Quantum key distribution in 50-km optic fibers

In this paper, we report our recent experiment of long-distance fiber-optic "plug and play" quantum cryptography system wherein a Faraday-Mirror was used to compensate for the polarization mode dispersion and phase drifts. The pulse-biased coincident gate single-photon detection technique was used to effectively reduce the noises from the detrimental Rayleigh backscattering. We have achieved a quantum key distribution system with the working distance of 50 km, which was tested to be stable in more than 6 hours' continuous work. And we also demonstrated the practical quantum communication in a local area network using the TCP protocol. Copyright by Science in China Press 2004.