Paul Jouguet

Establishing an information-theoretic secret key between the two communicating parties of a quantum key distribution (QKD) system is only possible when an accurate characterization of the quantum channel and proper device calibration routines are combined. Indeed, security loopholes caused by inappropriate calibration routines have been shown for discrete-variable QKD. Here, we propose and provide experimental evidence of an attack targeting the local oscillator calibration routine of a continuous-variable QKD system. The attack targets the classical local oscillator pulses during the QKD run in order to modify the trigger pulses used at the detection stage. This allows the eavesdropper to bias the shot noise estimation usually performed using a calibrated relationship. This loophole can be used to perform an undetected intercept-resend attack. We characterize the loophole and suggest possible countermeasures.

We experimentally implement a quantum coin flipping protocol that guarantees a strictly better security than classically possible against an all powerful adversary over a distance suitable for communication in metropolitan area networks. The implementation is based on a practical plug’n'play system, originally designed for quantum key distribution. Furthermore, we show that our protocol can be combined with quantum coin flipping protocols that provide almost perfect security against adversaries with limited resources and hence enhance them with a level of unconditional security. Our results offer a powerful theoretical and experimental toolbox for future secure quantum communications.