  We present a time-multiplexed interferometer based on Faraday mirrors, and
apply it to quantum key distribution. The interfering pulses follow exactly the
same spatial path, ensuring very high stability and self balancing. Use of
Faraday mirrors compensates automatically any birefringence effects and
polarization dependent losses in the transmitting fiber. First experimental
results show a fringe visibility of 0.9984 for a 23km-long interferometer,
based on installed telecom fibers.
