Research On Multiplexing Technology In Quantum Key Distribution System

Quantum communication is a cross discipline which is the combination of quantum mechanics with communication theory,having characteristics of high efficiency and absolute security.Quantum cryptography communication based on quantum key distribution is the fastest developing and the most practical quantum communication technology.By quantum key distribution using Heisenberg uncertainty principle and quantum state non-cloning theorem a series of shared key between the two parties can be built.The key bits are used to encrypt the classical information so as to realize secure transmission of information.With the advancement of practical point to point quantum key distribution technology,quantum communication is gradually developing towards large-scale networks so that more users can realize secure quantum communication.Therefore,quantum multiplexing technology becomes an urgent requirement.In this dissertation,we focus on the research of optical quantum signal multiplexing technology.Firstly,we briefly describe the fundermentals of quantum communication,including the concept and the common logical operation of quantum bit.Three typical quantum key distribution protocols,i.e.BB84,B92 and E91,are introduced in detail.The functional structure and the classification of quantum communication network are illustrated.In addition,we introduce three quantum multiplexing technologies such as wavelength division multiplexing(WDM),code division multiplexing(CDM)and orthogonal frequency division multiplexing(OFDM).Secondly,we propose a multiplexing and demultiplexing schemes based on quantum state fusion and fission.In the scheme,the optical quantum state fusion operation and a quantum circuit to convert two photons' polarization mode into a single photons' temporal-polarization mode(multiplexing),and the optical quantum state fission operation and a quantum circuit to convert one temporal-polarization mode of one photon into two polarization modes of two photons with the help of an ancilla photon(demultiplexing).We apply this multiplexing method into the quantum key distribution system with BB84 protocol and analyze the system performance under the influence of polarization misalignment and detectors' dark counts.The results show that the quantum bit error rate is reduced from 10% ~3% to 5% ~2%,the key rate is increased by 3~10 times and the communication distance is increased by 6~30Km when polarization misalignment noise parameter take different values.Thirdly,we discuss the quantum time division multiplexing(TDM)technology and design a new type of quantum time division multiplexing circuit and quantum time slot switching circuit.Furthermore,the TDMA multi-user quantum key distribution process using BB84 protocol is introduced.We make a comparison between the QKD scheme based on TDM and the one based on quantum state fusion/fission multiplexing.The simulation results show that both of the two schemes can enhance the performance of QKD system and the QKD scheme based on quantum state fusion/fission multiplexing has more advantages than the one based on TDM in terms of counting rate,quantum bit error rate,the final key rate and transmission distance.Finally,we summarize this dissertation and put forward some suggestions for the further work.