| The most mature research direction in quantum information is quantum key distribution(QKD).As a branch of QKD,continuous-variable(CV)QKD has also received extensive attention due to its unique advantages.CV-QKD uses coherent detection instead of single-photon detection,which can be implemented using classical optical communication devices.The selective filtering effect of the local oscillator(LO)in the coherent detection makes CV-QKD easier to integrate with classical optical communication networks than discrete-variable(DV)QKD.The security of CV-QKD based on Gaussian-modulated coherent states(GMCS)has been well proven,but the difference between the actual devices and the theoretical model will lead to security loopholes,making the generated keys insecure.It is worth noting that the LO used in the implementation of CV-QKD has not been included in the theoretical security proof.If the LO passes through the quantum channel,it will affect the security of CV-QKD.However,in early experiments,both the signal and LO were generated by the transmitter.In this way,although a stable phase reference can be easily established,the LO is easily attacked,because the LO follows the signal through the quantum channel.In order to fundamentally beat these attacks,the local LO(LLO)scheme was proposed.Yet,this scheme faces the challenge of establishing a stable phase reference.This thesis introduces the corresponding theoretical basis and basic realization of CV-QKD,reviews the theoretical and experimental development of CV-QKD over the recent years,discusses my main work during the doctoral period.My work is focused on studying the LLO scheme of CV-QKD,especially the establishment of the phase reference in this scheme.The main research task is to improve the phase estimation method of the LLO scheme,give the methods of frequency locking and frequency difference measurement between two lasers.In addition,the GMCS protocol is improved to increase the key rate and a rigorous calibration method of detection efficiency is proposed to improve the practical security.In terms of protocol improvement,two improved double Gaussian modulation protocols are given in the thesis.Compared with the original protocol,these two protocols can achieve better parameter estimation without consuming the raw keys.In terms of the improvement of the phase estimation method,this thesis discusses how to achieve better phase estimation under different detection methods,different system repetition rates,and different optical designs.In order to establish the phase reference of the two lasers in the LLO scheme,this thesis presents two frequency locking schemes:optical injection locking and heterodyne optical phase-locked loop.These two schemes can also be used for twin-field quantum key distribution.When the light pulse is at the single-photon level,the method of laser self-interference can be used to measure the frequency difference between the two lasers.In the calibration of detection efficiency,this thesis analyzes the shortcomings of the conventional calibration method and gives a rigorous calibration method,which closes security loopholes caused by the overestimated detection efficiency. |
