Long-distance stable “plug and play” quantum cryptography system
Chunyuan Zhou, Guang Wu, Xiuliang Chen, Hexiang Li, Heping Zeng · Frontiers in Optics · 2003
We have recently constructed a fiber-optic “plug and play” quantum cryptography system as shown in Fig.1, wherein a Faraday-Mirror was used to compensate the polarization mode dispersion and phase drifts. It worked at 1550 nm and was based on the B92 protocol[1]. We have achieved stable quantum key distribution in a standard telecom fiber with different working-distances of 10, 25 and 50 km, respectively, by using some specifically developed techniques to guarantee long-term stability of our system, such as polarization-independent phase-modulation for stable phase encoding and decoding, hybrid coincidence for efficient detection of single photons with coincidence-gated quenching of the single-photon detectors followed by coincident counting of the avalanche signals, and synchronized detection of single-photon pulses by coincidence counting gated with proper delay and width. These unique techniques greatly enhanced the fringe visibility of single-photon interference. And we have also demonstrated the application of our system in cryptographic communication by sending coded pictures from Alice to Bob using the sifted quantum keys.