Monitoring coincident clicks in differential-quadrature-phase shift QKD to reveal detector blinding and control attacks

Side-channel attacks manipulating single-photon detectors (SPDs) have known to be loopholes in realistic quantum key distribution (QKD) systems. Although measurement-device-independent (MDI) QKD schemes have been proposed and studied to avoid those loopholes, they are not easy to implement in practice because they Require some synchronization between signals sent from two distant parties. In this paper, we propose a new countermeasure against a side-channel attack (control blinding and controlling attacks). It utilizes coincident clicks in differential-quadrature-phase shift (DQPS) QKD systems. Our scheme requires no change in the system configuration of the conventional protocol. Unlike MDI-QKD, side-channel attacks can be found without difficulty in practical implementations.