Improved Coherent One-Way Quantum key Distribution for High-Loss Channels

Abstract

The coherent one-way (COW) quantum key distribution (QKD) is a highly practical quantum communication protocol that is currently deployed in off-the-shelve 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 the intersignal 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 the 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.