Online Seminars on Quantum Information in Poland. Warsaw time zone.

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Pretty good simulation of all quantum measurements by projective measurements in finite-dimensional quantum systems

Date: Wednesday, July 12, 2023
Time: 14:00
Host: ICTQT Seminar, room 319

Speaker: Michał Oszmaniec (University of Warsaw)

Abstract

In quantum theory general measurements are described by so-called Positive Operator-Valued Measures (POVMs). In this work we show that in d-dimensional quantum systems an application of depolarizing noise with constant (independent of d) visibility parameter makes any POVM simulable by a randomized implementation of projective measurements that do not require any auxiliary systems to be realized. This result significantly limits the asymptotic advantage that POVMs can offer over projective measurements in various information-processing tasks, including state discrimination, shadow tomography or quantum metrology. We also apply our findings to questions originating from quantum foundations. First, we asymptotically improve the range of parameters for which Werner and isotropic states have local models for generalized measurements (by factors of d and log(d) respectively). Second, we give asymptotically tight (in terms of dimension) bounds on critical visibility for which all POVMs are jointly measurable. On the technical side we use recent advances in POVM simulation, the solution to the celebrated Kadison-Singer problem, and a method of approximate implementation of a class of “nearly rank one” POVMs by a convex combination of projective measurements, which we call dimension-deficient Naimark extension theorem. The talk will be based on upcoming joint work with Michał Kotowski (MIM UW)

A new definition of genuine multipartite nonlocality

Date: Wednesday, July 5, 2023
Time: 14:30
Host: Quantum Information and Quantum Computing Working Group, Meeting ID: 844 2780 6931
Zoom link: https://us06web.zoom.us/j/84427806931
Passcode: nisq

Speaker: Owidiusz Makuta (CFT-PAS)

Abstract The conventional definition of nonlocality pertains solely to two subsystems, which falls short in fully describing the potential of quantum mechanics observed in multipartite systems. This role is better fulfilled by a variant of nonlocality called Genuine Multipartite NonLocality (GMNL). However, as researchers delved deeper into the concept of GMNL, an intriguing observation regarding its behaviour in certain scenarios came to light. It was discovered that it is possible to start with a measurement statistic that does not exhibit GMNL, and by grouping specific events together, end up with a GMNL measurement statistic. To this end, a new definition of GMNL based on the network scenario was introduced called LOSR-GMNL. In our work, we study this new definition with the aim of characterising the set of quantum states exhibiting LOSR-GMNL. We show that large classes of graph states for arbitrary, prime local dimension, as well as states that are sufficiently close to said states, are LOSR-GMNL.

A particle conserving approach to AC-DC driven interacting quantum dots with superconducting leads

Date: Wednesday, June 21, 2023
Time: 12:15
Host: ICTQT Seminar, room 319
Zoom link: https://zoom.us/j/91536504866?pwd=SkkvVDByRmlmRTBWNGxWVlIzd0hFdz09

Speaker: Julian Siegl

Abstract The combined action of a DC bias and a microwave drive on the transport characteristic of a superconductor-quantum dot-superconductor junction is investigated. To cope with time dependent non-equilibrium effects and interactions in the quantum dot, we develop a general formalism for the dynamics of the density operator based on a particle conserving approach to superconductivity. Without invoking a broken U (1) symmetry, we identify a dynamical phase connected to the coherent transfer of Cooper pairs across the junction. In the weak coupling limit, we show that besides quasiparticle transport, proximity induced superconducting correlations manifest in anomalous pair tunneling involving the transfer of a Cooper pair. The resulting generalized master equation in presence of the microwave drive showcases the characteristic bichromatic response due to the combination of the AC Josephson effect and an AC voltage. Analytical expressions for all harmonics in the driving frequency of both the current and the reduced dot operator are given for arbitrary driving strength. For the net DC current the resulting photon assisted processes give rise to rich current-voltage characteristics. In addition to photon assisted subgap transport we find regions of total current inversion in the stability diagram. There, the junction acts as a pump with the net DC current flowing against the applied DC bias. The first harmonic of the current, being closely related to the nonlinear dynamic susceptibility of the junction, is discussed at finite applied DC bias.

