Implementation security of quantum key distribution due to polarization-dependent efficiency mismatch

Superconducting nanowire single-photon detector (SNSPD) is a promising candidate for achieving high-rate quantum key distribution (QKD) over long distances, and it has been widely employed in recent QKD experiments. However, SNSPD is naturally polarization sensitive, which, if unchecked, could leave a back door for an eavesdropper to perform quantum hacking. Here, we experimentally study the polarization dependency on the detection efficiency of SNSPD and propose a quantum hacking attack to exploit this vulnerability. We experimentally characterize the polarization-dependent efficiencies of different SNSPDs and perform risk analysis on the security of a standard phase-encoding QKD implementation. Our experimental data and security analysis show that, if the effect of polarization-dependent efficiency is unnoticed, an eavesdropper can successfully hack the QKD implementation using the polarization-rotation attack. Countermeasures are also discussed. The result is important for the future design of secure implementations of QKD.