Robust twin-field quantum key distribution through sending-or-not-sending

TL;DR

This paper introduces a method to accurately calculate the secure key rate of the SNS twin-field QKD protocol considering source errors, demonstrating robustness and maintaining positive key rates over long distances despite intensity fluctuations.

Contribution

The study presents the first practical approach to account for source errors in TF-QKD, enhancing its security analysis and robustness in real-world conditions.

Findings

Key rate remains positive with ±9.5% intensity fluctuation over 511 km.

Method improves security analysis by incorporating source errors.

Robustness of SNS protocol against source intensity fluctuations is confirmed.

Abstract

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 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 $\pm 9.5\%$ intensity fluctuation. This shows that the SNS protocol is robust against source errors.