Research On Reconciliation Of Continuous Variable Quantum Key Distribution Based On Post-selection

Continuous variable quantum key distribution(CVQKD)is a hot topic in quantum cryptography.However,there are some errors in the key between the authorized users due to the attenuation of the quantum states and the presence of noise in quantum channel.The reconciliation protocol is used to correct the errors.Thus we study the reconciliation protocols based on systematic Polar codes and post-selection for CVQKD in this thesis.The main results are as follow:(1)The noiseless linear amplification(NLA)can improve the transmission distance of CVQKD,but the implementation of NLA is difficult and complex.To solve the problem,a post-selection method is proposed to extract the reliable data as NLA,where the higher probability to be kept is given for the larger measured value.Numerical simulations show that the proposed method can keep the larger measured key while obtain the smaller measured with a certain probability.(2)For the CVQKD protocol based on Gaussian modulation,a reverse reconciliation protocol is presented,where a multi-level coding and multi-stage decoding(MLC/MSD)method is used with systematic Polar codes.In the proposed protocol,Bob and Alice use uniform quantization to quantify the obtained continuous key to multi-level binary sequences.Bob encodes each level of the binary sequences by systematic Polar code with a suitable code rate,and sends the frozen bit information to Alice.Alice decodes the binary key with the side information from Bob by using MSD algorithm.Numerical simulations show the reconciliation efficiency is improved in comparison with that protocol without post-selection.(3)Finally,a four-state CVQKD protocol based on the post-selection method is presented.Numerical simulations show that the protocol can choose more reliable key efficiently.Then,a reverse reconciliation is discussed for the proposed four-state CVQKD,where Bob can encode the binary key directly and send the syndrome to Alice.Simulations shows that the reconciliation efficiency is improved when the four-state CVQKD protocol is with the post-selection method.