Principle Experiments And Technical Researches Of Quantum Key Distribution

The information security concerns many aspects of life. It also relates to the nation-al security, since some important institutions like financial systems and military have become information-based. Information security relies on the modern cryptography. While, ciphers based on mathematical computational complexity are always threatened by the increasingly powerful computation, especially the incoming quantum computer with great expectations. Luckily, the one-time-pad (OTP) cipher has been proved to be secure and unbreakable, and it can provide unconditional security even under the threat of quantum computations. Its secure key exchange can be realized by quantum key distribution (QKD), and their combination guarantees the unconditional security of communications. Since 1984, QKD has made many great achievements and become more and more mature and practical.Here is a brief introduction about several works during my PHD studies:1. Polarization fluctuations of fiber channels will strongly affect the performance of "polarization-sensitive" QKD. But there are still lack of corresponding measure-ments and statistical analyses of polarization. Thus, we performed long hours of polarization measurements on a buried wide-area fiber network and aerial fibers. Statistical and spectral analyses on measurement results were given, which can provide important guidances to polarization compensations of QKD. It is worth mentioning that a buried fiber under test crossed the Changjiang river, and the first polarization measurement on aerial fibers installed along the Ultrahigh-Voltage-Direct-Current (UHVDC) power line was performed.2. Fast polarization recovery and automatic compensation systems were built based on low-loss electro-mechanical fiber squeezers. Due to the low insertion loss, the fiber squeezer has been widely used in QKD. For the limitation of its finite retardation range, a novel constraint scheme was proposed. Compared to tradi-tional schemes like "resetting", it has merits of simplicity, facile realization and no need of calibrations. Based on fiber squeezers, the combination of constraint scheme and gradient algorithm accomplished the continuous polarization track-ing on the level of 10 rad/s, which is among the best tracking results achieved by fiber squeezers. And statistical results about the polarization recovery time provide helpful guidances to interrupted polarization compensations of QKD.3. The first proof-of-principle demonstration of loss-tolerant and anti-noise deter-ministic QKD based on entanglement was realized. This experiment is also the first realization of two-way QKD based on entanglement and telecom fiber.4. A detailed analysis was given about Hong-Ou-Mandel interference between two phase-independent weak coherent pulses, based on quantum optics. Since it is immune to phase noises from environments, the potential of applying it to distance sensing was discussed.