Design And Experimental Study Of Quantum Receivers In Space Optical Communication System

Coherent states are best signal carriers for communications;they are easy to prepare and manipulate,they are resilient to losses and can maximize the information transmitted in the communication channel,and they allow for communications over long distances.However,coherent states cannot be discriminated without error owing to their nonorthogonality.In terms of the symbol error rates of a communication system,the performance limits of conventional classical receivers(direct detection,homodyne detection and heterodyne detection)are bounded by the shot noise,known as the standard quantum limit(SQL).Quantum mechanics sets the ultimate error limits for communications,which are referred as the Helstrom limit.The Helstrom limit originates from the measurement uncertainty of nonorthogonal states,which is remarkably lower than the SQL.Physical realizations of quantum receivers to exceed the SQL and approach the Helstrom limit remain a major challenge.In this thesis,the free space laser communication system is taken as the research object,and physical realizations of quantum receivers as the research content.On the basis of reviewing the development of quantum receivers and their theoretical bases,the deep explorations on physical implementations and verification experiments of quantum receivers.The main achievements of this paper includes:(1)Quantum receivers for quadrature amplitude modulation(QAM)in laser communication systems.Based on the existing schemes of quantum receivers for binary modulation and multiple phase shift keying(PSK)modulation,we systematically study Bondurant quantum receivers,adaptive measurement quantum receivers and classical-quantum hybrid quantum receivers for the QAM signal alphabet.First,we generalize Bondurant quantum receivers,and give the probability model of the Bondurant quantum receiver of QAM sets.Secondly,we analyze the separate effects of non-ideal parameters including the sub-unity quantum efficiency and the dark count of the single-photon detector(SPD),the transmittance of the beam splitter,as well as the mode mismatch between the signal field and the local oscillating field on symbol error rates of adaptive measurement quantum receivers.After that,we point out that the impacts on the symbol error rates due to the above four imperfections,especially the mode mismatch factor,can be suppressed effectively by using the photon-number-resolving detector(PNRD)instead of the on-off SPD.Besides,the finite PNR capability of the PNRD is taken into consideration.Finally,we present a classical-quantum hybrid receiver and give a detailed analysis of its performance for 16-QAM.The hybrid receiver could outperform the SQL with a reduced number of partitions by only using the on-off SPD.On this basis of the above studies,some suggestions can be given on the design and engineering implementation of quantum receivers via comparing three quantum receivers.(2)Quantum receivers for the discrimination of arbitrary coherent states.We mainly study the sequential waveform nulling(SWN)quantum receiver and the optimal-control-strategy-based adaptive quantum receiver.We improve the SWN quantum receiver from two aspects:the displacement strategy and the probing strategy,and analyze the impacts of these two improvements on error performances of the SWN quantum receiver.In order to further reduce error probabilities of quantum receivers for discriminating arbitrary coherent states,we apply the optimal control theory to the design of quantum receivers,and then constructively and concretely establish the analytical model of the adaptive feedback control quantum receiver.Meanwhile,a suboptimal feedback control quantum receiver based on the maximum a posterior probability(MAP)criterion is obtained to avoid the"dimension disaster" in the feedback control strategy optimization,whose performance is still superior to the SQL.(3)Joint-detection quantum receivers for the binary coded signals.We first review the model of the optical communication system and start our researches with the simplest binary coded signals.Solutions to get the SQL and the Helstrom limit are given for binary coded signals.Then,we extend joint-detection quantum receivers using the conditional pulse nulling(CPN)strategy to a more general case-binary coded signals.Symbol error rates of such quantum receivers are reduced below the SQL by optimizing the CPN control strategy through the dynamic programming(DP)algorithm.(4)Experimental research of quantum receivers.On the basis of previous experimental demonstration works,we propose an experimental scheme for quantum receivers based on a polarization interference,and demonstrate experiments on the Kennedy receiver for BPSK signals and the CPN joint-detection quantum receiver for binary coded signals to validate our theoretical analyses.Experimental results show that these two quantum receivers can break the SQL under experimental conditions.