Publications by authors named "Vladyslav C Usenko"

To establish a scalable and secure quantum network, a critical milestone is advancing from basic point-to-point quantum key distribution (QKD) systems to the development of inherently multi-user protocols designed to maximize network capacity. Here, we propose a quantum passive optical network (QPON) protocol based on continuous-variable (CV) systems, particularly the quadrature of the coherent state, which enables deterministic, simultaneous, and high-rate secret key generation among all network users. We implement two protocols with different trust levels assigned to the network users and experimentally demonstrate key generation in a quantum access network with 8 users, each with an 11 km span of access link.

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We address minimization of information leakage from continuous-variable quantum channels. It is known, that regime of minimum leakage can be accessible for the modulated signal states with variance equivalent to a shot noise, i.e.

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Two-mode squeezed states are scalable and robust entanglement resources for continuous-variable and hybrid quantum information protocols that are realized at a distance. We consider the effect of a linear cross talk in the multimode distribution of two-mode squeezed states propagating through parallel similar channels. First, to reduce degradation of the distributed Gaussian entanglement, we show that the initial two-mode squeezing entering the channel should be optimized already in the presence of a small cross talk.

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We address the applicability of quantum key distribution with continuous-variable coherent and squeezed states over long-distance satellite-based links, considering low Earth orbits and taking into account strong varying channel attenuation, atmospheric turbulence and finite data ensemble size effects. We obtain tight security bounds on the untrusted excess noise on the channel output, which suggest that substantial efforts aimed at setup stabilization and reduction of noise and loss are required, or the protocols can be realistically implemented over satellite links once either individual or passive collective attacks are assumed. Furthermore, splitting the satellite pass into discrete segments and extracting the key from each rather than from the overall single pass allows one to effectively improve robustness against the untrusted channel noise and establish a secure key under active collective attacks.

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Article Synopsis
  • This study explores the use of bright multimode coherent states of light for continuous-variable quantum key distribution (QKD) and tests the effectiveness of homodyne detection.
  • It demonstrates that signal modes can be effectively chosen by aligning them with a local oscillator, which helps reduce quadrature noise from unmatched modes.
  • The findings suggest that a QKD protocol using these bright states is viable, especially when modulation is applied to either all modes or just the matched ones, paving the way for practical quantum communication systems similar to classical optics.
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Transmittance fluctuations in turbulent atmospheric channels result in quadrature excess noise which limits applicability of continuous-variable quantum communication. Such fluctuations are commonly caused by beam wandering around the receiving aperture. We study the possibility to stabilize the fluctuations by expanding the beam, and test this channel stabilization in regard of continuous-variable entanglement sharing and quantum key distribution.

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Article Synopsis
  • The study focuses on homodyne detection of bright nonclassical light states and their potential in quantum communication.
  • Unmatched light modes increase noise, hindering the detection of Gaussian entanglement at a macroscopic scale.
  • When mode-matching is adequate, multimode quantum key distribution using bright beams becomes possible, combining quantum tech with classical optical methods.
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Quantum key distribution enables two remote parties to grow a shared key, which they can use for unconditionally secure communication over a certain distance. The maximal distance depends on the loss and the excess noise of the connecting quantum channel. Several quantum key distribution schemes based on coherent states and continuous variable measurements are resilient to high loss in the channel, but are strongly affected by small amounts of channel excess noise.

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