  We report the first demonstration of quantum key distribution over a standard
telecom fiber exceeding 100 km in length. Through careful optimisation of the
interferometer and single photon detector, we achieve a quantum bit error ratio
of 8.9% for a 122km link, allowing a secure shared key to be formed after error
correction and privacy amplification. Key formation rates of up to 1.9 kbit/sec
are achieved depending upon fiber length. We discuss the factors limiting the
maximum fiber length in quantum cryptography.
