Research On Light Source And Data Acquisition System In High-speed Quantum Key Distribution

Quantum Key Distribution(QKD)is a secure communication technology based on the basic principles of Quantum mechanics,which has great potential for military and commercial applications.The first QKD protocol was proposed in 1984.After more than three decades of rapid development,QKD has made great progress in both theory and experiment and is gradually moving towards industrialization.A practical QKD system needs to have a sufficient secure bit rate(SKR).When the implementa-tion scheme and quantum channel of a QKD system are determined,one of the most important means to improve the SKR is to improve the repetition frequency.For the light source,this thesis first studies the DC characteristics of the semicon-ductor laser,designs a temperature control and current control circuit with high accuracy and low temperature drift,and measures the temperature drift of the laser by interferom-etry accurately.next,in order to obtain high-speed light pulses,this thesis studies the nature of gain-switched semiconductor lasers and their requirements for electrical pulse signals from the rate equation,and designs a high-speed laser pulse driving circuit.Be-cause the spectral width of the light source has a significant impact on the measurement-device independent QKD(MDI-QKD),the chirp of the light pulse is characterized in this paper using a homemade tunable laser.Combining the above research results and the light filtering technique,light pulses with an extinction ratio of 29.5 dB were ob-tained at a repetition frequency of 1.25 GHz,and a HOM interference visibility of 0.484 was achieved.In terms of modulator driving,this thesis designs a multi-channel high-speed DC-coupled modulator driver.In the case of outputting a 5GHz return-to-zero random code,the highest output amplitude of our circuit can reach 7.5V.To make the multi-channel SerDes work together better in this circuit,this thesis designs an automatic alignment scheme,which can achieve an alignment accuracy of 2.5ps without using high-precision TDC.It is sufficient to meet the experimental demand of the current GHz high-speed QKD.This paper also designs an amplifier circuit with a non-floating ground output,which has now achieved a 5 V swing at 2 GHz repetition frequency and can be applied to the modulators in QKD that need to be connected to the ground.In terms of data acquisition,in response to the new challenges brought by the in-crease in system clock frequency,photon counting rate,and the channels of single-photon detector,this thesis designs a high-speed data acquisition system based on high-speed SerDes in FPGA for SNSPD,which can realize the reception and real-time co-incidence for the signal of 32-channel single-photon detector at the 2.5GHz QKD.The system uses a 10GHz sampling clock,and in principle,it can be directly applied to a QKD system up to 10GHz without changing the hardware.The research work in this thesis supports several high-speed QKD experiments,and the main innovations of it are summarized as follows:1.We designed a semiconductor laser controller with low noise and low tempera-ture drift,and the drift of temperature is accurately measured by the interference method.By using gain switching technology,a 1.25GHz pulse laser with an extinction ratio of 29dB is achieved.A self-made tunable laser is used to char-acterize the chirp of the optical pulse and the influence of the chirp is reduced by the optical filtering technique.2.We obtained 4 amplitude modulator drive signals at 5GHz repetition frequency through multi-channel waveform synthesis,and designed a high-speed non-floating DC-coupled amplifying circuit.Use the characteristics of flip-flops,we have realized the automatic alignment of the multi-channel SerDes signals with an alignment accuracy of 2.5ps.3.We make GTX receive a 10GHz signal with single-ended DC connections suc-cessfully and can carry out the phase shift of picosecond level.Based on this,we designed an acquisition and coincidence system for the multi-channel high-speed single-photon detector,which supports QKD systems with a repetition frequency of up to 10 GHz in principle.