Recent development of technologies for quantum communication
Akihisa Tomita · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2005
One of our major target is quantum key distribution (QKD,) which is closest to the practical use. Though QKD can be performed only with current technology, there still remain many thing to be improved. One of the most important devices that determine the system performance is photon detectors. It limits the transmission distance in optical fiber communication wavelength. A photon detector combining the two avalanche photon diodes (APD) has been demonstrated for qubit discrimination in 1550 nm. Spikes accompanied with the signals in gated-mode were canceled by balanced output from the two APDs. The balanced APD photon detector shows very low dark count rate (2x10-7) counts/pulse) with 10 % detection efficiency. We have also developed a highly stable interferometer on a planer-lightwave-circuit (PLC.) We have achieved single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of the two integrated-optic asymmetric Mach-Zehnder interferometers, and the balanced gated-mode photon detectors. The observed visibility was more than 80 % after 150-km transmission, which refers to a quantum bit error rate of less than 10 %. We will also describe temperature insensible QKD system and high speed (100 bps) key transmission over 40 km fiber.