  We demonstrate a robust, compact and automated quantum key distribution
system, based upon a one-way Mach-Zender interferometer, which is actively
compensated for temporal drifts in the photon phase and polarization. The
system gives a superior performance to passive compensation schemes with an
average quantum bit error rate of 0.87% and a duty cycle of 99.6% for a
continuous quantum key distribution session of 19 hours over a 20.3km installed
telecom fibre. The results suggest that actively compensated QKD systems are
suitable for practical applications.
