Simple security proof of coherent-one-way quantum key distribution

TL;DR

This paper presents a practical and secure implementation of coherent-one-way quantum key distribution (COW-QKD) over 100 km by introducing a two-pulse vacuum state as a decoy, enhancing security against coherent attacks.

Contribution

It introduces a simple modification to COW-QKD with a two-pulse vacuum decoy, providing a rigorous security proof against coherent attacks over longer distances.

Findings

COW-QKD can be securely implemented over 100 km.

The two-pulse vacuum decoy enhances security without complicating setup.

The method provides an analytical upper bound on phase error rate.

Abstract

Coherent-one-way quantum key distribution (COW-QKD), which requires a simple experimental setup and has the ability to withstand photon-number-splitting attacks, has been not only experimentally implemented but also commercially applied. However, recent studies have shown that the current COW-QKD system is insecure and can only distribute secret keys safely within 20 km of the optical fiber length. In this study, we propose a practical implementation of COW-QKD by adding a two-pulse vacuum state as a new decoy sequence. This proposal maintains the original experimental setup as well as the simplicity of its implementation. Utilizing detailed observations on the monitoring line to provide an analytical upper bound on the phase error rate, we provide a high-performance COW-QKD asymptotically secure against coherent attacks. This ensures the availability of COW-QKD within 100 km and establishes theoretical foundations for further applications.