Warsaw Quantum Computing Group Meetings – CFT PAN – Centrum Fizyki Teoretycznej Polskiej Akademii Nauk

Full nonlocality of non-maximally entangled states

cft — Mon, 23 Feb 2026 11:04:35 +0000

Abstract:
It is a well-established fact that some quantum correlations are
nonlocal, meaning they cannot be described by a local hidden variable
model. Beyond this, certain quantum correlations are so strong that
they cannot be reproduced even if one allows only a small fraction of
the rounds to admit a local hidden variable description. Such
correlations are called fully nonlocal and they give rise to Bell
inequalities in which the quantum bound saturates the non-signaling
bound. A famous example is the Peres–Mermin square, in which the
underlying state is a four-dimensional maximally entangled state.
Surprisingly, examples of full nonlocality are rare and typically
restricted to maximally entangled states. In this work, we show that
in every local Hilbert space dimension d≥4 there exist non-maximally
entangled pure states that are fully nonlocal. In fact, we derive
simple criteria ensuring full nonlocality that depend only on the
smallest and largest Schmidt coefficients. Furthermore, we show that
all pure entangled states can be activated to exhibit full nonlocality
in the many-copy scenario.

Remote guests are invited via Zoom:
Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1
Meeting ID: 842 4891 1743
Passcode: 394021

Random unitaries in extremely low depth

cft — Mon, 19 Jan 2026 07:27:17 +0000

Abstract:

We prove that random quantum circuits on any geometry, including a one-dimensional line, can form approximate unitary designs over n qubits with logarithmic depth in n. In a similar spirit, we construct pseudorandom unitaries in one-dimensional circuits with polylogarithmic depth, and in all-to-all connected circuits with polyloglogarithmic depth. In all three settings, the dependence on n is optimal and yields an exponential improvement over previously known results.

These shallow quantum circuits have low complexity and generate only short-range entanglement, yet they are indistinguishable from unitaries of exponential complexity. Our construction proceeds by gluing together local random unitaries acting on patches of qubits of logarithmic or polylogarithmic size, thereby forming a global random unitary on all n qubits.

In the case of approximate unitary designs, the local unitaries are drawn from existing constructions of approximate unitary k-designs and therefore inherit optimal scaling in k. For pseudorandom unitaries, the local components are drawn from existing PRU constructions.

Applications of our results include showing that classical shadows generated by one-dimensional logarithmic-depth Clifford circuits are as powerful as those obtained from deep circuits, demonstrating a superpolynomial quantum advantage in learning low-complexity physical systems, and establishing quantum hardness for recognizing phases of matter with topological order.

Remote guests are invited via Zoom:
Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1
Meeting ID: 842 4891 1743
Passcode: 394021

CET, Room D

Certifying Majorana Fermions with Elegant-Like Bell Inequalities and a New Self-Testing Equivalence

cft — Tue, 13 Jan 2026 07:18:37 +0000

Abstract:
Bell inequalities provide a fundamental tool for probing nonlocal correlations, yet their quantum bound is rarely accessible analytically. In my talk, I will present a general construction of Bell inequalities whose quantum bound can be computed exactly. This framework generalizes the Clauser–Horne–Shimony–Holt and Gisin’s elegant inequalities, and yields Bell expressions maximally violated by any number of pairwise anticommuting Clifford observables together with the corresponding maximally entangled state. Under suitable assumptions, these inequalities also enable device-independent certification of Majorana fermions, viewed as multiqubit realizations of Clifford algebra generators. Finally, I will highlight an additional equivalence (beyond local isometries and transposition) that must be included in self-testing: partial transposition applied to the shared state and measurements, which can leave all observable correlations invariant.

Remote guests are invited via Zoom:
Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1
Meeting ID: 842 4891 1743
Passcode: 394021

Self-Testing Tilted Strategies for Maximal Loophole-Free Nonlocality

cft — Thu, 11 Dec 2025 13:00:49 +0000

Abstract:
The degree of experimentally attainable nonlocality, as gauged by the loophole-free or effective violation of Bell inequalities, remains severely limited due to inefficient detectors. We address an experimentally motivated question: Which quantum strategies attain the maximal loophole-free nonlocality in the presence of inefficient detectors? For any Bell inequality and any specification of detection efficiencies, the optimal strategies are those that maximally violate a tilted version of the Bell inequality in ideal conditions. In the simplest scenario, we demonstrate that the quantum strategies that maximally violate the doubly-tilted versions of Clauser-Horne-Shimony-Holt inequality are unique up to local isometries. We utilize Jordan’s lemma and Grobner basis-based proof technique to analytically derive self-testing statements for the entire family of doubly-tilted CHSH inequalities and numerically demonstrate their robustness. These results enable us to reveal the insufficiency of even high levels of the Navascues-Pironio-Acin hierarchy to saturate the maximum quantum violation of these inequalities.

Remote guests are invited via Zoom:
Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1

Meeting ID: 842 4891 1743
Passcode: 394021

Quantum computing with single photons

cft — Wed, 26 Nov 2025 12:36:18 +0000

Abstract: Quandela is a start-up dedicated to building a photonic
quantum computer based on quantum dot single photon sources. In this
talk, I will review Quandela’s technology and discuss paths for
scaling to a fault-tolerant quantum computer. I will also present
recent results on how to design and run quantum algorithms for near-
and mid-term processors based on linear optics.

