Upper security bounds for coherent-one-way quantum key distribution

TL;DR

This paper establishes fundamental upper bounds on the secret key rate of the coherent-one-way quantum key distribution protocol, revealing limitations that challenge its suitability for long-distance secure communication.

Contribution

It derives the first simple upper security bounds for COW QKD, showing the protocol's key rate scales quadratically with transmittance, and exposes its insecurity in current implementations.

Findings

COW QKD's key rate scales at most quadratically with transmittance.

Current COW implementations are insecure for long-distance QKD.

The results challenge the practicality of COW protocol for extended secure communication.

Abstract

The performance of quantum key distribution (QKD) is severely limited by multi-photon pulses emitted by laser sources due to the photon-number splitting attack. Coherent-one-way (COW) QKD has been introduced as a promising solution to overcome this limitation, and thus extend the achievable distance of practical QKD. Indeed, thanks to its experimental simplicity, the COW protocol is already used in commercial applications. Here, we derive simple upper security bounds on its secret key rate, which demonstrate that it scales at most quadratically with the system's transmittance, thus solving a long-standing problem. That is, in contrast to what has been claimed, this approach is inappropriate for long-distance QKD transmission. Remarkably, our findings imply that all implementations of the COW protocol performed so far are insecure.