Optimization And Analysis Of Phase Coding Quantum Key Distribution System

With the speedy development of information technology, more and more attentions has been paid to communication security. The security of classical communication is based on the difficulty of computation. But with the development of the quantum computation, especially the proposed shor quantum algorithm for integer factorization problem, the systems in practice will be aggressed easily. Quantum communication is a rapidly developing communication technology in recent years. The security of quantum communication is based on the laws of quantum mechanics. Quantum No-Cloning Theorem, measurement-collapse theory and Heisenberg uncertainty principle ensure that any adversary from the third party would be detected by the communicating parties. Therefore, quantum key distribution can provide unconditional security key. Quantum key distribution quickly raised people’s attentions because of its huge application prospects in the field of military and commercial. There are a variety of transport protocols on quantum key distribution currently. Its unconditional security has been proved strictly. Quantum key distribution will become more and more mature in the future.In fiber-based phase coding quantum key distribution systems, the light beam would be affected by environmental conditions when it is propagating in the fiber. The variations of temperature, thermal stress, mechanical stress, vibration has influence on both polarization and phase of transmitted single-photons. These factors would limit the practical application of quantum key distribution. Based on single-photons phase coding quantum key distribution system, this paper analyzes the key issues which affect the long-term stable operation of experimental system. The main contents include:1. The cancellation of low-frequency optical phase noise in fiber transmission. The optical phase noise which arises from fiber transmission is one of the main sources of signal impairment in communication system. We use low-frequency noise signal as feedback signal, and load it on AOM to cancel phase noise at the local end. By using a low pass filter we extract DC-20kHz frequency region as the feedback signal, the compressed the optical phase noise is close to the background level when the transmission distance are 25,50, and 75 km.2. The compression of fiber-induced polarization fluctuation in real time. We precisely control polarization state of the light field using dynamic polarization controller based on Genetic algorithm. It is shown that the system parameters could be quickly optimized and the polarization of signal beam could be locked in a long term at the single-photons level. In our phase-modulated quantum key distribution system, the average photon number are less than 0.1 photons per pulse. In our experiment, we lock the polarization state at the optimum point and keep the stable transmission in a long time for 25km optical fiber.Through these parts of optimization, we reduced fiber-induced noise and stabilized the photon number used for communications code. Finally we improved the performance of quantum key distribution system.