The Evaluation Of Continuous Variable Quantum Communication Channel

Quantum communication as a novel transmission mode can be divided into discrete variable quantum communication(DVQC)and continuous variable quantum communication(CVQC).There are many defects in DVQC,such as no ideal single-photon source,small capacity,weak signals,susceptible to interference and signal attenuation easily,so the development and application of DVQC is limited.The CVQC overcomes these defects.It has the characteristics of large information capacity,strong signals and flexible modulation.So it fits long distance transmission and is highly appreciated.However,the communication fidelity is severely limited due to finite entanglement and sensitivity to loss.In this thesis,the channel evaluation and error-correction scheme for CVQC are studied to enhance the fidelity.The main work of this thesis is to propose the scheme of CVQC channel evaluation and fidelity optimization.Firstly,the typical optimization schemes of quantum communication are introduced,including high-fidelity scheme of quantum teleportation(QT)through noisy channels using weak measurement and continuous variable quantum teleportation system(CVQTS)toward multistep quantum operations.We choose quantum neural network(QNN)to process quantum information in the communication system based on the comparison between classical neural network and QNN.Quantum logic circuit of QNN is built and indirect QNN learning structure is proposed,which can correct the outputs distortion on the premise of secret communication.Secondly,a high fidelity scheme of QT based on QNN is proposed.The indirect QNN learning structure is introduced to track the outputs of QT and correct the distortions in time.To ensure the security of QT,only 10%of teleported information is extracted for the training of QNN parameters.Then the outputs are corrected to achieve high-fidelity teleportation.A random series of numbers ranged in[0,?]are generated as inputs to simulate the properties of our teleportation scheme.The simulation results show that the fidelity of QT system is approximately improved to 1 by the error-correction of QNN.At last,the above scheme is generalized to optimize the fidelity of CVQTS.To achieve the teleportation in two-dimension Hilbert space,the coherent states are split into N modes by an array of N-1 beam splitters(N-splitter)and re-composed in output side after teleported by QT.However,high fidelity communication can't be achieved for limited number of beam splitters.A novel CVQTS scheme based on QNN is proposed in this thesis to implement the high-efficient and security communication.QNN tracks the inputs and outputs real-timely,then restores the distortion outputs in time.The results demonstrate that our scheme is capable of enhancing the fidelity close to 1 for almost all teleported information.QNN consists of simple quantum logic gates.CVQTS scheme based on QNN can be applied to any other inputs and improves the maneuverability and implementability in the experiment.