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

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Past events

Hybrid Quantum-Classical Codes

Date: Monday, November 28, 2022
Time: 14:15
Host: Quantum Chaos and Quantum Information (Jagiellonian University), Meeting id: 896 0129 1160
Zoom link: https://us05web.zoom.us/j/89601291160?
Passcode: please contact albertrico23 at gmail.com

Speaker: Andrew Nemec (Duke University, USA)

Abstract Hybrid codes simultaneously encode both quantum and classical information together, giving an advantage over coding schemes where the quantum and classical information are transmitted separately. We construct the first known families of hybrid codes that are guaranteed to provide an advantage over quantum codes, as well as also giving a construction of hybrid codes from subsystem codes that allow for different minimum distances for the encoded quantum and classical information . We also show how hybrid codes can be applied to the problem of faulty syndrome measurements and lead to the construction of new quantum data-syndrome codes.

Graph Approach to Entanglement Generation in Linear Quantum Networks

Date: Wednesday, November 23, 2022
Time: 14:00
Host: ICTQT, room 317
Zoom link: https://zoom.us/j/7763535903?pwd=Q1VwQWhHME9GcjBJaUhEZWoxazhDdz09
Passcode: 743020

Speaker: Seungbeom Chin (ICTQT / Sungkyunkwan University)

Abstract We propose a graph method to systematically search for schemes that obtains genuine entanglement in arbitrary N-partite linear quantum networks (LQNs). While the indistinguishability of quantum particles is widely used as a resource for the generation of entanglement, it is challenging to devise a suitable LQN that carries a specific entangled state. Our research presents a mapping process of arbitrary LQNs to graphs, which provides an organized strategy for designing LQNs to generate multipartite entanglement with and without postselection. This talk is based on Quantum 5 (2021), 611 and arXiv:2211.04042.

Mutually unbiased measurements

Date: Wednesday, November 23, 2022
Time: 12:30
Host: Center for Theoretical Physics Colloquium
Zoom link: https://zoom.us/j/82380380442?pwd=Z3IyeEhlZmFHU1B2M2VUVVJhODkrUT09
Passcode: 134595

Speaker: Máté Farkas (ICFO, Barcelona)

Abstract Mutually unbiased bases (MUBs) correspond to measurements in quantum theory that are complementary: if a measurement in a basis yields a definite outcome on a given quantum state, then a measurement in a basis unbiased to the first one yields a uniformly random outcome on the same state. Simple examples of MUBs are photon polarisation measurements in the horizontal and vertical directions, or spin measurements in the z and x directions of a spin-1/2 particle. Their complementary property makes MUBs highly useful in various quantum information processing tasks, such as quantum state tomography, communication tasks, Bell inequalities, and quantum cryptography.

In this talk—after an introduction to MUBs and their use in quantum information—I will introduce a generalisation of MUBs termed mutually unbiased measurements (MUMs). MUMs retain the complementary property of MUBs in a “device-independent” manner: in order to define MUMs, one does not need to refer to the Hilbert space dimension (the number of degrees of freedom, which is not an observable property), only to the outcome number of the measurements (an operational property). I will discuss the mathematical characterisation and constructions of MUMs, and the fundamental similarities and differences between MUBs and MUMs. Then, I will introduce a family of Bell inequalities tailored to MUMs, and show how to use these inequalities for device-independent quantum cryptography, as well as how to use these Bell inequalities to tackle a long-standing open problem on the number of MUBs in a given Hilbert space dimension.

All quantum spectra in one shot

Date: Wednesday, November 23, 2022
Time: 12:30
Host: ICTQT, room 317
Zoom link: https://zoom.us/j/7763535903?pwd=Q1VwQWhHME9GcjBJaUhEZWoxazhDdz09
Passcode: 743020

Speaker: Carlos L. Benavides-Riveros (MPI for the Physics of Complex Systems, Dresden / University of Trento)

Abstract Determining the properties of the excitations in quantum many-body systems is a fundamental problem across almost all sciences. For instance, quantum excited states underpin new states of matter, support biological processes such as vision, or determine optoelectronic properties of photovoltaic devices. Yet, while ground-state properties can be determined by rather accurate computational methods, there remains a need for theoretical and computational developments to target excited states efficiently. Inspired by the duplication of the Hilbert space used to study black-hole entanglement and the electronic pairing of conventional superconductivity, we have recently developed a new variational scheme to compute the full spectrum of a quantum many-body Hamiltonian, rather than only its ground or the lowest-excited states. An important feature of our proposed scheme is that these spectra can be computed in a one-shot calculation. The scheme thus provides a novel variational platform for excited-state physics. In the talk, I will show an explicit calculation for a Fermi-Hubbard Hamiltonian, based on a unitary coupled-cluster ansatz. Since our approach is suitable for efficient implementation on quantum computers, we believe this “variational quantum diagonalizer” has the potential to enable unprecedented calculations of excited-state processes of quantum many-body systems.
The talk is based on C. L. Benavides-Riveros et al., Phys. Rev. Lett. 129, 066401 (2022)

