Advanced unambiguous state discrimination attack and countermeasure strategy in a practical B92 QKD system

Even though unconditional security of B92 quantum key distribution (QKD) system is based on the assumption of perfect positive-operator-valued measures, practical B92 systems only utilize two projective measurements. Unfortunately, such implementation may degrade the security of the B92 QKD system due to Eve's potential attack exploiting the imperfection of system. In this paper, we propose an advanced attack strategy with an unambiguous state discrimination (USD) measurement which makes practical B92 QKD systems insecure even under a lossless channel. In addition, we propose an effective countermeasure against the advanced USD attack model by monitoring double-click events. We further address a fundamental approach to make the B92 QKD system tolerable to attack strategies with USD measurements using a multi-qubit scheme.