Research On High-efficiency Reconciliation Protocol For Continuous-variable Quantum Key Distribution

Quantum key distribution(QKD)provides a means of sharing secret keys between two legal parties based on quantum mechanics.Combined with one-time-pad(OTP)encryption algorithm,QKD can provide a communication services with unconditional security.QKD includes the discrete variable QKD(DV-QKD)and the continuous variable QKD(CV-QKD).CV-QKD has the advantages of simplicity and efficiency in preparation and detection of quantum states,low manufacturing cost and easy compatibility with classical optical communication facilities.These advantages make CV-QKD a research hotspot in the field of quantum cryptography in recent years.As an essential procedure in the post-processing of CV-QKD system,the performance of reconciliation directly affects the secret key rate and secure distance of CV-QKD system.CV-QKD systems have different quantum channel environments due to different transmission distances.The long-distance CV-QKD system has a relatively low signal-tonoise ratio(SNR)channel,and the short-distance CV-QKD system has a relatively high SNR channel.It is necessary to design the reconciliation scheme for CV-QKD systems with different channel environments.Based on the in-depth investigation of the existing work about reconciliation,this paper studies the high-efficient reconciliation protocols for high-SNR CV-QKD system and low-SNR CV-QKD system respectively,so as to improve the performance of reconciliation and then finally achieve the purpose of improving the secret key rate of CV-QKD systems.This research can evidently promote the largescale application of CV-QKD system,both theoretically and practically.The main works and contributions of this dissertation are as follows:(1)In order to solve the problem that the large-scale data interaction in the decoding process of multidimensional reconciliation will make the miniaturization of long-distance CV-QKD system face the challenges of tight storage,tight communication resources and high consumption of secret key in authentication,a low-interaction multidimensional reconciliation protocol is proposed.Based on the discovery that the distance between vectors is closely related to their initial decoding information,a novel initial decoding message computation method for multidimensional reconciliation,which does not need the norm information from the encoder,is derived by fully utilize the the joint probability distribution of the raw data,and then the low-interaction multidimensional reconciliation protocol is designed by combining with this formula.The experimental results demonstrate that the improved scheme can greatly decrease the communication traffic and storage resource consumption of the reconciliation procedure and then decrease the secret key consumption for authentication almost without degradation in the reconciliation efficiency,hence it is conducive to the miniaturization of long-distance CV-QKD system.(2)In order to make the low-SNR CV-QKD system cope with the changing quantum channel environment,a spinal-based adaptive reconciliation protocol is proposed.In view of the uniqueness of CV-QKD system with two channels: quantum channel and classical authentication channel,by deeply analyzing the communication model of traditional communication system with spinal code,an equivalent transfer method of noise is designed to construct virtual channel.Based on this transfer method,the spinal-based adaptive reconciliation protocol is proposed.The leaked information in reconciliation process is quantitatively analyzed from the perspective of information theory,which proves the security of the novel reconciliation protocol.The experimental results show that the spinal-based adaptive reconciliation protocol can inherently achieve a high-efficiency and adaptive reconciliation in a broad range of SNRs.Particularly,compared with the existing adaptive reconciliation schemes,the proposed protocol does not need to design and store the matrices which greatly reduces the difficulty of implementing adaptive coordination in error correction code design and storage.(3)In order to reduce the serious quantization loss caused by noise in slice error correction(SEC),an highly quantified SEC protocol,named Rotated-SEC(RSEC),is proposed.In view of the similarity between the quantized values in the last slice of SEC and the spherical-rotated results of multidimensional reconciliation,an efficient estimator for the quantized sequences is deduced by drawing on the idea of rotation estimation in multidimensional reconciliation.Based on this new estimator,the RSEC protocol is designed.Considering that the polar code has been strictly proven to achieve the Shannon capacity,and the recursive structure of its encoding and decoding gives them good practical performance for SEC protocol of the diversified code rate,the implementation scheme of the proposed protocol is designed based on polar code.The experimental results in the high-SNR channel environment show that the RSEC protocol reduces the quantization loss and then improves the quantization efficiency and reconciliation efficiency,which allow the CV-QKD system obtain a higher secret key rate.(4)In order to make the high-SNR CV-QKD system cope with the changing quantum channel environment,a Cascade-based adaptive SEC reconciliation protocol is proposed.In view of the fact that the error-correction rate of the Cascade algorithm is a concave function of the bit error rate of sequence,this work sort the sequence in each slice according to the absolute value of likelihood ratio information and divide the sorted sequence into several subsequences with different bit error rates for error correction with Cascade algorithm,so as to make full use of the Jason inequality property of the concave function of Cascade algorithm in error-correction rate.According to this strategy,the Cascadebased adaptive SEC reconciliation protocol is designed to realize high-efficiency adaptive reconciliation and avoid the arduous tasks in design and storage of error correction code.In order to improve the error correction ability of cascade algorithm,strategies such as random scrambling,parity calculation,and backtracking are optimized.To maximize the performance of the protocol,an experimental scheme is designed to find out the optimal number of subsequence partition.The experimental results show that the Cascade-based adaptive reconciliation protocol can always achieve high reconciliation efficiency and keep good adaptability when the channel environment changes in the large range of high SNR.