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Środowiskowe Seminarium z Informacji i Technologii Kwantowych

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do roku 2023/2024 Seminarium Kwantowa Informacja | kanał YouTube

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Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Michał Oszmaniec (CFT PAN)

Optimal interferometric certification of multi-photon indistinguishability

Multiphoton indistinguishability is a resource for photonic quantum technologies, but its characterisation can be resource-intensive. Here we present an efficient interferometric protocol certifying closeness (in Ulhmann fidelity) of an unknown N-photon state to the state of perfectly indistinguishable photons. The protocol uses a single Fourier interferometer followed by particle-number measurements and a simple classical post-processing. The sample complexity of certifying fidelity to the perfectly indistinguishable case has optimal sampling complexity and is independent of the number of photons N, unlike previous approaches which exhibit exponential scaling. Our efficient method, based on a generalisation of the Hong-Ou-Mandel test, brings the rigorous and operationally grounded certification of multiphoton indistinguishability within reach of current-day photonic technologies.

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Yink Loong Len (National Cheng Kung University, Taiwan)

Various efforts in implementing quantum information processing under realistic considerations

We will share the recent direction and progress in our research group, which mainly focus on implementation of quantum information processing under realistic considerations. Specifically, we will discuss an example with quantum thermometry, and another one on fusion-based quantum computing.

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Marco Genoni (University of Milan)

Attaining Quantum Metrology Enhancement in Monitored Dissipative Time Crystals

We investigate quantum-enhanced parameter estimation through continuous monitoring in open quantum systems that exhibit a dissipative time crystal phase. We first analytically derive the global quantum Fisher information (QFI) rate for boundary time crystals (BTCs), demonstrating that within the time-crystal phase, the ultimate precision exhibits a cubic scaling with the system size. We then generalize this finding to a broader class of dynamics, including the transverse collective dephasing (TCD) model, which achieves a time-crystal phase through a closing Liouvillian gap without requiring a dissipative phase transition. We proceed to numerically demonstrate that this maximal global QFI rate is experimentally attainable for both the BTC and TCD models, even at finite system sizes, via continuous homodyne and photodetection. Moving towards practical implementations, we analyze the precision limits under inefficient detection, revealing a critical difference: for BTC dynamics, inefficiencies asymptotically restore a classical scaling, and only a constant-factor quantum advantage remains possible. In contrast, for TCD dynamics, a super-classical scaling is still in principle observable, and our numerical simulations confirm its presence, even under inefficient measurement conditions.

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Karol Horodecki (Uniwersytet Gdański)

Quantification of the energy consumption of entanglement distribution

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Maciej Ogrodnik (Wydział Fizyki, Uniwersytet Warszawski)

Time-phase encoded quantum key distribution with the temporal Talbot effect detection

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Gediminas Juzeliūnas (Department of Physics, Vilnius University, Lithuania)

Two-dimensional sub-wavelength topological lattices for ultracold atoms

Ultracold atoms provide a versatile platform for simulating topological and many-bodyphenomena in condensed matter and high-energy physics. The use of atomic darkstates (long-lived superpositions of atomic internal ground states immune to atom-lightcoupling) offers new possibilities for such simulations. Making the dark states positiondependent allows for the generation of a synthetic magnetic field for ultracold atomsadiabatically following the dark states [1]. Recently, two-dimensional (2D) dark-statelattices have been considered [2-3].Here, we present a general description of 2D topological dark state lattices elucidatingan interplay with the sub-wavelength lattices [4]. In particular, we demonstrate that onecan create a 2D Kronig-Penney lattice representing a periodic set of 2D subwavelengthpotential peaks affected by a non-staggered magnetic flux. Away from these patches ofthe strong magnetic field, there is a smooth magnetic flux of the opposite sign,compensating for the former peaks. While the total magnetic flux over an elementarycell is zero, the system supports topological phases due to the smooth backgroundmagnetic flux, where the particle moves in a nearly constant magnetic field, resemblingthe Landau problem. This work paves the way for experimental exploration oftopological phases in dark-state optical lattices, offering new possibilities forsimulating quantum Hall systems, fractional Chern insulators and related stronglycorrelated phases.

