Research On Optical System For Practical Continuous-variable Quantum Key Distribution And Related Theory Of Detector

With the rapid development of informatization,people pay more and more attention to information security.The security of modern cryptography mainly depends on the complexity of mathematics,and the emergence of quantum computers brings serious safety loophole to this cryptosystem based on computational complexity.Quantum Key Distribution(QKD)enables both parties to share information-theoretically secure keys in insecure channels.Combined with the one-time pad cryptosystem,it can realize unconditional-secure quantum securcy communication.QKD will have promising application prospects in the field,for instance,national defense,government affairs,finance,electric power and big data,etc.QKD empleies the quantum state as the carrier of the key,and the unconditional security of the key is guaranteed by the basic principles of quantum mechanics.Any potential eavesdropping behavior will disturb the quantum state with the carried key,which will be found by both legitimate communication parties.At present,there are two main technical routes for QKD: Discrete-Variable QKD(DV-QKD)and Continuous-Variable QKD(CV-QKD),both have their own advantages.CV-QKD has the advantages of easy preparation of light source,higher theoretical secure key rate in medium-short distances,compatibility with existing optical communication networks,and strong resistance to additional noise,and it is suitable for the construction of quantum communication networks in metropolitan areas.Since DV-QKD was proposed,it has attracted the researcher attention.After rapid development for decades,at present,a number of quantum secure communication verification networks have been built at domestic and foreign,and they have gone toward practicality.CV-QKD was proposed late,in recent years,significant breakthroughs have been made in theoretical and experimental research of CV-QKD,and demonstrations have been realized from the laboratory to the field environment.In order to promote the practical process of CV-QKD,it is necessary to design a system with high-integration and high-speed.The Balance Homodyne Detector(BHD)is the key component of the CV-QKD system,and determines the upper limit of the repetition rate of the system.FPGA is a large-scale programmable device with low-power-consumption,high-integration and high-flexibility.Most of the engineering prototypes of QKD are based on FPGA implementation.This paper focuses on how to design an integrated and practical CV-QKD system at theory and experiment aspects,the main contents as follows:1.The design of an FPGA-based CV-QKD optical system.First,according to the implementation process and optical path module of the experimental system,the overall structure of the system is designed.Then design the optical path module of Alice and Bob and each module of the optical control system of both ends,and design the clock and data transmission control between the each hardware of the optical control system at both ends,as well as the clock synchronize between them.Next,according to the requirements of the optical path module of the system,combined with the output of the optical control system,the system related electronic modules are designed,mainly including the amplifier module,high-speed pulse generator,clock signal recovery module and high-speed BHD module.Finally,the optical path modules at both ends are integrated designed.Our designed optical system paves the way for the practical CV-QKD system.2.The implement of key algorithms in the practical CV-QKD system.We propose an averaging method to find the extinction operating point and achieve the locking of the bias operating points of the LN-MZ intensity modulator.This algorithm is simple,and the locking result is less different from the polynomial fitting,and the locked extinction ratio can reach more than 40 d B.The polarization locking of the signal and the local oscillator(LO)in the practical CV-QKD system is implemented,and the locking polarization extinction ratio is greater than 27 d B.The two arms of the BHD in the practical CV-QKD system are automatically balanced,the relative phase compensation between the signal and LO and Gaussian modulation of the signal are implemented.The performance of the system related electronic modules are tested,and the test results meet the design requirements.Through comprehensive analysis of the results of the above algorithms,it shows that the design of the entire optical system is feasible.3.The research on the influence mechanism of the finite BHD bandwidth and signal modulation pattern on the performance of practical CV-QKD systems.The model of the channel parameters estimation in CV-QKD under the finite BHD bandwidth and the different signal modulation pattern is established,and the lower bound of secret key rate against collective attack is derived in the finite-key-regime under finite-bandwidth BHD.The numerical simulation results shows that insufficient BHD bandwidth can significantly affect the precise estimation of channel parameters and therefore deteriorate the performance of the CV-QKD,and the effect is relatively larger for the large duty cycle pulses.By comparing the two different signal modulation patterns,the root raised-cosine pulse modulation and rectangular pulse modulation,it is found that the former is less affected on the channel parameters estimation of the system and can improve system performance.Through analyzing the situation where the BHD bandwidth is determined,it is found that there is an optimal repetition rate to maximize the security key rate of the system.Meanwhile,for the large duty cycle pulse,the longer the transmission distances,the higher the BHD bandwidth is required.The above research results can provide useful guidance for the design of high speed practical CV-QKD systems and the proposed theory model can be used to analyze the impact of BHD bandwidth on other CV-QKD protocols.