1-km free-space quantum key distribution experiments between portable optical terminals for space communications

摘要

Quantum cryptography is a new technique for transmitting the information whose security is guaranteed by the laws of physics. In such systems, a single photon is used for the quantum information, however, the transmission distance in the optical fibers is limited by the absorption of photons in an optical fiber in which the maximum demonstrated range is about 140 km. Free-space quantum key distribution (QKD) between an optical ground station and a satellite will be a possible solution to extend the distance for the quantum network beyond further than that with optical fibers. The laser communication demonstration between the National Institute of Information and Communications Technology (NICT) optical ground station and a low earth orbit satellite was successfully conducted in 2006. For such space communication links, the free-space quantum cryptography is considered to be an important application in the future. A free-space QKD system was developed in NICT. A prototype system for free-space quantum cryptography with the free-space polarization tracking scheme was developed, and the clock rate of QKD was improved from 20 MHz to 100 MHz clock rate. 1-km QKD experiment was conducted between two buildings over downtown area in Tokyo. By using the measured parameters, the Decoy state QKD performances aiming for the satellite QKD are examined and the results are presented.