Hacking coherent-one-way quantum key distribution with present-day technology

TL;DR

This paper demonstrates that present-day technology can implement zero-error attacks on coherent-one-way quantum key distribution, potentially compromising its security over long distances by exploiting unambiguous state discrimination.

Contribution

The authors introduce two practical USD receivers using linear optics and detectors to evaluate the security of COW QKD against zero-error attacks under realistic conditions.

Findings

Zero-error attacks can potentially break COW QKD security.

Practical USD receivers can be realized with current optical technology.

Security of COW QKD is vulnerable even with realistic experimental imperfections.

Abstract

Recent results have shown that the secret-key rate of coherent-one-way (COW) quantum key distribution (QKD) scales quadratically with the system's transmittance, thus rendering this protocol unsuitable for long-distance transmission. This was proven by using a so-called zero-error attack, which relies on an unambiguous state discrimination (USD) measurement. This type of attack allows the eavesdropper to learn the whole secret key without introducing any error. Here, we investigate the feasibility and effectiveness of zero-error attacks against COW QKD with present-day technology. For this, we introduce two practical USD receivers that can be realized with linear passive optical elements, phase-space displacement operations and threshold single-photon detectors. The first receiver is optimal with respect to its success probability, while the second one can impose stronger restrictions on the protocol's performance with faulty eavesdropping equipment. Our findings suggest that zero-error attacks could break the security of COW QKD even assuming realistic experimental conditions.