszak
Abstract
Port-based teleportation (PBT) is a quantum teleportation protocol, in which the parties exploit joint measurements performed on $N$ shared $d$-dimensional maximally entangled pairs (the resource) and the state to be teleported, with the addition of the one-way classical communication. The lack of correction in the last step is an essential feature distinguishing PBT from standard quantum teleportation. In my talk I shall consider the idea of entanglement recycling, i.e. the repeated use of the same resource for multiple rounds of PBT. The question is how the resource degrades after one or multiple uses. To answer it, we analyse the structure of the measurement employed in the protocol (the square-root-measurement, to be precise), depending greatly on the symmetries present in the system. In particular, as the result we evaluate its roots and compositions. These findings allow us to present the explicit formula for the recycling fidelity involving only group-theoretic parameters describing irreducible representations of the symmetric group $S(n)$. Additionally, I shall present the analysis of the resource degradation in the optimal PBT.
Speaker: Jordan Cotler (Harvard University)
Abstract
We introduce a theoretical framework to study experimental physics using quantum complexity theory. This allows us to address: what is the computational complexity of an experiment? For several ‘model’ experiments, we prove that there is an exponential savings in resources if the experimentalist can entangle apparatuses with experimental samples. A novel example is the experimental task of determining the symmetry class of a time evolution operator for a quantum many-body system. Some of our complexity advantages have been realized on Google’s Sycamore processor, demonstrating a real-world advantage for learning algorithms with a quantum memory.
References: ArXiv:2111.05881 ArXiv:2111.05874 ArXiv:2112.00778
Speaker: Alejandro Jenkins (ICTQT)
Abstract
The question of how heat is persistently transported from the Sun’s photosphere (at about 6,000 K) to the much hotter corona (at about 10^6 K) is one of the great open puzzles in astrophysics. Using the quantum Markovian master equation, 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 small magnetic fields. Energy is mostly carried by megahertz Alfven waves that scatter elastically until they reach a height at which they can dissipate via mode conversion. This model gives the right power flux for coronal heating and may account for “chromospheric evaporation” leading to impulsive heat transport into the corona.
Speaker: H. Chau Nguyen (University of Siegen, Germany)
Speaker: Erik Aurell (KTH Royal Institute of Technology, Stockholm)
Abstract
A well-studied model in open quantum system theory is a system interacting with a thermal bath of harmonic oscillators at finite temperature. This provides a quantum mechanical model of a classical resistive element in a circuit, and includes as famous examples the Caldeira-Leggett theory of quantum Brownian motion, and the “spin-boson model”. Such environments however also include baths of thermal photons and phonons, and putative baths of gravitons. As long as the environment consists of harmonic oscillators interacting linearly with the system, and starting in a thermal state, the environmental degrees of freedom can be integrated out using the Feynman-Vernon method.
I will first present the open system dynamics of a test particle interacting linearly with a thermal bath of photons, following [1] and [2]. I will then discuss the resulting energy change of the bath (quantum heat) using the Feynman-Vernon approach. I will discuss what one can say if the bath temperature is very low or zero, i.e. if the test particle interacts with the electromagnetic vacuum.
I will then consider a test particle interacting with a gravitational field quantized in the weak-field (linear) approximation. I will review the recent theory of Parikh, Wilczek and Zahariade [3] describing an arm of a gravitational wave detector interacting with this kind of quantized gravitational field. Following Parikh et al I will show that an effective nonlinear friction force follows analogously to the way ordinary friction appears in the Caldeira-Leggett theory. I will discuss the random force from the vacuum on the test particle, and the heating of such a gravitational vacuum by the interaction with the test particle.
I will end by discussing what this says or does not say about the entropy production in the electro-magnetic vacuum and gravitational vacuum.
[1] Heinz-Peter Breuer and Francesco Petruccione, “Destruction of quantum coherence through emission of bremsstrahlung”, Phys. Rev. A 63, 032102 (2001)
[2] Heinz-Peter Breuer and Francesco Petruccione, Theory of Open Quantum Systems (2002), Chapter 12
[3] Maulik Parikh, Frank Wilczek and George Zahariade, “Signatures of the quantization of gravity at gravitational wave detectors”, Phys. Rev. D 104, 046021 (2021)
Speaker: Marcin Łobejko (University of Gdańsk)
Speaker: Filip Maciejewski (CFT PAN Warszawa)
Abstract
We introduce operational distance measures between quantum states, measurements, and channels based on their average-case distinguishability. To this end, we analyze the average Total Variation Distance (TVD) between statistics of quantum protocols in which quantum objects are intertwined with random circuits and subsequently measured in a computational basis. We show that for circuits forming approximate 4-designs, the average TVDs can be approximated by simple explicit functions of the underlying objects, which we call average-case distances. The so-defined distances capture average-case distinguishability via moderate-depth random quantum circuits and satisfy many natural properties. We apply them to analyze the effects of noise in quantum advantage experiments and in the context of efficient discrimination of high-dimensional quantum states and channels without quantum memory. Furthermore, based on analytical and numerical examples, we argue that average-case distances are better suited for assessing the quality of NISQ devices than conventional distance measures such as trace distance and the diamond norm.
The talk is based on recent preprints: arXiv:2112.14283 and arXiv:2112.14284.