Long Distance Optical Fiber Quantum Cryptography System

Quantum key distribution (QKD) allows two users, Alice and Bob, to communicate in absolute security in the presence of an eavesdropper, Eve. Unlike conventional cryptography, the security of QKD is based on the fundamental laws of quantum mechanics, rather than unproved computational assumptions. It is probably one of the most rapid growing and most exciting fields of physics within the last years. Here I report the recent results of our theoretical work about QKD and the detailed application using optical fiber.Theoretically, we propose a innovative experiment scheme of optical fiber QKD based on Mach-Zehnder interferometer and Sagnac loop. Also, we discuss thoroughly the polarization control in the quantum channel. The scheme combines the advantages of previous QKD systems. And it can also compensate automatically the birefringence effect of the fibers and suppress polarization dependent losses occuring during transmission. Thus the anti-disturbed abililty of the system is guaranteed.Experimentally, we have realized a long distance QKD system based on the scheme above. It works at the wavelength of 1310nm and the transmission frequency of 500KHz. Experimental results show a fringe visibility of 99.5% over a 25-km optical fiber link and the system maintains a long-term stability. The overall performances show the possibility of buliding a practical QKD system with high efficiency, long-term stability and low quantum bit error rate. Most apparatus of the experimental system are made by ourselves, including the driver of phase modulator, single photon detector and data collecting and processing system, etc.