Emilien Lavie

Measurement-device-independent quantum key distribution (MDI-QKD) is the only known QKD scheme that can completely overcome the problem of detection side-channel attacks. Yet, despite its practical importance, there is no standard approach towards proving the security of MDI-QKD. Here, we present a simple numerical method that can efficiently compute almost-tight security bounds for any discretely modulated MDI-QKD protocol. To demonstrate the broad utility of our method, we use it to analyze the security of coherent-state MDI-QKD, decoy-state MDI-QKD with leaky sources, and a variant of twin-field QKD called phase-matching QKD. In all of the numerical simulations (using realistic detection models) we find that our method gives significantly higher secret key rates than those obtained with current security proof techniques. Interestingly, we also find that phase-matching QKD using only two coherent test states is enough to overcome the fundamental rate-distance limit of QKD. Taken together, these findings suggest that our security proof method enables a versatile, fast, and possibly optimal approach towards the security validation of practical MDI-QKD systems.

Characterising the input-output photon-number distribution of an unknown optical quantum channel is an important task especially in the case of quantum cryptography. In practice, this would require true photon number sources and photon-number-resolving detectors, but these technologies are still work-in-progress. In this work, we propose an efficient technique called double-decoy method which can provide relevant partial information of the input-output photon-number distribution, including the fraction of events in which the unknown quantum channel accepts and outputs a single photon. These detections correspond to events in which the transmitted single photon survives a basis-independent filter just before the measurement. We apply the double-decoy method to quantum key distribution (QKD) and show that it can substantially reduce the background noise and systematic error at the privacy amplification level, thereby improving the current secret key rate and achievable distance for standard QKD protocols. We also believe that several applications beyond cryptography will benefit from this technique.