Time-division single-photon Sagnac interferometer for quantum key distribution

A single-photon Sagnac interferometer for quantum cryptography was studied using time-division phase modulation. The interferometer was arranged to allow coherent laser pulses to travel clockwise and counter-clockwise along the same fiber loop. High visibility of the single-photon interference was observed over a long-distance Sagnac loop. Time-divison phase encoding and decoding can be realized by controlling the applied electric pulses on integrated phase modulators in the Sagnac loop, which makes this system suitable for a practical quantum cryptography system.