``Plug and play'' systems for quantum cryptography
  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. 
A. Muller, T. Herzog, B. Huttner, W. Tittel, H. Zbinden and N. Gisin   (Group of Applied Physics, University of Geneva)