Patent ID: 7583803

Claim:
A quantum key distribution (QKD) system for form forming a quantum key, comprising: optically coupled first and second QKD stations configured to form the quantum key from quantum signals exchanged therebetween; in the first QKD station, a fast optical switch optically coupled to first and second round-trip optical paths; an optical modulator arranged in the first optical path and adapted to provide a selective random phase modulation to quantum signals passing therethrough; wherein the fast optical switch is: (i) switchable to a first operating state that divides an incident quantum signal to travel over the respective first and second optical paths, thereby forming a modulated first quantum signal and an unmodulated second quantum signal; and (ii) switchable to second and third operating states that respectively output the modulated first quantum signal and the unmodulated second quantum signal onto an optical fiber link in the order in which the first and second quantum signals are received from the first and second optical paths.