Quantum transfer of interacting and entangled qubits

Date: Monday, June 19, 2023
Time: 14:15
Host: Quantum Chaos and Quantum Information (Jagiellonian University)
Zoom link: https://us05web.zoom.us/j/89601291160?
Passcode: please contact albertrico23 at gmail.com

Speaker: Tony Apollaro (University of Malta)

Abstract The transfer of quantum information between different locations is key to many quantum information processing tasks. Whereas, the transfer of a single qubit state has been extensively investigated, the transfer of a many-body system configuration has insofar remained elusive. We address the problem of transferring the state of n interacting qubits [1]. Both the exponentially increasing Hilbert space dimension, and the presence of interactions significantly scale-up the complexity of achieving high-fidelity transfer. By employing tools from random matrix theory and using the formalism of quantum dynamical maps, we derive a general expression for the average and the variance of the fidelity of an arbitrary quantum state transfer protocol for n interacting qubits. We find that the average fidelity decreases with the amount and the type of entanglement in the sender state [2]. Finally, by adopting a weak-coupling scheme in a spin chain, we obtain the explicit conditions for high-fidelity transfer of 3 and 4 interacting qubits. [1] Tony J G Apollaro et al, Quantum transfer of interacting qubits, 2022 New J. Phys. 24 083025 https://iopscience.iop.org/article/10.1088/1367-2630/ac86e7 [2] Tony J G Apollaro et al, Entangled States Are Harder to Transfer than Product States, Entropy 2023, 25(1), 46; https://doi.org/10.3390/e25010046

Numerical modeling of quantum dynamical processes

Date: Friday, June 16, 2023
Time: 12:15
Host: IFTiA Seminar, room 361

Speaker:  Janek Kozicki (Gdańsk University of Technology)

Abstract In this talk I present a high-precision (15, 18 or 33 decimal places) C++ implementation of quantum dynamics time propagation algorithms for both time-independent and time-dependent Hamiltonian with an inhomogeneous source term. Moreover I present an extension of both algorithms for time propagation to handle arbitrary number of coupled electronic levels. I have performed a careful validation of these implementations comparing my results with numerous reference results such as: a forced harmonic oscillator with an inhomogeneous source term or an atom in an intense laser field. Next, I apply these new algorithms to calculate the rovibrational predissociation in the KLi molecule and compare it with experiment. In doing so I have shown that the KLi quasi-bound state decays exponentially in time and I have described a robust method to calculating the widths of quasi-bound vibrational levels. Also I have calculated the decay in the NaRb dimer using three coupled electronic states. I have performed a fit of this decay to a modified power-law and I have shown that this fit is better than a fit to the exponential decay. I hope that those results are of considerable relevance to the design of experiments.

Quantum transfer of interacting and entangled qubits

Date: Thursday, June 15, 2023
Time: 12:00
Host: ICTQT Seminar, room 319
Zoom link: https://zoom.us/j/91536504866?pwd=SkkvVDByRmlmRTBWNGxWVlIzd0hFdz09

Speaker: Tony Apollaro (University of Malta)

Abstract The transfer of quantum information between different locations is key to many quantum information processing tasks. Whereas, the transfer of a single qubit state has been extensively investigated, the transfer of a many-body system configuration has insofar remained elusive. We address the problem of transferring the state of n interacting qubits [1]. Both the exponentially increasing Hilbert space dimension, and the presence of interactions significantly scale-up the complexity of achieving high-fidelity transfer. By employing tools from random matrix theory and using the formalism of quantum dynamical maps, we derive a general expression for the average and the variance of the fidelity of an arbitrary quantum state transfer protocol for n interacting qubits. We find that the average fidelity decreases with the amount and the type of entanglement in the sender state [2]. Finally, by adopting a weak-coupling scheme in a spin chain, we obtain the explicit conditions for high-fidelity transfer of 3 and 4 interacting qubits. [1] Tony J G Apollaro et al, Quantum transfer of interacting qubits, 2022 New J. Phys. 24 083025 https://iopscience.iop.org/article/10.1088/1367-2630/ac86e7 [2] Tony J G Apollaro et al, Entangled States Are Harder to Transfer than Product States, Entropy 2023, 25(1), 46; https://doi.org/10.3390/e25010046

