Róbert Trényi

Coherent-one-way (COW) quantum key distribution (QKD) held the promise of distributing secret keys over long distances with a simple experimental setup while being robust against the photon-number splitting attack. Indeed, there are already commercial products implementing this scheme, and long distance realizations over 300 km have been reported recently. Surprisingly enough, however, here we show that its asymptotic secret key rate scales at most quadratically with the system's transmittance, thus solving a long standing problem. This means that COW is actually inappropriate for long distance QKD transmission. This is done by deriving the optimal zero-error attack, which is a type of attack where the eavesdropper does not introduce any error, but still prevents Alice and Bob from distilling a secure key. In doing so, we also show, for instance, that all implementations of the COW scheme reported so far in the scientific literature are insecure.