  We report a demonstration of quantum key distribution (QKD) over a standard
telecom fiber exceeding 50 dB in loss and 250 km in length. The differential
phase shift QKD protocol was chosen and implemented with 2 GHz system clock
rate. By careful optimization of the 1-bit delayed Faraday-Michelson
interferometer and the use of the super-conducting single photon detector
(SSPD), we achieved a quantum bit error rate below 2% when the fiber length was
no more than 205 km, and of 3.45% for the 260 km length fiber with 52.9 dB
loss. We also improved the quantum efficiency of SSPD to obtain high key rate
for 50 km length.
