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Free-space quantum key distribution system with synchronously delayed classical signals

Two essential features of free-space quantum key distribution system are its timing pulses and frequency tracking devices. We make use of these two features to design a new type of quantum key distribution system with synchronously delayed classical signals. This scheme is much more secure since its synchronously randomizing of frequency and arriving time of quantum signals makes Eves have almost no opportunity to catch the quantum signals in time. Besides, this scheme can improve the intrinsic photon utilization efficient from 50% to 100%. Because the scheme breaks the symmetry between Bob and Eve before open discussion, it extends the concept of quantum key distribution and increases our choice of protocol bases.