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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

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.

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

Leonardo Novo (INL Braga)

Multiphoton interference with partially distinguishable photons

In this talk, I will start by discussing the boson sampling problem and how it led to one of the first claims of an experimental demonstration of a quantum computational speedup. A boson sampler is a non-universal quantum computer that is able to interfere multiple photons over many optical modes and detect where the photons are at the output. A major source of imperfections in such devices comes from the fact that the photons are not exactly identical and have small differences in their internal degrees of freedom. I will discuss techniques that can be used to distinguish an ideal boson sampler and one involving partially distinguishable photons and require only an efficient classical postprocessing of the experimental data. Moreover, I will discuss the complex relation between the phenomenon of boson bunching and particle distinguishability, showing different situations where lower Hong-Ou-Mandel visibilities between the input photons can lead to higher bunching, contrary to common expectation.

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

Marzena Szymańska (University College London, CFT PAN)

Novel Non-equilibrium Phenomena in Quantum Fluids of Light

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

Michał Siemaszko (MIMUW)

Neuromorphic computing based on quantum memristor

Machine learning has recently developed novel approaches, mimicking the synapses of the human brain to achieve similarly efficient learning strategies. Such an approach retains the universality of standard methods, while attempting to circumvent their excessive requirements, which hinder their scalability. In this landscape, quantum (or quantum inspired) algorithms may bring enhancement. However, high-performing neural networks invariably display nonlinear behaviours, which poses a challenge to quantum platforms, given the intrinsically linear evolution of closed systems. One possible architecture which can overcome this issue is the use of quantum memristors.

In this talk, I will begin by introducing the general concept of neuromorphic computing, then discuss how photonic quantum memristors fit within this framework, and finally present experimental results demonstrating neuromorphic computing with a photonic quantum memristor (based on arXiv:2504.18694.).

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

Ana Predojević (Stockholm University, Sweden)

Quantum light sources: entanglement generation and characterization

Further development of quantum communication demands novel sources of quantum light, entanglement characterization methods, and improved communication protocols. Single quantum dots are established emitters of single photons and entangled photon pairs. By means of resonant excitation they generate photon pairs with high efficiency, low multi-photon contribution, and suitable for entangling schemes such as time-bin entanglement. The entanglement of photons generated by quantum dots can be employed in free-space and fibre-based quantum communication. However, the achievable degree of entanglement and the source readiness to be deployed in quantum communication protocols depend on additional functionalities, including high photon collection efficiency. I will present engineered photonic systems that allow for entangled photon pair sources to be more efficient. Also, I will introduce novel methods to characterize sources of entangled photons.

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