Research And Implementation Of Software In Free Space MDI-QKD Experiment

Quantum information is a new research field combining quantum mechanics,electronic information science and computer science.Among them,the most widely used is the quantum key distribution which has a solid theoretical basis and has been proved by practice.It USES the quantum non-cloning principle and the quantum uncertainty principle in quantum mechanics to realize the unconditional and absolutely secure communication.However,the imperfection of the devices used in the actual quantum key distribution system may have a defect that is not completely consistent with the security assumption,and it is vulnerable to the attack of eavesdroppers,which leads to a great decline in its security.MDI-QKD(MDI-QKD,Measurement-Device-Independent Quantum Key Distribution)is a solution to the most serious vulnerability in the detection side of the actual system,which makes the experimental system immune to various forms of attacks on the detection side and greatly improves the security of the system.In the experiment of MDI-QKD with free space as the channel,in order to improve its key generation rate,in addition to ensuring the high performance of each experimental equipment,correlation processing algorithm and software are particularly important.This paper mainly focuses on the design and implementation of related software for the free-space MDI-QKD experimental system.The main work includes:(1)Time synchronization scheme and implementationIn the free-space MDI-QKD system,although the high-precision atomic clock can be used as the time benchmark,the quantum pulse period in the high-speed QKD system is very small,even shorter in nanoseconds,so the delay time needs to be measured precisely and compensated timely by the synchronization signal and the software algorithm.In addition,the distance between Alice and Bob respectively and the measurer Charlie is not the same.In order to ensure that the quantum pulses at both ends arrive at the same time to generate interference,time synchronization and compensation are also needed.This paper designs and implements the corresponding algorithm and software for the time synchronization requirements of free-space MDI-QKD system,and finally achieves the time synchronization accuracy of less than 50 ps.(2)Scheme and realization of base siftingCompared with the BB84 protocol,MDI-QKD protocol participants by the two parties(launch party Alice and Bob)to the recipient to expand to three parties(launch party Alice,Bob and measuring Charlie),base vector alignment you need to conform to identify the transmitted pulse time,due to the combination of vector quantity are many,therefore in the process of QKD data post-processing base vector matching scheme need to design and implement.This paper studies and implements the base vector comparison software of MDIQKD system,and tests it in the experimental system.The test results show that the software processing results are consistent with the experimental parameters of the system.(3)Wave-front correction scheme and realizationIn addition,in the MDI-QKD system,the transmitting distance of the link is the sum of the distances between Alice and Bob and Charlie respectively as the transmitting party is changed from the single party of Alice to the two parties of Alice and Bob,which inevitably leads to a longer transmission link and more serious energy attenuation of the link.In the free space QKD system,atmospheric turbulence is an important factor affecting link efficiency.It distorts the phase plane of the light wave when it reaches the pupil surface of the receiving telescope,resulting in the decrease of the received energy.The link efficiency of free space QKD system can be improved effectively by correcting and compensating the phase distortion of the wave-front.In this paper,Zernike polynomials are used to describe the wave-front,and a dual-channel CNN neural network scheme based on deep learning is proposed to predict the Zernike coefficient.The experimental results show that the expected average error of the overall prediction is no more than 10% of the real coefficient,and the wave-front can be effectively restored.