Study On Key Technologies Of Multi-user Quantum Communication

Since the first quantum protocol was published in the80’s of the20thcentury,quantum communication has obtained significant breakthroughs in theory andapplication. Because quantum communication has the characteristic of unconditionalsecurity in theory, more and more research institutions have begun to focus on makingresearch on quantum communication, and even some companies have, in amarket-oriented way, developed equipments and key apparatuses of quantumcommunication. The development trend of quantum communication will be thecyberization of quantum, with the research on the quantum communication networkfaced with many problems, which include some difficulties unresolved in quantumcommunication.Research fields of quantum communication are separated into three parts from theview of communication connection: sending terminal, receiving terminal and channel.The key point and difficulty of research on the sending terminal is the preparation ofthe quantum state, the research on the receiving terminal is focused on the detection ofstates and the research on the channel is focused on the construction of the model andthe analysis of noise. Based on the theory of the quantum communication network, thispaper has done researches on the multi-user quantum channel, multi-user quantumstates, multi-user quantum detection, protocol of multi-user quantum key distributionand safety of quantum key distribution. The main contributions of this work can besummarized as follows:1. The model of the multi-user quantum channel is constructed and a novelscheme for multi-user quantum detection is suggested.Based on the analysis of the forms and characteristics of the quantum state in quantumkey distribution, the features of the real quantum channel is analyzed and incombination with theories of classical multi-user detection, a study of multi-userquantum detection is made, a scheme for multi-user quantum detection is suggested. Inthis scheme, by observing the intensity of the single-photon pulse based on differentbases at the receiving terminal, according to the relation between the Stokes parametersand the rotation angle at some point on the Poincare sphere, the deflection angle of thepolarization state is obtained. Then, with the average value of the deflection angle in aspan, detection operators are adjusted and multi-user quantum detection is realized. The validity of this scheme is verified and this scheme’s advantage of reducing theerror rate is proved by measuring the actual optical fiber channel.2. A novel scheme for multi-user quantum key distribution is derived.Coding schemes for the single user quantum state are analyzed. Through simulation,the relative merits of these schemes are analyzed; the noise of the real quantumchannel is studied, and based on the concrete conditions of multi-user quantum keydistribution and the theory of the optical field, the error rate formula for multi-userquantum detection in a real quantum channel is derived. A scheme for multi-userquantum key distribution is derived. Through the combination of the concept ofOptical Code Division Multiple Access (OCDMA) and the theory of quantum coding,the polarization coding of quantum states leads to the random address codes, which areused to distinguish different user pairs and to phase-code the information with respectto the quantum states. Simulation result indicates that the scheme is effective and safe.3. The secure threshold formula for the BB84protocol is derived.The security of different quantum key distribution protocols and several differentmeans of eavesdrop and their effects upon the quantum key distribution are analyzed.The quantum channel serves as an open system, and together with the eavesdropoperation and noise interference, forms a closed system. A model of the quantumchannel is suggested, on the basis of which the security of the quantum key distributionprotocol is analyzed. The secure threshold formula for the BB84protocol is derived.Simulation result indicates that this formula is effective for judging whether or notthere is any eavesdrop during the process of quantum key distribution.