Employing long fibre-optical Mach-Zehnder interferometers for quantum cryptography with orthogonal states

It is experimentally demonstrated, that a long distance actively stabilised Mach-Zehnder fibre optical interferometer can be used to reliably implement the GV95 quantum cryptography protocol employing orthogonal states. A proof-of-principle experiment using an interferometer composed of 1 km of spooled optical fibres connecting Alice and Bob is performed. The active stabilisation against phase drifts is done with a classical channel wavelength-multiplexed with the quantum channel and a highly stable visibility implying a quantum bit error rate of 2.2% is shown.