Certification of unknown quantum measurements

Date: Monday, November 21, 2022
Time: 14:15
Host: Quantum Chaos and Quantum Information (Jagiellonian University), Meeting id: 896 0129 1160
Zoom link: https://us05web.zoom.us/j/89601291160?
Passcode: please contact albertrico23 at gmail.com

Speaker: Zbigniew Puchala (IITiS Gliwice)

Quantum Error Correction with Superconducting Transmon Qubits

Date: Wednesday, November 16, 2022
Time: 15:30
Host: Team-Net Quantum Computing Colloquium
Zoom link: https://us06web.zoom.us/j/81247873600
Passcode: teamnet

Speaker: Barbara Terhal (QuTech, TU Delft & Forschungszentrum Juelich)

Abstract We review the goals of quantum error correction, in particular the use of the surface code. We discuss some of the results and challenges in experimental quantum error correction, in particular with respect to qubit leakage.

Computable and operationally meaningful multipartite entanglement measures

Date: Monday, November 14, 2022
Time: 14:15
Host: Quantum Chaos and Quantum Information (Jagiellonian University)
Zoom link: https://us02web.zoom.us/j/85911849629
Passcode: subspace

Speaker: Nicolás Gigena (University of Warsaw)

Abstract We introduce a family of multipartite entanglement measures called “concentratable entanglements”. These are connected to previously defined entanglement measures and have an operational interpretation in terms of probabilistic concentration of entanglement into Bell pairs. Furthermore, we show that these quantities can be estimated on a quantum computer by implementing a parallelized SWAP test, opening a research direction for measuring multipartite entanglement measures on quantum devices.

An elegant scheme of self-testing for multipartite Bell inequalities

Date: Thursday, November 10, 2022
Time: 12:00
Host: ICTQT, room 317
Zoom link: https://zoom.us/j/7763535903?pwd=Q1VwQWhHME9GcjBJaUhEZWoxazhDdz09
Passcode: 743020

Speaker: Ekta Panwar (UG/ICTQT)

Abstract The predictions of quantum theory are incompatible with local-causal explanations. This phenomenon is called Bell non-locality and is witnessed by the violation of Bell-inequalities. The maximal violation of certain Bell-inequalities can only be attained in an essentially unique manner. This feature is referred to as self-testing and constitutes the most accurate form of certification of quantum devices. While self-testing in bipartite Bell scenarios has been thoroughly studied, self-testing in the more complex multipartite Bell scenarios remains largely unexplored. This work presents a simple and broadly applicable self-testing argument for N-partite correlation Bell inequalities with two binary outcome observables per party. Our proof technique forms a generalization of the Mayer-Yao formulation and is not restricted to linear Bell-inequalities, unlike the usual sum of squares method. To showcase the versatility of our proof technique, we obtain self-testing statements for N party Mermin-Ardehali-Belinskii-Klyshko (MABK) and Werner-Wolf-Weinfurter-Zukowski-Brukner (WWWZB) family of linear Bell inequalities, Uffink’s family of N party quadratic Bell-inequalities, and the novel Uffink’s complex-valued N partite Bell expressions.

Nobel Prize in Physics 2022 – abandon all classical intuition all ye who enter here…

Date: Wednesday, November 9, 2022
Time: 12:30
Host: Center for Theoretical Physics Colloquium
Zoom link: https://zoom.us/j/82380380442?pwd=Z3IyeEhlZmFHU1B2M2VUVVJhODkrUT09
Passcode: 134595

Speaker: Rafał Demkowicz Dobrzański (University of Warsaw)

Device-independent quantum cryptography: disproving the sufficiency of Bell nonlocality and verifying the noise-robustness of current implementations

Date: Monday, November 7, 2022
Time: 14:15
Host: Quantum Chaos and Quantum Information (Jagiellonian University)
Zoom link: https://us02web.zoom.us/j/85911849629
Passcode: subspace

Speaker: Jan Kolodynski (Center of New Technologies, University of Warsaw) 

Abstract Device-independent quantum key distribution (DIQKD) constitutes the most pragmatic approach to quantum cryptography that does not put any trust in the inner workings of the devices. This is possible by constructing security proofs at the level of correlations being shared by the end-users, leveraging from the phenomenon of Bell nonlocality. In particular, quantum nonlocality allows one then to lower-bound the asymptotically achievable key rates, even in the presence of the most general eavesdropping attacks. However, only recently first proof-of-principle implementations of DIQKD have been demonstrated, as the device-independent framework imposes very stringent requirements on the noise tolerance, even in the absence of any eavesdroppers. In our works, we follow a complementary approach in which we propose an easy-to-optimise attack on any DIQKD protocol, with help of which we construct upper bounds on the asymptotic key. On one hand, it allows us to disprove a long-standing conjecture that any form of Bell nonlocality is sufficient for distributing secret keys in a device-independent manner. On the other, it allows us to verify that current state-of-the-art implementations already operate very close to the ultimate noise thresholds, and cannot be thus improved by resorting to more profound security-proof techniques.

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