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Frontiers in Physics

Alternative schemes for twin-field quantum key distribution with discrete-phase-randomized sources

Huaicong Li, Chun-Mei Zhang

Published Mar 27, 2023• 1 citation
Publisher

Abstract

The twin-field quantum key distribution (TF-QKD) protocol and its variants can overcome the well-known rate-loss bound without quantum repeaters, which have attracted significant attention. Generally, to ensure the security of these protocols, weak coherent states with continuous randomized phases are always assumed in the test mode. However, this assumption is difficult to meet in practice. To bridge the gap between theory and practice, we propose two alternative discrete-phase-randomized (DPR)-twin-field quantum key distribution protocols, which remove the phase sifting procedure in the code mode. Simulation results show that when compared with previous discrete-phase-randomized-twin-field quantum key distribution protocols, our modified protocols can significantly improve the secret key rate in the low channel loss range, which is very promising for practical twin-field quantum key distribution systems.

DOI
10.3389/fphy.2023.1140156

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The twin-field quantum key distribution (TF-QKD) protocol and its variants can overcome the well-known rate-loss bound without quantum repeaters, which have attracted significant attention. Generally, to ensure the security of these protocols, weak coherent states with continuous randomized phases are always assumed in the test mode. However, this assumption is difficult to meet in practice. To bridge the gap between theory and practice, we propose two alternative discrete-phase-randomized (DPR)-twin-field quantum key distribution protocols, which remove the phase sifting procedure in the code mode. Simulation results show that when compared with previous discrete-phase-randomized-twin-field quantum key distribution protocols, our modified protocols can significantly improve the secret key rate in the low channel loss range, which is very promising for practical twin-field quantum key distribution systems.
Due to the single-photon interference at a third untrusted party, the twin-field quantum key distribution (TF-QKD) protocol and its variants can beat the well-known rate-loss bound without quantum repeaters and related experiments have recently been implemented. In general, quantum states in these schemes should be randomly switched between the code mode and the test mode. To adopt the standard decoy-state method, the phases of coherent-state sources in the test mode are assumed to be continuously randomized. However, such a crucial assumption cannot be satisfied well in experimental implementations. In this paper, to bridge the gap between theory and practice, we propose a TF-QKD variant with discrete-phase-randomized sources both in the code mode and the test mode and prove its security against collective attacks. Our simulation results indicate that, with only a small number of discrete phases, the performance of discrete-phase-randomized sources can overcome the rate-loss bound and approach that of continuous-phase-randomized sources.
The twin-field quantum key distribution (TF-QKD) protocol and its variants provide promising solutions to long-distance information-theoretic-secure communication, which can surpass the fundamental rate-loss bound without quantum repeaters. Different from other variants, the discrete-phase-randomized TF-QKD variant only needs to modulate weak coherent sources with random discrete phases, which avoids the necessity of modulating continuously randomized phases and can be implemented with current technology. However, the discrete-phase-randomized variant compromises the performance of TF-QKD under high channel losses and error rates. To overcome this dilemma, we propose to adopt the advantage distillation method to improve the performance of discrete-phase-randomized TF-QKD, and analyze its security in the asymptotic case. Without changing the optical hardware of TF-QKD systems, the proposed scheme can improve the secret key rate under high channel losses and error rates, and extend the tolerable channel loss as well.
Abstract Twin-field quantum key distribution (TF-QKD) and its variant protocols are highly attractive due to the advantage of overcoming the rate-loss limit for secret key rates of point-to-point QKD protocols. For variations of TF-QKD, the key point to ensure security is switching randomly between a code mode and a test mode. Among all TF-QKD protocols, their code modes are very different, e.g. modulating continuous phases, modulating only two opposite phases, and sending or not sending signal pulses. Here we show that, by discretizing the number of global phases in the code mode, we can give a unified view on the first two types of TF-QKD protocols, and demonstrate that increasing the number of discrete phases extends the achievable distance, and as a trade-off, lowers the secret key rate at short distances due to the phase post-selection.
Twin-field (TF) quantum key distribution (QKD) can overcome fundamental secret-key-rate bounds on point-to-point QKD links, allowing us to reach longer distances than ever before. Since its introduction, several TFQKD variants have been proposed, and some of them have already been implemented experimentally. Most of them assume that the users can emit weak coherent pulses with a continuous random phase. In practice, this assumption is often not satisfied, which could open up security loopholes in their implementations. To close this loophole, we propose and prove the security of a TFQKD variant that relies exclusively on discrete phase randomization. Remarkably, our results show that it can also provide higher secret-key rates than an equivalent continuous-phase-randomized protocol.
Twin-field (TF) quantum key distribution (QKD) can overcome fundamental secret-key-rate bounds on point-to-point QKD links, allowing us to reach longer distances than ever before. Since its introduction, several TFQKD variants have been proposed, and some of them have already been implemented experimentally. Most of them assume that the users can emit weak coherent pulses with a continuous random phase. In practice, this assumption is often not satisfied, which could open up security loopholes in their implementations. To close this loophole, we propose and prove the security of a TFQKD variant that relies exclusively on discrete phase randomization. Remarkably, our results show that it can also provide higher secret-key rates than an equivalent continuous-phase-randomized protocol.
Abstract Twin-Field quantum key distribution (TF-QKD) and its variants, e.g. phase-maching QKD, sending-or-not-sending QKD, and no phase post-selection TFQKD promise high key rates at long distance to beat the rate distance limit without a repeater. The security proof of these protocols are based on decoy-state method, which is usually performed by actively modulating a variable optical attenuator together with a random number generator in practical experiments, however, active-decoy schemes like this may lead to side channel and could open a security loophole. To enhance the source security of TF-QKD, in this paper, we propose passive-decoy based TF-QKD, in which we combine TF-QKD with the passive-decoy method. And we present a simulation comparing the key generation rate with that in active-decoy, the result shows our scheme performs as good as active decoy TF-QKD, and our scheme could reach satisfactory secret key rates with just a few photon detectors. This shows our work is meaningful in practice.