Joint Seminar on Quantum Information and Technologies
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until 2023/2024 Quantum Information Seminar | YouTube channel
2024-12-12 (Thursday)
room 0.06, Pasteura 5 at 11:15 
Michał Oszmaniec (CFT PAN)Saturation and recurrence of quantum complexity in random local quantum dynamics
Quantum complexity is a measure of the minimal number of elementary operations required to approximately prepare a given state or unitary channel. Recently, this concept has found applications beyond quantum computing -- in studying the dynamics of quantum many-body systems and the long-time properties of AdS black holes. In this context Brown and Susskind conjectured that the complexity of a chaotic quantum system grows linearly in time up to times exponential in the system size, saturating at a maximal value, and remaining maximally complex until undergoing recurrences at doubly-exponential times. In this work we prove the saturation and recurrence of complexity in two models of chaotic time evolutions based on (i) random local quantum circuits and (ii) stochastic local Hamiltonian evolution. Our results advance an understanding of the long-time behaviour of chaotic quantum systems and could shed light on the physics of black hole interiors. From a technical perspective our results are based on a quantitative connection between spectral gaps of random walks on the unitary group and the property of approximate equidistribution, which turns out to be crucial for establishing saturation and recurrence.
The talk is based on a joint work with Marcin Kotowski, Nick Hunter Jones and Michał Horodecki, preprint arXiv:2205.09734 (accepted for publication in Physical Review X).
The talk is based on a joint work with Marcin Kotowski, Nick Hunter Jones and Michał Horodecki, preprint arXiv:2205.09734 (accepted for publication in Physical Review X).
2024-12-05 (Thursday)
room 0.06, Pasteura 5 at 11:15
Maximilian Lock (IQOQI Vienna)The Emergence of Irreversibility in Quantum Theory: Entropy and Measurement
The second law of thermodynamics states that the entropy of an isolated system can only increase over time, thereby distinguishing the past from the future. This seems to conflict with the reversible evolution of isolated quantum systems, which preserves the von Neumann entropy. However, counterintuitively, many observables in large isolated systems do reach equilibrium, despite the unitary evolution of the system's state. We characterise the extent to which any observable exhibits this emergent irreversibility, as determined by the relationship between the microstates associated with the reversible evolution and the macrostates associated with the observable. We demonstrate how a version of the second law of thermodynamics can be recovered in isolated quantum systems, and analyse the fluctuations from equilibrium that reveal the underlying reversible dynamics, finding that these fluctuations diminish as the system size increases. We then explore the hypothesis that the apparent irreversibility of the quantum measurement process is a manifestation of the second law of thermodynamics, resulting in possible criteria for when a physical system constitutes an observer.
2024-11-28 (Thursday)
room 0.06, Pasteura 5 at 11:15
Andrey Rakhubovsky (Palacký University, Olomouc)Non-classical states of mechanical motion in levitated optomechanics
Optomechanics with levitated subwavelength dielectric nanoparticles(NPs) has attracted strong interest since its conception.Good isolation of the mechanical motion from the environment and therelatively high mass of the NPs make these systems especially suitablefor wide applications, including quantum sensing and fundamentalphysics tests.Inspired by the recent achievements of Gaussian control of the NPs'motion, we consider theoretically the prospects of reachingnon-classical and eventually quantum non-Gaussian states of theirmotion.In this talk, we will start with a general introduction to the fieldof optomechanics, with an emphasis on the peculiarities of thelevitated NPs.We will then consider a protocol to generate optomechanicalentanglement via pulsed two-mode squeezing interaction, and aprocedure to enhance the entanglement using Bayesian optimization ofthe control parameters.
2024-11-21 (Thursday)
room 0.06, Pasteura 5 at 11:15
Olivier Reardon-Smith (CFT PAN)Magic and adding things up: State of the art classical simulations of quantum computations
A surprising and non-intuitive feature of the universe is that there appear to be exactly two possible types of computer (up to polynomial equivalence) - classical and quantum computers. While quantum computers are widely expected to be faster than classical computers at certain tasks, for now classical computers are dramatically more reliable, more powerful and more available than their quantum counterparts. This motivates us to use classical computers to simulate quantum computers. In addition to being of obvious practical use, classical simulations of quantum computations have interesting theoretical implications. Intuitively those computations which may be efficiently simulated by a classical computer are somehow "less quantum" while those which are prohibitively expensive to simulate classically are "more quantum". I will discuss some ways of quantifying non-classicality in the form of "magic" resources, as well as some classical simulation algorithms whose run-times are determined by the amount of magic in the quantum computation being simulated.
2024-11-14 (Thursday)
room 0.06, Pasteura 5 at 11:15
Mateusz Mazelaniki (QOT CENT UW)Microwave sensing with Rydberg atoms - from classical radiometry to quantum-enhanced metrology
2024-11-07 (Thursday)
room 0.06, Pasteura 5 at 11:15
Zoltan Zimboras (Wigner Research Center & Algorithmiq)Myths around Quantum Computations before Full Fault Tolerance: What no-go theorems rule out and what they don't
2024-10-31 (Thursday)
room 0.06, Pasteura 5 at 11:30
Wojciech Górecki (University of Pavia)Mutual Information Bounded by Fisher Information
2024-10-24 (Thursday)
room 0.06, Pasteura 5 at 11:30
Dariusz Chruściński (UMK Toruń)Constraints for relaxation rates for open quantum systems
Seminar recording on Youtube
Relaxation rates provide important characteristics both for classical and quantum processes. Essentially they control how fast the system thermalizes, equilibrates, decohere, and/or dissipate. Moreover, very often they are directly accessible to be measured in the laboratory and hence they define key physical characteristics of the system. In my talk I show that relaxation rates for any Markovian evolution of an open system satisfy a universal tight constraint (valid for all quantum systems with finite number of energy levels). Some implications of this result are discussed as well.
Relaxation rates provide important characteristics both for classical and quantum processes. Essentially they control how fast the system thermalizes, equilibrates, decohere, and/or dissipate. Moreover, very often they are directly accessible to be measured in the laboratory and hence they define key physical characteristics of the system. In my talk I show that relaxation rates for any Markovian evolution of an open system satisfy a universal tight constraint (valid for all quantum systems with finite number of energy levels). Some implications of this result are discussed as well.
2024-10-17 (Thursday)
room 0.06, Pasteura 5 at 11:30
Konrad Banaszek (QOT CENT UW)Back from deep space: Optical communications in the photon starved regime
2024-10-10 (Thursday)
room 0.06, Pasteura 5 at 11:30
Łukasz Cywiński (IFPAN)