Research Of Polarization Multiplexing Encoding Scheme In Continuous-Variable Quantum Key Distribution

Light multiplexing technique,which includes wavelength division multiplexing,code division multiplexing,polarization multiplexing,orbital angular momentum multiplexing,etc.,has been widely applied in the field of optical communication as well as quantum secure communication.It's also a research fosus in the realm of quantum secure communication.Polarization multiplexing is the technique of randomly selecting one of the three Stokes parameters,namely S1,S2 or S3 as local oscillator(LO)and the other two as signals,and sending them together in the same beam in the quantum key distribution(QKD)system.Compared with the existing technique in quantum secure communication,in polarization multiplexing,the LO severs as a light mode filter which filters the stray light,weakens the atmospheric effect in the free space quantum channel,and avoids phase deviation.Therefore,in this paper,we focus on the study of different encoding schemes based on polarization multiplexing technique,and also give a short introduction to the application of polarization multiplexing in OAM-based QKD.Centered on polarization multiplexing technique,this paper includes three sections as follows.Firstly,a discrete modulation scheme of continuous-variable quantum key distribution(CV-QKD)based on polarization multiplexing technique is proposed,the experimental setup based on discrete-modulation is introduced,and the multiplexing and demultiplexing principles based on the three-plate Stokes parameters encoder and decoder are stated.Two small zones on the Poincare sphere are selected as the encoding zone,one of the three Stokes parameters is selected as LO,and the other two are selected as signals.A Static look-up-table mapping the objective polarization state into the driving voltage of the encoder is established,which boosts the encoding speed,and yields a maximum modulation frequency of 119 kHz.Secondly,RQNN model is applied to fulfill the task of coherent signal identification and restoration.A RQNN model appropriate to the experimental setup is configured after carefully analyzing the noise source.Using MATLAB script Node of Lab VIEW,RQNN is integrated to the data acquiring system,and performs real-time processing of the communication signal.After signal identification and restoration,the quantum bit error rate(QBER)of the CV-QKD system reaches a stable level of 4%,and the final error rate after data reconciliation is less than 10-6,which is lower than the QKD security threshold.Compared with the raw signal,the signal after RQNN experiences a lower QBER,thus channel capacity is enlarged,which proves the feasibility of this method.Thirdly,based on the discrete modulation of the continuous variable,this paper configures a Gaussian modulation scheme based on polarization multiplexing.Gaussian modulation protocol is introduced,and the encoding principle based upon the three-plate Stokes parameters encoder is presented.Relevant experiments are carried out which prove the feasibility of this scheme.Then,application of polarization multiplexing in OAM-based QKD is introduced,an encoding method based on OAM multiplexing is simulated on the MATLAB platform,which lays foundation on the study of OAM encoding in QKD system.