Experimental Research On Fiber Quantum Key Distribution

The history of cryptography can be traced back to ancient time, and has become a systematic scientific subject since the World War II. It is wildly used in many fields, such as military and diplomacy in national security, ATM password and email protection in every day's life.The modern cryptography can be divided into two categories, symmetric and asymmetric system (public-key system), based on whether the encoding and decoding password is the same. The major problem in symmetric cryptographic system is how to distribute the key between the legitimate users. The public-key system can avoid the key distributing problem, but its security is based on the computational complexity. The great improvement of PC and especially the arising of the distributing computation and the present of quantum computer, lead to fatal challenge to the public-key cryptographic system.As a combination of quantum mechanics and the classical cryptography, the security of quantum key distribution (QKD) is protected by the uncertainty principle and quantum non-cloning theorem, which means the key distribution session is absolutely secure and any action of eavesdropping can be detected by the legitimate users. Using "one-time-pad" encryption with the key distributed by QKD, the absolutely secure communication can be achieved. As the next generation cryptographic technology, QKD has been researched by a lot of experts for more than twenty years, and a lot of magnificent results have been shown both in theoretical and experimental field.This dissertation is the collection of the major research works and the scientific results of the experimental QKD by our research team in the past 4 years. The work is based on phase encoding QKD scheme and implemented in telecom fiber. The major results we get are outlined as follow:1. A QKD system with complete optical, electronic control hardware and the software is implemented. The system is based on the Faraday-Michelson interferometer, and the repetition rate of the optical pulses is enhanced to 1 MHz, which is 10 kHz in original system. The system can communicate with the host through USB2.0 interface and is "Plug and Play".2. We use the system declared in 1, and the decoy state protocol which is proposed in recent years, we fulfilled a series of decoy state QKD experiments, such as the 123 km two-state decoy QKD experiment in the lab, and the 32 km two-state decoy state QKD experiment in the dark fiber of CNC in Beijing. As far as we know from the pubic reports, the former is the longest two-state decoy experiment, and the latter is the unique field decoy state QKD experiment in the world until the dissertation is finished.3. Heralded single photon source (HSPS) is the new light source which is wildly researched by a lot of groups. The HSPS is based on the spontaneous parameter down-converted procedure of the optical crystal of waveguide. Utilizing the correlation of the two emission photons, HSPS uses one photon as heralding photon and the other one as the signal photon so as to get better photon distribution than currently used weak coherent source theoretically. We combined the decoy state protocol and the HSPS, and experimentally demonstrated that the scheme can reach the performance cloest to the ideal single photon source in all exisiting schemes.4. The QKD network is one of the research focuses nowadays. Using the "Quantum Router", which is based on the wavelength routing theorem and is proposed by the researchers of our team, we implemented a star topology four-user QKD network in the backbone optical network of CNC in Beijing in Mar. to Apr. of 2007. The successful testament of the network demonstrated the network scheme is suitable to be integrated into existing metropolitan commercial optical network. The network is the first QKD network in China and is also the only full-connection QKD network without trusted relay setup in the world as far as we know. Further more we also investigated the crosstalk in the network both in theory and experiment and some useful results have been acquired.