Quantum cryptographic device using single-photon phase modulation

We report a particular implementation of a quantum cryptographic device ope rating at 1540-nm wavelength and involving interference between phase-modul ated sidebands produced by a pair of phase modulators in the transmitting a nd receiving modules. The principle of operation is described in terms of b oth classical and quantum optics. The method has been demonstrated experime ntally using a strongly attenuated semiconductor laser diode. Single photon interference has been obtained with a fringe visibility greater than 90%, indicating that the system can be used for quantum key distribution. [S1050 -2947(99)06608-1].