  A hierarchical metropolitan quantum cryptography network upon the inner-city
commercial telecom fiber cables is reported in this paper. The seven-user
network contains a four-node backbone net with one node acting as the subnet
gateway, a two-user subnet and a single-fiber access link, which is realized by
the Faraday-Michelson Interferometer set-ups. The techniques of the quantum
router, optical switch and trusted relay are assembled here to guarantee the
feasibility and expandability of the quantum cryptography network. Five nodes
of the network are located in the government departments and the secure keys
generated by the quantum key distribution network are utilized to encrypt the
instant video, sound, text messages and confidential files transmitting between
these bureaus. The whole implementation including the hierarchical quantum
cryptographic communication network links and corresponding application
software shows a big step toward the practical user-oriented network with high
security level.
