Research On Phase Encoding Quantum Key Distribution System

Cryptography is an ancient discipline. It is extremely important since it appears. It plays an important role in the military, commercial and even people’s daily lives. Experienced manual encryption stage on ancient period, machine encryption stage on recent period and computer based on encryption stage on modern period, the development of tradiontial cryptography can be said to have gradually come to the end. This is because the security of tradiontial cryptography is ensured by the complexity of algorithm in public key cryptography system. Although it is possible to increase the complexity of algorithm to improve the security, however this encryption algorithm is unsecure in principle. And there are substantial threat to current encryption algorithm expecially with the development of high performance computing and parallel distributed computing, and even the concept of quantum computers.Since the encryption algorithm used now is unsecure, then looking for an absolutely one becomes urgent needs. In fact one time pad proposed by Vernam was proved to be absolutely secure in 1917. But there is difficulty on distributing large amount of secure key between two locations. However the emergence of quantum key distribution(QKD) can overcome this difficulty. Quantum key distribution is based on the principles of quantum mechanics, and can complete the distribution of security key in public channel. QKD is an unconditional secure encryption operation combined with one time pad.The work of this thesis is the research for the key technology of high speed quantum key distribution system.Firstly it is difficult to avoid the effect of polarization state of light because of intrinsic birefringence in single-mode fiber. It is often solved by adjusting the polarization controller according to the feedback to maintain the polarization state of the light. But our QKD system gives another completely different way. The using of Faladay-Michelson interferometer structure makes phase modulating completely free of the effect of polarization. It greatly improves the system stability. The phase coding scheme utilizing of high speed DAC has characteristics such as good linearity, high flexibility, easy to control, easy to find the Vπ voltage of phase modulator.Then is the synchronizing scheme of phase coding QKD system and design of automatic scanning system operating parameters. Maintaining synchronization between quantum channel and synchronizing channel in previous QKD system relies on the changes in length of the channel. Adding a clock delay control module in FPGA helps our system maintaining synchronization without changing length of the channel. And it greatly enhances the practicality of the system. Besides, QKD operating parameters can be finded after system power on by automatic running QKD program. This also makes our QKD system closer to the ultimate practical QKD devices.Lastly considering the system design for the future practical QKD communication network node, and flexible and complex high level part of BB84 protocol more suitable for software implementing, an embedded CPU subsystem is also included in QKD system design. XC5CFX70T FPGA integrated with a PPC440 CPU core can minimize the cost to complete the design. This scheme shows great advantage contrast to PC based QKD control system. SoC QKD control electronics system with XC5VFX70T as the core is a superior solution to the quantum crtptography communication network node design.This thesis not only did research on the key technology of high speed phase coding QKD system, also implemented a prototype of the QKD system. And QKD experiments had been done. The prototype had continuously operated over 17hours on the transmission fiber distance of 40km with 125MHz repetition rate. And the QKD prototype gets 2.7% QBER and 329.1kbps sifted key rate with 0.5 photons per pulse intensity. When the repetition rate rises up to 200MHz which is current single phonton detector limit, the QKD prototype gets obviously poor performance limited by high frequency response of phase modulator and excessive demands to phase modulating voltage from currently used F-M interferometer. The QBER is up to 6%, and sifted key rate is down to 216.2kbps.