Zero-error attack against coherent-one-way quantum key distribution

Abstract Coherent-one-way (COW) quantum key distribution (QKD) held the promise of distributing secret keys over long distances with a simple experimental setup. Indeed, this scheme is currently used in commercial applications. Surprisingly, however, it has been recently shown that its rate scales at most quadratically system’s transmittance and, thus, not appropriate for distance QKD transmission. Such pessimistic result was derived by employing so-called zero-error attack, which eavesdropper does introduce any error, but still legitimate users system cannot distill secure key. Here, we present an improved attack against COW-QKD significantly outperforms previous approaches, and provides stronger restrictions on maximum achievable absence errors. This translates into upper bound more than order magnitude lower previously known bounds.