The Photonics Research Institute (PRI) of the National Institute of Advanced Industrial Science and Technology (AIST), one of independent administrative institutions, has developed a thermoelectrically cooled single-photon detector boasting of its fastest operational speed in the world in the wavelength band for optical fiber communication, 1550 nm, based on the AIST-developed epoch-making single-photon detecting technique, discharge pulse counting, and succeeded in demonstrating the worldÅfs fastest quantum key distribution at 1550 nm. The repetition rate of single-photon detection was 10 MHz. The key rate was 45 kbit/s when using a 10.5 km long optical fiber as a quantum channel.