| The method of encrypting information with a key protects the security of information transmission,and secure key distribution is the basis of this method.Quantum key distribution has received extensive attention in recent years because it can realize the unconditionally secure distribution of the key.Among them,continuous-variable quantum key distribution has become one of the important research directions in the field of quantum key distribution because of its advantages of deterministic preparation,detection of quantum states,and good compatibility with existing communication networks.However,due to the limitations of the quantum no-cloning theorem and the lack of mature quantum relays,it is difficult to realize a continuous-variable quantum cryptography network on a global scale using optic fiber.Under the current technical conditions,building a quantum cryptography network on a global scale based on satellite is a feasible solution,and related research has developed rapidly in recent years.However,at this stage,this solution faces problems such as poor performance of the uplink protocol,the need to increase the number of users,the lack of comprehensive theoretical reference support,and the need to simplify the satellite,which limits its actual implementation.Therefore,it is necessary to conduct research on these problems and propose solutions to promote the actual realization of quantum cryptography networks.The thesis focuses on the related problems in the process of building a satellite continuous-variable quantum cryptography network,focusing on two aspects of protocol performance improvement and practicality.The main work is as follows:(1)There is a lack of comprehensive theoretical reference and support for the system design and equipment selection of satellite-ground continuousvariable quantum key distribution.This thesis analyzes the impact of atmospheric effects,setup noise,and eavesdropper attack capabilities on security.Atmospheric effects involve free-space diffraction,atmospheric extinction,atmospheric turbulence,and background light noise;in terms of setup noise,theoretically model various noises generated by various setup devices,and establish their relationship with the continuous-variable quantum key distribution.Finally,according to the attack ability of the eavesdropper(that is,the number of side channels and noises that can be accessed or controlled),the protocol is divided into five trust levels,and the composable key rate of the protocol is analyzed under the conditions of different satellite altitudes and zenith angles.The results show that the lower the trust level of the protocol,the higher the security requirements,and the lower the composable key rate generated by the system.In addition,this thesis also analyzes the"line-of-sight" security when eavesdroppers conduct a passive attack.This work can provide theoretical reference and support for the actual realization of the satellite-groud system.(2)To solve the problem that the maximum allowable zenith angle of the uplink protocol is limited due to the poor quality of uplink signal transmission,this thesis proposes an uplink continuous-variable quantum key based on the Braunstein-Kimble teleportation protocol.By deploying the Bell-state measurement module at the ground station,deploying the Einstein-Podolsky-Rosen state at the satellite,and distributing entanglement to the ground through the downlink,this protocol realizes the transmission of the quantum state from the ground station to the satellite without going through the uplink,avoiding the atmospheric effects in the uplink.After estimating the parameters of the protocol using the tail bound,the composable key rate is calculated.The results show that the teleportationbased scheme can simultaneously increase the maximum allowable zenith angle and the composable key rate of the protocol compared with the original scheme.This work provides a solution for the uplink protocol to operate in the case of large zenith angles.(3)Aiming at the problem that the number of users needs to be increased,this thesis uses a star continuous-variable measurement-deviceindependent protocol to build an inter-satellite multi-user LAN.In this scheme,the users send the coherent state to the central relay and perform multiparty Bell-state measurement.The measurement results are broadcast by the central relay to each user,which is the reference of displacement operations.The data after displacement can be used to generate keys due to its correlation.To further improve the area of LAN,a quantum error correction scheme based on the Gottesman-Kitaev-Preskill code is used to correct the Gaussian error in the channel,and the residual noise and key rate after using the scheme are simulated and analyzed.The results show that the Gottesman-Kitaev-Preskill code using approximate Gottesman-KitaevPreskill states can effectively correct the additive Gaussian noise generated in the channel,and improve the key rate and transmission distance of the protocol.This work provides a solution for the construction of inter-satellite quantum key distribution LAN.(4)Aiming at the problem that the structure of the satellite needs to be simplified,this thesis proposes an all-digital clock synchronization scheme based on the symbol timing-recovery method,which uses a simplified implementation to realize the clock synchronization of continuous-variable quantum key distribution.The scheme oversamples the quantum signal and processes the samples based on the feedback interpolation reconstruction algorithm to recover the clock information and symbol information simultaneously,without additional hardware modules.The performance of the feedback interpolation reconstruction algorithm using Lagrangian interpolation and parabolic interpolation is simulated and analyzed.The results show that the continuous-variable quantum key distribution protocol using timing synchronization technology for clock synchronization can generate a secret key,and the residual noise of clock synchronization after using Lagrangian interpolation and parabolic interpolation under the 3.68 dB loss is about 0.08 SNU and 0.065 SNU.This work provides a solution to the clock synchronization problem in quantum cryptography networks. |
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