Remote guests are invited via Zoom:
Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1
Meeting ID: 842 4891 1743
Passcode: 394021

Discrete conformal field theories with dual-unitary circuits

cft — Mon, 17 Nov 2025 12:06:27 +0000

Dear Colleagues,
We are pleased to announce the next seminar:

Date: Nov 19, 2025 3;15 PM CET, ONLINE ONLY
Speaker: Lluis Masanes (UCL)

Abstract:
I will introduce discrete conformal field theories based on a
construction with dual-unitary circuits. Using holography, we will
interpret these discrete CFTs as toy theories of quantum gravity,
which transparently display the processes of black-hole formation and
evaporation.

Remote guests are invited via Zoom:
Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1
Meeting ID: 842 4891 1743
Passcode: 394021

Quantum and Classical Simulations of Non-Equilibrium Quantum Many-Body Dynamics

cft — Thu, 30 Oct 2025 07:48:19 +0000

Speaker: Marek Rams (Instytut Fizyki Teoretycznej, Uniwersytet Jagielloński w Krakowie)

Abstract:

Simulating dynamical processes in quantum many-body systems poses significant challenges. Much hope for advancing our understanding of such phenomena lies in the development of dedicated quantum simulators and quantum computers. In this context, we are currently witnessing a race for supremacy between advanced numerical algorithms—based on neural and tensor networks—and the current generation of quantum computers. In this presentation, I will explore aspects of this race through the lens of one of the simplest many-body models: the quantum Ising model on a square lattice, driven out of equilibrium by a time-dependent transverse field.

Time and Place: Nov 05, 2025, 02:00 PM Warsaw, IF PAN, room D

Remote guests are invited via Zoom:

Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1
Meeting ID: 842 4891 1743
Passcode: 394021

Self-testing Quantum States and Measurements in Quantum Networks

cft — Tue, 21 Oct 2025 10:24:47 +0000

Abstract:

Bell nonlocality—the existence of correlations that cannot be reproduced by means of local hidden variable models—is one of the most distinctive features of quantum theory. Indeed, the pioneering experiments to verify the existence of this phenomenon were awarded a Nobel Prize in Physics in 2022. What is more, beyond its fundamental interest, Bell nonlocality has been recognized as a valuable resource for various applications within device-independent quantum information processing, where observers process information without placing trust into the devices themselves. One of such applications that has recently gained substantial interest, driven by the recent rapid development of new quantum technologies, is the device-independent certification of entangled quantum states and the measurements performed on them, commonly referred to as self-testing. In this talk I will present our recent results [1,2] exploiting Bell nonlocality to design self-testing schemes for arbitrary quantum states and measurements. Our results are formulated within the framework of quantum networks in which many observers share correlations, possibly quantum, which are distributed by multiple sources. The key ingredient of our method is the construction of a family of Bell inequalities that enable the self-testing of a tomographically complete set of measurements performed by an arbitrary number of observers, as well as an arbitrary number of maximally entangled pair of particles shared among them.

References:

[1] S. Sarkar, C. Datta, S. Halder, R. Augusiak, Self-testing composite measurements and bound entangled state in a single quantum network, Phys. Rev. Lett. 134, 190203 (2025).

[2] S. Sarkar, A. C. Orthey, Jr., R. Augusiak, A universal scheme to self-test any quantum state and extremal measurement, arXiv:2312.04405.

Time and Place: Oct 22, 2025, 02:00 PM Warsaw, room D

Remote guests are invited via Zoom:

Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1

Optimal sample complexities for learning functions on the unitary group, and more

cft — Mon, 06 Oct 2025 08:36:24 +0000

Abstract:
Estimating properties of unknown unitary operations is a fundamental task in quantum information science. While full unitary tomography requires a number of samples to the unknown unitary scaling linearly with the dimension (implying exponentially with the number of qubits), estimating specific functions of a unitary can be significantly more eﬃcient. In this paper, we present a unified framework for the sample-eﬃcient estimation of arbitrary square integrable functions f : U(d) → C, using only access to the controlled-unitary operation. We first provide a tight characterization of the optimal sample complexity when the accuracy is measured by the averaged bias over the unitary U(d). We then construct a sample-eﬃcient estimation algorithm that becomes optimal under the Probably Approximately Correct (PAC) learning criterion for various classes of functions.

Based on: https://scirate.com/arxiv/2509.05710

I will then explore some other topics related to my earlier works, provided time permits.

Time and Place: Oct 8, 2025, 02:00 PM Warsaw, room 203

Remote guests are invited via Zoom:

Zoom link: https://us06web.zoom.us/j/84248911743?pwd=ZsiAOQbRgYCm5IFsArOnb18Fj5IsZh.1

All classes of informationally complete symmetric measurements in finite dimensions

dev — Wed, 15 Jan 2025 06:53:10 +0000

Abstract:

A broad class of informationally complete symmetric measurements is introduced. It can be understood as a common generalization of symmetric, informationally complete (SIC) POVMs and mutually unbiased bases (MUBs). Additionally, it provides a natural way to define two new families of mutually unbiased symmetric measurement operators in any finite dimension. I show a general method of their construction, together with an example of an optimal measurement. Finally, I analyze the properties of symmetric measurements and provide applications in entropic relations and entanglement detection.

Time: Jan 8, 2025 02:30 PM Warsaw
Zoom link: https://us05web.zoom.us/j/83922907300?pwd=S2Y2RiCKa7sD55bf3Rp2i6aamQuSkN.1
Meeting ID: 839 2290 7300
Passcode: 41FiTq

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