[1] N. Goldman, G. Juzeliūnas, P. Öhberg, and I. B. Spielman, Rep. Prog. Phys., 77,126401 (2014).
[2] E. Gvozdiovas, I. B. Spielman, and G. Juzeliūnas, Phys. Rev.. A, 107, 033328 (2023).
[3] S. Nascimbene and J. Dalibard, 135, 153402 (2025).
[4] D. Burba and G. Juzeliūnas, Phys. Rev. Research 7, 043090 (2025).

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Nathan Walk (Freie Universität Berlin, Germany)

Network advantages for quantum cryptography

Rapid advancements in photonic, atomic and solid-state quantum information experiments have seen a steady increase in the sophistication of quantum communication networks now and in the near future. This presents two significant theoretical challenges: the increasing complexity of modelling and analysis and the quest for new protocols that optimally exploit such multipartite networks. This talk will present several results on the performance advantages for multipartite entanglement in quantum cryptography. First, we prove the security of a variant of the GHZ-state based secret sharing protocol against general attacks, including participant attacks which break the security of the original GHZ scheme [1]. We then identify parameters for a performance advantage of multipartite protocols over any bipartite protocols for both secret sharing and conference key agreement over bottleneck (star) networks. Secondly, we will show how recent advances in the study of nonlocality over so-called broadcast network can be exploited to improve the robustness of device-independent random number generation (DI-RNG). We present a theoretical analysis and proof-of-principle experimental demonstration of DI-RNG in a tripartite broadcast network using initial bipartite states that are Bell local [2]

[1] Memmen, J., Eisert, J. & Walk, N. Advantage of multi-partite entanglement for quantum cryptography over long and short ranged networks. arXiv:2312.13376 (2023)
[2] Polino, E. et al. Experimental quantum randomness enhanced by a quantum network. arXiv:2412.16973 (2024).

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Rafał Demkowicz-Dobrzański (IFT UW)

Quantum Metrology - an almost perfect theory with just a few cracks

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Piotr Grochowski (Palacky University Olomouc)

Phase-insensitive force sensing

Quantum sensing with continuous-variable systems offers a versatile platform for reaching fundamental precision limits. In this work, we develop a general theoretical framework for phase-insensitive sensing with bosonic modes subject to displacements with random phase, applicable to both single-mode and multimode scenarios. We derive analytical bounds on the achievable precision in terms of first-order correlations and average excitations, revealing how non-Gaussian resources can maximize sensitivity while maintaining robustness to decoherence. This framework provides a unified perspective for understanding force and amplitude estimation in experimentally relevant, lossy, and phase-randomized environments.

Specializing to the single-mode case [1], we find that excitation-number–resolving measurements remain optimal and that N-spaced Fock states and number-squeezed Schrödinger cat states can approach the fundamental bound while mitigating decoherence. In the multimode, distributed setting [2], we show that first-order correlations enable a collective quantum advantage that scales linearly with total excitations, even without a shared phase reference. Multimode states with definite joint parity saturate this limit and can be probed efficiently via local parity measurements. Our results provide experimentally accessible strategies for enhancing phase-insensitive quantum sensing across mechanical, optical, microwave, and hybrid continuous-variable platforms.
[1] PTG, Radim Filip, arXiv:2505.20832, to appear in Phys. Rev. Lett.
[2] PTG, Radim Filip, in prep.

Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 11:15

Jan Kochanowski (Inria, Institut Polytechnique de Paris)

Efficient Quantum Measurements: Computational Max- and Measured Rényi Divergences and Applications

Quantum information processing is limited, in practice, to efficiently implementable operations. This motivates the study of quantum divergences that preserve their operational meaning while faithfully capturing these computational constraints. Using geometric, computational, and information theoretic tools, we define two new types of computational divergences, which we term computational max-divergence and computational measured Rényi divergences.In this talk I will give an overview of the construction of our framework, based on cones of efficient binary measurements, and will detail their applications to hypothesis testing, quantum resource quantification and entanglement distillation/ cost.

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