Quantum key distribution with coherent laser pulse train: Security without monitoring disturbance

Conventional quantum key distribution (QKD) schemes determine the amount of leaked information through estimation of signal disturbance. Here we present a QKD protocol based on an entirely different principle, which works without monitoring the disturbance. The protocol is implemented with a laser, an interferometer with a variable delay, and photon detectors. It is capable of producing a secret key when the bit error rate is high and the communication time is short.