  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.
