The quantum key distribution (QKD) systems (200, 300, 400) of the
invention includes first and second QKD stations (Alice and Bob)
according to the present invention, wherein either one or both QKD
stations include fast optical switches (120, 220, 310, 320). Each fast
optical switch is respectively optically coupled to two different
round-trip optical paths (OP1 and OP2 at Alice, OP3 and OP4 at Bob) of
different length that define respective optical path differences
OPD.sub.A and OPD.sub.B, wherein OPD.sub.A=OPD.sub.B. By switching the
fast optical switches using timed switching signals (S1-S3 at Alice,
S4-S6 at Bob) from their corresponding controllers (CA at Alice, CB at
Bob), the quantum signals--which can be single-photon or weak-coherent
pulses--can be generated from a single optical pulse (112), randomly
selectively encoded while traversing the optical paths in Alice and Bob,
and then interfered and measured (detected) at Bob. The QKD system has
less loss as compared to conventional fiber-based QKD systems because it
omits at least some of the beamsplitters in conventional QKD systems.