The Optical Fiber QKD System And Real-Time Phase Compensation Application

Information security is a signification issue. Communication security becomes more and more important. Quantum cryptography is the combination of quantum physics and cryptography. It is a new kind of cryptography, which is based on the Heisenberg's uncertainty principle and the no-cloning theorem of unknown quantum state. It can be theoretically proved to be unconditionally secure. Thanks to its absolute security, quantum cryptography will have wide practical use in many fields, such as military, diplomacy, communication and Electronic Commerce. However, the polarization and phase states of photons in the fiber will change and result to phase shift according to the environmental changes. Though such changes are not serious in a short instance, they cannot be ignored in the long distance transmission. Phase drift cased by environment is the main source of QBER(Quantum Bit Error Rate) and instability in the double M-Z interferometer QKD(quantum key distribution) system. The phase drift consist of both the high-frequency and the low-frequency vibration, the former due to environment noise usually is conducted via atmosphere and can be isolated from vibration by technological measures(e.g, with wrapped-foam or rubber pillow), the latter is from Building-vibration, it changes slowly with a serious impact on M-Z interferometer, its period is 3±0.32min(min level).So it adds the quantum bit error rate and as a result limits the communication distance.In this thesis, the compensating technology of the phase drift in the Quantum key Distribution (QKD) system is discussed, and a novel real-time tracking and compensating method for phase drift, which is from low-frequency vibration, is proposed, a new improved setup of the QKD system based on double M-Z interferometer is introduced. The compensating technology is theoretically analyzed. And the experimental results are given to prove the theoretical result and they show good coherence.The thesis is consisted of 5 chapters. The complete content as follows: Chapter1: This chapter introduces the existing problem in the classical cryptography, the origin and development of quantum cryptography, and current research status, including the net.Chapter2: This chapter introduces the currents compensating technology in the QKD system, and analyzes theirs existing problem.Chapter3: This chapter discusses the double M-Z quantum cryptography system based on phase modulation technology in details. A novel real-time tracking and compensating method for phase drift is introduced, according to the single photon statistic counts. It also analyzes the quantum coding and encoding, communication process and phase drift compensation in theory with optic matrix.Chapter4: The real-time tracking and auto-compensating unit applied to double M-Z QKDS is designed. And the DAQ and the software system is developed. With the improved scheme, the QKD system based on double M-Z interferometer can operate stably for 24 hours, key exchange with an error below 6% over 75 kilometers has been achieved at lab.Chapter5: Summarize this thesis, and give an expectation for the future development.