| Quantum key distribution is a technique that is based on the principles of quantum physics and can guarantee the establishment of secure shared keys between two legitimate communication parties through an unknown channel.Quantum key distribution can be divided into two categories:discrete-variable and continuous-variable according to the dimensionality of encoding from quantum states.The continuous-variable quantum key distribution protocols based on coherent states have attracted much attention because of easy preparation of quantum states,low-cost system implementation and compatibility with classical optical communication systems.At present,the continuous-variable quantum key distribution protocol can be implemented in practice by two ways:one scheme is that the local oscillator(LO)is transmitted by the quantum signal in the channel which called Transmitted-LO system;the other scheme is that the LO is set at the receiver which called Local-LO system.The former system has mainly three disadvantages:the first one is that the Lo is vulnerable to Eve’s attack;the second is that the system can not reach the shot-noise-limited detection;the third is that the system structure is complex.The latter system has the advantage of eliminating the above-mentioned disadvantages and has achieved rapid development in recent years.However,the current local-LO system still has some problems,such as the practical security problem has not been completely solved,the signal damage caused by system performance limitations and so on,which seriously hinders the practical implementation of local-LO continuous-variable quantum key distribution system.This dissertation focuses on the practical system of local-LO continuousvariable quantum key distribution using optical fiber as medium.It studies and analyzes the practical security problems caused by non-orthogonal measurement,the stability of secret key rate and the precision of phase estimation in local-LO continuous-variable quantum key distribution system,and promote the process of practical implementation of local-LO continuous-variable quantum key distribution system.The main innovative work achieved is as follows:1.Aiming at the practical security problem caused by non-orthogonal measurement in practical local-LO continuous-variable quantum key distribution system,this dissertation analyze and point out the measurement angular error introduced by current coherent detection scheme,which would introduce security loopholes,and propose a solution.Continuous-variable quantum key distribution system is based on coherent detection,which requires the quadrature of two regular components.Due to the non-ideality of measurement in practical system,it leads to the possibility of non-orthogonal measurement and introduces the measurement angular error.By modeling and analyzing the measurement angular error in the practical local-LO system based on Gaussian modulation coherent state,the current model will cause the overestimation of the secret key rate(when the measurement angular error is 5°,the secret key rate will be overestimated by more than 10%at the transmission distance of 50 km,and the eavesdropper will exploit this part of the key rate to conceal the attack),which would introduce security loopholes.In this dissertation,an estimation scheme of measurement angular error based on actual measurement values and a post-processing compensation scheme without adding additional equipment are derived based on the Entanglement-based model equivalent to the Prepareand-Measure model,which can effectively close the security loopholes caused by measurement angular error,and achieve the purpose of enhancing the security of the practical local-LO system.2.In view of the stability problem of secret key rate caused by polarization drift in the local-LO continuous-variable quantum key distribution experimental system,this dissertation propose a polarization loss compensation scheme based on Kalman filter algorithm in a whole practical experiment.The current local-LO system usually only adopts the polarization controller to recover the polarization.The controlled state of polarization will drift With long-time operation,resulting in polarization loss and signal damage.As a result,the secret key rate would decrease with the loss of signal.This dissertation uses Kalman filter algorithm with high tracking ability to implement polarization loss compensation as well as recover signal.The compensation scheme can be realized through digital signal processing without adding additional equipment,which simplifies the complexity of the system.Experimental results show that the secret key rate can be maintained at 110 kbps(±3%)at the transmission distance of 20 km optical fiber.It is proved that this scheme can effectively solve the problem of signal damage caused by polarization drift in the practical local-LO system,which also improves the stability of the secret bit rate,and contribute to the long-term operation of the local-LO experimental system.3.To solve the problem that the phase estimation accuracy is sensitive to the channel additive noise in the local-LO continuous-variable quantum key distribution system based on discrete-modulation,this dissertation propose a phase estimation scheme based on unscented Kalman filter algorithm with a proof-ofprinciple experiment.The accuracy of phase estimation scheme used in current local-LO discrete-modulation system is limited by the signal-to-noise ratio of pilot signal.With the decrease of signal-to-noise ratio of pilot signal,the effect of phase compensation will precipitously decrease,resulting in signal damage and the decrease of secret key rate.In order to solve the problem of signal damage in the process of phase compensation,this dissertation uses Kalman filter algorithm to filter the noise in phase estimation,which can effectively improve the accuracy of phase estimation.This dissertation analyzes the feasibility of the scheme by theoretical simulation,and tests the phase estimation capability of the scheme in practical optical fiber links.Experimental results show that the system cannot extract effective key rate in the traditional phase estimation scheme under the condition that the signal-to-noise ratio of pilot signal is 0 dB but can still have a high value of secret key rate in this scheme.It effectively solves the problem that the precision of phase estimation of local-LO discretemodulation system is sensitive to the additive noise of channel and has high practicability. |
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