Free-space quantum key distribution experiment with polarization basis tracking between portable optical terminals

A free-space quantum key distribution system is being developed by the National Institute of Information and Communications Technology (NICT) in Koganei, Japan. Quantum cryptography is a new technique for transmitting information where the security is guaranteed by the laws of physics. In such systems, a single photon is used for the quantum information; however, since the transmission distance in optical fibers is limited by the absorption of photons by the fiber, the maximum demonstrated range has been about 100 km. Free-space quantum cryptography between an optical ground station and a satellite is a possible solution to extend the distance for a quantum network further beyond the limits of optical fibers. At NICT, a laser communication demonstration between the NICT optical ground station and a low earth orbit satellite was successfully conducted in 2006. For such space communication links, free-space quantum cryptography is considered an important application for the future. A prototype system for free-space quantum cryptography using a free-space polarization tracking scheme has been developed by NICT, and the preliminary test results for the quantum key distribution with the polarization tracking are presented here.