The Research Of Key Technology For Free-space High-speed Real-time Quantum Key Distribution

In classical cryptography,the one-time-pad encryption has been proven to be able to ensure secure communication between two distant sites in principle.Because the keys of one-time-pad encryption are used only once and must be generated in real time for continuous communication,the core problem is the secure key distribution between the two sites.Quantum key distribution(QKD)is the intersection of quantum mechan-ics and classical informatics.Based on the quantum non-cloning theorem and quan-tum superposition theorem,QKD can provide theoretically unconditionally secure key distribution between two distant sites.In the past several years,QKD has been grad-ually developed to longer distances and higher key rates.The experimental results of"Micius" satellite have verified the feasibility of satellite-based quantum network,and laid the foundation for the global quantum communication network.Satellite-to-ground QKD employs a quantum satellite as a trusted relay to distribute secure keys to two distance sites on the ground.In order to meet the demands of satellite-based quantum network,it is necessary to realize the high-speed real-time QKD sys-tems.The main factors limiting the secure key rates are the repetition frequency of QKD optical source and the channel attenuation in a satellite-to-ground QKD system.Under the condition that the channel attenuation is unchanged,increasing the repeti-tion frequency of the QKD optical source is capable to improve the secure key rates effectively.This paper aims at realizing the high-speed real-time QKD system for satellite-to-ground applications and conducts some researches of key technologies in practical QKD systems.The paper is mainly organized in the following four parts.Firstly,we study the intensity fluctuations of generated optical pulses based on the gain switching semiconductor laser method.The theoretical analysis and simulation results show that the optical pulse intensity fluctuations make a great influence on the system secure key rate,especially in the case of large channel attenuations.By adopting the external laser injection method,the intensity fluctuations of the optical pulses get effective suppression,and the system secure key rate can be significantly increased.Considering a typical QKD system with 40 dB attenuation,the secure key rate of the system is increased by 51.89%,when the relative intensity fluctuations of the optical pulses are reduced from 1.59%to 1.15%.Secondly,we study the phase locking between multi-channel signals in high-speed QKD system.The method combining time-to-digital conversion(TDC)and phase in-terpolation(PI)is adopted to lock the relative delays between multiple high-speed serial transceivers in a closed loop.The delays between multi-channel signals can be adjusted within a specific range.Based on the internal carry-chain of the Kintex Ultrascale 040 FPGA,the realized TDC can achieve 10.2 ps time resolution and 18 ps measurement precision.These results can be further improved by the multiple-measurement method.The delays between multiple high-speed serial transceivers are locked at a peak-to-peak value of 14.7 ps and 2.5 ps RMS.The presented method of multi-channel phase align-ments has been successfully applied in practical QKD systems,and can get potential applications in some large nuclear physical systems.Thirdly,we study the high-precision time synchronization in the high-speed QKD system.In real-time QKD systems,it is necessary to have a laser communication chan-nel to support data interaction in real time.Different from previous time synchronization schemes,we utilize the laser communication channel to deliver the synchronous signal.Based on the low-jitter CDR method,we have achieved high-precision time synchro-nization with a full width at half maximum of 30 ps,between the sender and receiver.In addition,the presented method is insensitive to optical power variations through adopt-ing the timing discrimination method of the optical detection front-end(including the APD,transimpedance amplifier and limiting amplifier).The measured results show that the absolute position of the synchronous signal shifts only 70 ps when the received optical power varies from-41.4 dBm to-23.8 dBm.Meanwhile,the sensitivity of the optical communication system can reach-41.4dBm.Finally,we develop a high-speed real-time QKD system,including QKD optical source,laser communication,key post-processing and other auxiliary modules.This paper mainly concentrates on the QKD optical source and laser communication.The system attenuation of typical satellite-to-ground link changes from-47.5dB to-38.5dB,and we conduct a desktop experiment to simulate this attenuation and evaluate the per-formance of our high-speed real-time QKD system.The experimental results show that the quantum bit error rate varies from 1.9%to 0.8%when the system attenuation is from-47.5 dB to-38.5 dB.The simulated secure key rate is 397.5 bit/s at-47.5 dB and 4.6 kbit/s at-38.5 dB.The main innovations of this paper are as followed,1.The external laser injection method can reduce the intensity fluctuations of the generated light pulses adopting the gain-switching semiconductor laser method and improves the secure key rate of the QKD system effectively,especially un-der large attenuation conditions.Considering a typical QKD system with 40 dB attenuation,the secure key rate is increased by 51.89%when the relative intensity fluctuations of the optical pulses are reduced from 1.59%to 1.15%.2.By adopting the TDC based on FPGA carry chain and the phase interpolation,the relative phases between multi-channel high-speed serial signals can be locked to specific values as need.The peak-to-peak value of phase locking is 14.7 ps and the corresponding RMS is 2.5 ps.3.A high-precision time synchronization scheme embedded in laser communica-tion is realized.The full width at half maximum of synchronization precision is 30 ps when the received optical power is constant.The absolute position of the synchronous pulse shifts only 70 ps when the receiving optical power varies from-41.4 dBm to-23.8 dBm.4.A high-speed real-time QKD system is realized towards satellite-to-ground appli-cations.When the system attenuation is from-47.5 dB to-38.5 dB,the measured quantum bit error rate is from 1.9%to 0.8%and the simulated secure key rate is 397.5 bit/s at-47.5 dB and 4.6 kbit/s at-38.5 dB.