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Frontiers in Optics

Unbiased No-Switching Continuous-Variable Quantum Key Distribution

Yiming Bian, Fengkai Sun, Yichen Zhang

Published Jan 1, 2021• 0 citations
Publisher

Abstract

We report a practical no-switching continuous-variable quantum key distribution protocol with unbiased quadrature detection, which shows the potential to compensate for the asymmetry imperfections of practical heterodyne detector.
DOI
10.1364/fio.2021.jth5a.54

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Continuous-variable quantum key distribution protocol using coherent states and heterodyne detection, called the no-switching protocol, is widely used in practical systems due to the simple experimental setup without basis switching and easy assessment to phase information. The security of an ideal no-switching protocol has been proved against general attacks in the finite-size regime and composable security framework, whose heterodyne detector consists of a beam splitter with transmittance of $50\mathrm{%}$ and two ideal homodyne detectors. However, the transmittance of a beam splitter is inaccurate and the two detectors always have different quantum efficiency and electronic noise, which introduce asymmetry into the heterodyne detection, and further lead to the mismatch between the ideal protocol and practical systems, thereby overestimating the secret key rate and resulting in a practical security loophole. In this paper, we close this loophole by proposing a modified no-switching protocol with biased quadrature detection, where the asymmetry of the heterodyne detection is modeled to match the practical systems, and the security of the protocol is analyzed in asymptotic and finite-size regimes. Further, an optimization strategy is proposed to achieve the optimal secret key rate by adjusting the transmittance of the beam splitter. Simulation results show the necessity of considering the asymmetry in heterodyne detection and the effectiveness of the optimization, which provides a promising way to realize a practical secure and high-performance no-switching system.
The protocol of continuous-variable quantum key distribution based on quadrature modulation of coherent states and heterodyne quadrature detection, which does not rely on sharing either a local oscillator or pilot tones, is proposed. Using unidimensional single-quadrature encoding, we suggest alignment of the measurement bases relying on the asymmetry of the phase-space distribution of the modulated states. By rotating the bases so that the maximum quadrature variance is observed, the remote communicating party can identify the signal quadrature direction. Depending on the coherence time of the signal and the repetition rate of the system, the necessary statistics can be accumulated to minimize misalignment between the preparation and measurement bases. We address the issue of residual bases misalignment in the suggested shared-reference-frame-independent continuous-variable quantum key distribution protocol and show that it is feasible with state-of-the-art techniques. The result paves the way for low-cost and simplified quantum communication schemes, which are independent of any shared phase reference and are therefore immune to potential quantum hacking attacks on the local oscillator or pilot tones.
The review examines the essential characteristics of continuous-variable quantum key distribution protocols. In these protocols, information is encoded into the complex amplitude of the optical field, which can have a continuous range of values, unlike a qubit. An advantage over many other quantum key distribution protocols is provided by the fact that the practical implementation of these protocols does not require sources and single-photon detectors. Additionally, they offer a relatively high rate of secret key generation. However, continuous-variable protocols require a specific analysis of secrecy and special consideration for the influence of noise that may arise during practical implementation. This presents a broad area for further research.