Improved coherent one-way quantum key distribution for high-loss channels

TL;DR

This paper introduces a simple, secure variant of the coherent one-way quantum key distribution protocol, enabling practical high-loss channel deployment with minimal hardware changes and comparable key rates to existing protocols.

Contribution

A new, secure variant of COW-QKD is proposed and proven secure in the infinite-key limit, with modifications that maintain high key rates in high-loss channels.

Findings

The modified protocol is secure in the infinite-key limit.

Key rate is comparable to existing upper bounds and BB84.

Deployment in high-loss networks is feasible with minimal changes.

Abstract

The coherent one-way (COW) quantum key distribution (QKD) is a highly practical quantum communication protocol that is currently deployed in off-the-shelves products. However, despite its simplicity and widespread use, the security of COW-QKD is still an open problem. This is largely due to its unique security feature based on inter-signal phase distribution, which makes it very difficult to analyze using standard security proof techniques. Here, to overcome this problem, we present a simple variant of COW-QKD and prove its security in the infinite-key limit. The proposed modifications only involve an additional vacuum tail signal following every encoded signal and a balanced beam-splitter for passive measurement basis choice. Remarkably, the resulting key rate of our protocol is comparable with both the existing upper-bound on COW-QKD key rate and the secure key rate of the coherent-state BB84 protocol. Our findings therefore suggest that the secured deployment of COW-QKD systems in high loss optical networks is indeed feasible with minimal adaptations applied to its hardware and software.