Catalysis in quantum optics

Date: Monday, June 12, 2023
Time: 14:15
Host: Quantum Chaos and Quantum Information (Jagiellonian University)
Zoom link: https://us05web.zoom.us/j/89601291160?
Passcode: please contact albertrico23 at gmail.com

Speaker: Alexssandre de Oliveira Junior  (Faculty of Physics, UJ)

Abstract Catalysis plays a key role in many scientific areas, most notably in chemistry and biology. In this talk, I will present a catalytic process in a paradigmatic quantum optics setup, namely the Jaynes-Cummings model, where an atom interacts with an optical cavity. The atom plays the role of the catalyst, and allows for the deterministic generation of non-classical light in the cavity. Considering a cavity prepared in a “classical” coherent state, and choosing appropriately the atomic state and the interaction time, we obtain an evolution with the following properties. First, the state of the cavity has been modified, and now features non-classicality, as witnessed by sub-Poissonian statistics or Wigner negativity. Second, the process is catalytic, in the sense that the atom is deterministically returned to its initial state exactly, and could then in principle be re-used multiple times. We investigate the mechanism of this catalytic process, in particular highlighting the key role of correlations and quantum coherence.

A de Finetti theorem for quantum causal structures

Date: Wednesday, June 7, 2023
Time: 14:00
Host: ICTQT Seminar, room 411 New Rectorat
Zoom link: https://zoom.us/j/91536504866?pwd=SkkvVDByRmlmRTBWNGxWVlIzd0hFdz09

Speaker: Fabio Costa (University of Queensland)

Abstract What does it mean for a causal structure to be “unknown”? Can we even talk about “repetitions” of an experiment without prior knowledge of causal relations? And under what conditions can we say that a set of processes are independent and identically distributed (i.i.d.)? Similar questions for classical probabilities, quantum states, and quantum channels are beautifully answered by “de Finetti theorems”, which connect a simple and easy-to-justify condition—symmetry under exchange—to a very particular multipartite structure: a mixture of identical states/channels. Practically, they provide the foundations for principle-based Bayesian methods, e.g., in tomography. Apart from the foundational relevance, de Finetti representations for general causal structures would be useful in the analysis of multi-time, non-Markovian processes, with applications to state-of-the-art quantum devices. At face value, it appears that each causal structure or assumption on causal structure requires its own de Finetti theorems. Fortunately, I will show that each scenario can be mapped to a linear constraint on quantum states. By proving a de Finetti representation for states subject to a sufficiently large class of constraints, we can derive all the desired results for a broad class of processes.

Exploring General Relativistic Approaches to Modeling Large-Scale Structure Formation in the Universe

Date: Wednesday, June 7, 2023
Time: 12:30
Host: CENTER FOR THEORETICAL PHYSICS COLLOQUIUM
Zoom link: https://zoom.us/j/82380380442?pwd=Z3IyeEhlZmFHU1B2M2VUVVJhODkrUT09
Passcode: 134595

Speaker: Ismael Delgado Gaspar (National Centre for Nuclear Research)

Abstract The current era of precision cosmology has provided us with high-quality observational data across astrophysical and cosmological scales. Interpreting these data requires robust modeling of self-gravitating systems. However, the use of exact solutions of Einstein’s equations and non-linear approaches has been less favored in examining the observations. In this talk, we explore different relativistic approaches to model the formation of large-scale structures, with an emphasis on exact solutions and the relativistic Zeldovich approximation (RZA). We show how the dynamical freedom of the Lemaıtre-Tolman-Bondi (LTB) and Szekeres exact solutions allows for a realistic description of three-dimensional networks of cold dark matter structures. We discuss the “silent property” exhibited by these solutions and its use in developing alternative approaches. Specifically, we establish a connection between the LTB/Szekeres models and RZA, allowing us to develop a new general relativistic method. By exploring these various methods, we aim to enhance our understanding of the role of curvature and relativistic corrections in the formation of large-scale structures.

Entanglement detection with trace polynomials

Date: Monday, June 5, 2023
Time: 14:15
Host: Quantum Chaos and Quantum Information (Jagiellonian University)
Zoom link: https://us05web.zoom.us/j/89601291160?
Passcode: please contact albertrico23 at gmail.com

Speaker: Albert Rico  (Faculty of Physics, UJ)

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