Explicit attacks on passive side channels of the light source in the BB84 decoy state protocol

Abstract Various imperfections of the hardware in quantum key distribution (QKD) lead to discrepancies between the theory and real-world devices. An eavesdropper can use these discrepancies to compromise the QKD security, thus legitimate sides need to estimate the influence of imperfections to prevent the loss of security. Here we investigate explicit eavesdropping attacks on passive side channels of the light source for a BB84 decoy states QKD protocol. We consider an optimal phase-covariant cloning attack, followed by unambiguous state discrimination and a joint measurement. Our calculations provide a hint that for state-of-the-art light sources the inherent presence of passive side channels still allows the protocol to be secure, albeit with the reduced secret key rates.