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Optics Letters

Hybrid protocol for sending-or-not-sending twin-field quantum key distribution

Xu Hai, Xiao-Long Hu, Xun‐Li Feng, Xiang‐Bin Wang

Published Jun 24, 2020• 6 citations
Publisher

Abstract

We propose a hybrid protocol for sending-or-not-sending (SNS) twin-field quantum key distribution: replacing the signal source by heralded single-photon source (HSPS) in the original SNS protocol, while decoy sources are still unchanged. Numerical simulation shows that after adopting this HSPS, the performance in key rate and secure distance is much improved.

DOI
10.1364/ol.399137

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We propose a hybrid protocol for sending-or-not-sending (SNS) twin-field quantum key distribution: replacing the signal source by heralded single-photon source (HSPS) in the original SNS protocol, while decoy sources are still unchanged. Numerical simulation shows that after adopting this HSPS, the performance in key rate and secure distance is much improved.
Twin-field quantum key distribution (TF-QKD) has attracted considerable attention because it can exceed the basic rate-distance limit without quantum repeaters. Its variant protocol, sending or not-sending quantum key distribution (SNS-QKD), not only fixes the security vulnerability of TF-QKD, but also can tolerate large misalignment errors. However, the current SNS-QKD protocol is based on the active decoy-state method, which may lead to side channel information leakage when multiple light intensities are modulated in practice. In this work, we propose a passive decoy-state SNS-QKD protocol to further enhance the security of SNS-QKD. Numerical simulation results show that the protocol not only improves the security in source, but also retains the advantages of tolerating large misalignment errors. Therefore, it may provide further guidance for the practical application of SNS-QKD.
Twin-field quantum key distribution (TF-QKD) can overcome the fundamental rate-loss limit in the point-to-point QKD without a quantum repeater and achieve high key rates over long distances. However, TF-QKD protocol and its variants commonly use weak coherent pulses (WCPs) as light sources. The vacuum part of WCP accounts for a large proportion, resulting in the lower photon detection rate of the communication receiver, which leads to a shorter transmission distance of the secret key. In this paper, we propose an active-decoy TF-QKD protocol with heralded single-photon source (HSPS), which can achieve higher key rate and longer transmission distance than the active-decoy TF-QKD protocol with WCPs. Also, in order to resist the side channel attacks caused by actively modulating intensity of the pulses, we propose a passive-decoy TF-QKD protocol with HSPS to enhance the security, which can perform almost as good as the active-decoy TF-QKD protocol with WCPs.
Twin-field quantum key distribution (TF-QKD) is proposed to achieve a remote key distribution with a maximum secure transmission distance up to over 500 km. Although the security of TF-QKD in its detection part is guaranteed, there are some remaining problems in the source part. The sending-or-not-sending (SNS) protocol is proposed to solve the security problem in the phase post-selection process; however, the light source is still assumed to be an ideal coherent state. This assumption is not satisfied in real-life QKD systems, leading to practical secure issues. In this paper, we discuss the condition that the photon number distribution (PND) of the source is unknown for the SNS protocol, demonstrate that the security analysis is still valid under a source with unknown PND, and show that with light source monitoring, the performance of the SNS protocol can remain almost unchanged.
The sending-or-not-sending (SNS) protocol is one of the most major variants of the twin-field (TF) quantum key distribution (QKD) protocol and has been realized in a 511-km field fiber, the farthest field experiment to date. In practice, however, all decoy-state methods have unavoidable source errors, and the source errors may be non-random, which compromises the security condition of the existing TF-QKD protocols. In this study, we present a general approach for efficiently calculating the SNS protocol's secure key rate with source errors, by establishing the equivalent protocols through virtual attenuation and the tagged model. This makes the first result for TF QKD in practice where source intensity cannot be controlled exactly. Our method can be combined with the two-way classical communication method such as active odd-parity pairing to further improve the key rate. The numerical results show that if the intensity error is within a few percent, the key rate and secure distance only decrease marginally. The key rate of the recent SNS experiment in the 511-km field fiber is still positive using our method presented here, even if there is a [Formula: see text] intensity fluctuation. This shows that the SNS protocol is robust against source errors.
The sending-or-not-sending (SNS) protocol of the twin-field quantum key distribution can tolerate large misalignment error and its key rate can exceed the linear bound of repeaterless quantum key distribution. But the original SNS protocol requires the two users to use the same source parameters. Here we propose an SNS protocol with asymmetric source parameters and give the security proof of this protocol. Our asymmetric protocol has a much better performance than that of the original SNS protocol when the channel is asymmetric.
Abstract The sending-or-not-sending (SNS) protocol of the twin-field (TF) quantum key distribution (QKD) can tolerant large misalignment error and its key rate can exceed the linear bound of repeaterless QKD. The original SNS protocol and all variants of TF-QKD require perfect vacuum sources, but in the real world experiments there is no practical perfect vacuum source. Instead, experimenters use extremely weak sources to substitute vacuum sources, which may break the security of the protocol. Here we propose an SNS protocol with imperfect vacuum sources and give the non-asymptotic decoy-state analysis of this protocol. Our numerical simulation shows that when the imperfect vacuum sources are close to perfect vacuum sources, our protocol can obtain similar key rate as that with perfect vacuum sources. This is the first result that closes the potential security loophole due to imperfect vacuum of TF-QKD.