Research Of Real-time Processing Technology In Quantum Key Distribution

Quantum key distribution (QKD) system is the first practical engineering application in cryptography field. The key generated by QKD system is "absolutely secure" which is guaranteed by the non-cloning theorem and measuring collapse theorem in quantum physics. QKD is of great value for both military and civilian use. The first demonstration system of QKD came out in1991which was designed by Bennette etc. During about twenty years’ development, the structure of QKD became more and more complicate and the functionality became more and more mature. The record of distribution distance and generated key rate were kept refreshing at the same time. By now, QKD network has appeared up and became one of the research focuses. Some famous representations of QKD network are "SECOQC" in Europe,"Tokyo QKD network" in Japan,"all-pass quantum phone network" and ""Hefei-Luan-Shucheng inter-city network" in China.The developing direction of the QKD in the future is high-speed(high final key rate). The QKD of the next generation may adopt laser with photon-emitting frequency greater than1GHz and superconductor detector to acquire much higher key rate. Post-processing of QKD is the most dragging unit for the entire QKD because it has the biggest latency. Its goal is to eliminate the discrepency and improve the security level of the distributed key. It mainly contains four steps:basis comparison, identity authentication, error correction and privacy amplification. This thesis focuses on the real-time technology aimed to speed up the post process to satisfy the needs of high-speed QKD. Considering the demand of the small-size and convinience of the QKD device and the superiority of Field Programmable Gate Array (FPGA), we especially concentrate on FPGA-based real-time post process. This thesis researches on three kinds of real-time technology:error correction of high speed and high efficiency, identity authentication of high message throughput, high speed data link.As for error correction, two different methods are devoloped:expeditious error reconciliation based on Winnow protocol and fast error reconcilition based on LDPC code. The Winnow-based method performs well in both efficiency and speed, and it is the most mature error correction module used in middle or low speed QKD. The LDPC code-based method needs only one time of information exchange, so it has the greatest superiority in QKD with high communication latency. In addition, the method has huge potencial to be improved in both efficiency and speed compared to the Winnow-based method, so it has been considered as an altenetive algorithm in the future QKD. The Winnow-based method uses Hamming code as basic error correction tool which is the central point of the Winnow algorithm. We optimize the method by selecting suitable original segment length, choosing the proper iteration times, and proposing an efficient permutation using pseudo-random sequence. The LDPC code-based method utilizes QC-LDPC as the check matrix and semi-serial decoding algorithm as the decoder. The decoder avoides complex hyperbolic function computation, multiplication and division, and saves most ram resource cost compared to the standard BP algorithm. So it is especially suitable to be implemented in FPGA. We also propose a novel message clipping function for the decoder which could improve the efficiency of the error correction tremendously.As for identity authentication, we implemente the functionality using LFSR-based Toeplitz matrix. The security of the authentication is absolutely safe which is a basic requirement of QKD. The essence of the authentication is a high-dimension multiplicaiton of a vector by a matrix. In order to speed up the authentication, we propose first-step parallelization from the point of boosting the multiplicaiton of involving more lines and second-step parallelization from the point of boosting the multiplicaiton of involving more columns in one clock cycle.As for high speed data link, it was built by means of USB3.0bus. The data link is in charge of transferring information during post-processing, mainly containing: key sifting, error correction and identity authentication. The function of the USB3.0protocol is realized by FX3chip produced by Cypress Corporation. The chip provides an interface named "slave FIFO" mode of GPIF II, reducing the difficulty of implementation. We design the firmware, an interface module in FPGA to fit the GPIF II and a software in PC.This thesis makes exploratory research on post-processing technology in QKD. It mainly contains:error correction of high speed and high efficiency, identity authentication of high message throughput, high speed data link. The research achievements directly help to improve the performance of QKD. The developed modules have been successfully used in inner-city exemplary quantum network and can be futher used in inter-city quantum network. The topic of the thesis is valuable both theoretically and practically.The innovation of the thesis lies on:1. To fit with different final key rates, two different methods of error correction are developed. The Winnow-based error correction is suitable for QKD with middle or low final key rate. The method is built on FPGA and of high parallelization. It has been integrated in inner-city exemplary quantum network successfully. LDPC-code based error correction is based on QC-LDPC matrix and uses serial decoder. It is suitable for QKD with high or ultral-high final key rate. It can be integrated in QKD using lasers of emission frequency greater than1GHz.2. To satisfy the requierment of the security of QKD, intense research is made on indentity authentication. The identity authentication using LFSR-based Toeplitz matrix built on FPGA is specifically designed and realized. It’s "absolutely secure" and of high parallelization. It has been integrated in inner-city exemplary quantum network successfully.3. To meet the requirement of huge amout of classic data communication in QKD post-processing, intense research is made on high-speed data link. It is built by means of USB3.0bus and MAC and can be used in future high speed QKD.