Vladyslav Usenko

Integrating quantum key distribution (QKD) with classical data transmission over the same fiber is crucial for scalable quantum-secured communication. However, noise from classical channels limits QKD distance. We demonstrate the longest-distance continuous-variable QKD (CVQKD) over 120 km (20 dB loss) coexisting with a fully populated coarse wavelength division multiplexing system. Natural mode filtering of the local oscillator and phase noise mitigation enabled this without additional filtering or wavelength reallocation. Benchmarking against a commercial discrete-variable QKD system and considering finite-size effects confirms the feasibility of CVQKD as a plug-and-play solution for typical 80–100 km long-haul optical networks. Our results set a record distance for CVQKD, showing its potential for cost-effective, large-scale deployment in existing network infrastructure.

Continuous variable encoding of quantum information requires the deterministic generation of highly correlated quantum states of light in the form of quantum networks, which, in turn, necessitates the controlled generation of a large number of squeezed modes. In this work, we present an experimental source of multimode squeezed states of light at telecommunication wavelengths. Generation at such wavelengths is especially important as it can enable quantum information processing, communication, and sensing beyond the laboratory scale. We use a single-pass spontaneous parametric down-conversion process in a non-linear waveguide pumped with the second harmonic of a femtosecond laser. We demonstrate multiparty entanglement by measuring the state’s covariance matrix. Our measurements reveal significant squeezing in more than 21 frequency modes, with a maximum squeezing value exceeding 2.5 dB. We finally present a frequency-multiplexed quantum key distribution protocol and the expected key rates in bipartite and in multipartite scenarii.

Squeezed states of light promise significant advantages for enhancing the performance of continuous-variable quantum key distribution (CV-QKD) systems. These advantages include the ability to reach longer distances, tolerate higher levels of excess noise, and operate at lower information reconciliation efficiency. So far those advantages were only predicted in theory. In this work, we experimentally demonstrate a CV-QKD system over 40 km fibre using squeezed light achieving a secret key rate of 0.0318 bits per channel use, surpassing the equivalent coherent state system. Similar to state-of-the-art coherent state QKD systems our system employs digital signal processing for impairment compensation eliminating the need for complex locking mechanisms and enhancing its suitability for practical implementations.

We develop a novel multi-user protocol and report the first continuous-variable quantum passive optical network (CV-QPON), that supports secure key generation for eight users simultaneously. This is achieved considering practical PON topology with an 11 km span of access links. Depending on the trust assumptions about users we reach 1.5 Mbits/s and 2.1 Mbits/s of total network key generation. Novel CV-QPON protocol exploits the multi-user nature of the network allowing to extend the network size and enhance individual keys, thus offering a pathway toward establishing low-cost, high-rate, and scalable quantum access networks using standard telecom technologies that directly benefits from the existing access network infrastructure.

We address and theoretically analyze continuous-variable quantum key distribution (CV QKD) with noisy squeezed states. The noise in such states unavoidably emerges due to optical losses in the state preparation and has to be taken into account in any practical scenario, with the outcomes depending on the trust assumption on such noise. We show that the untrusted noise should pessimistically be allocated to the anti-squeezed (AS) quadrature and can break the security of the protocols already in the asymptotic regime. In the finite-size regime we analyze the impact of the AS noise on the parameter estimation, showing that it limits the performance of the protocols even if assumed trusted and requires the protocol modifications in terms of modulation and detection schemes. We also consider the noisy squeezed-state CV QKD in the channels with transmittance fluctuations (typical for the atmospheric channels) and show that even trusted AS noise can break the security of the protocols in this regime due to fluctuations-related channel excess noise, which has to be assumed untrusted. Our results demonstrate the importance of the squeezing purity in practical realizations of squeezed-state CV QKD.

We investigate estimation of fluctuating channels and its effect on security of continuous-variable quantum key distribution. We propose a novel estimation scheme which is based on the clusterization of the estimated transmittance data. We show that uncertainty about whether the transmittance is fixed or not results in a lower key rate. However, if the total number of measurements is large, one can obtain using our method a key rate similar to the non-fluctuating channel even for highly fluctuating channels. We also verify our theoretical assumptions using experimental data from an atmospheric quantum channel. Our method is therefore promising for secure quantum communication over strongly fluctuating turbulent atmospheric channels.

We investigate estimation of fluctuating channels and its effect on security of continuous-variable quantum key distribution. We propose a novel estimation scheme which is based on the clusterization of the estimated transmittance data. We show that uncertainty about whether the transmittance is fixed or not results in a lower key rate. However, if the total number of measurements is large, one can obtain using our method a key rate similar to the non-fluctuating channel even for highly fluctuating channels. We also verify our theoretical assumptions using experimental data from an atmospheric quantum channel. Our method is therefore promising for secure quantum communication over strongly fluctuating turbulent atmospheric channels.