Research On The Theoretical Schemes Of High-performance Continuous Variable Quantum Key Distribution

Quantum key distribution?QKD?is an important research direction in quantum secure communication and quantum information processing.Its theoretically unconditional security is guaranteed by the basic principles of quantum mechanics.QKD mainly includes two kinds of protocols: Discreet Variable?DV?protocol based on single photon coding and Continuous Variable?CV?protocol based on optical quadratures coding.Continuous variable quantum key distribution?CVQKD?has attracted extensive attention from researchers at home and abroad in recent years since its advantages of higher key rates,easier integration with traditional optical fiber networks,simple experimental deployment and high reliability,and it has achieved important results in both theoretical scheme design and experimental implementation.Currently,CVQKD schemes are mainly based on one-way Gaussian-modulated coherent-state?GMCS?protocols.However,due to the characteristics of the protocols and the imperfections of some devices,the further development of CVQKD is restricted to a certain extent,such as eavesdropper's attacks on the local oscillator and the loopholes of the detectors.In recent years,some new CVQKD schemes have been proposed.For instance,the plug-and-play dual-phase-modulated protocol can reduce the key loss caused by eavesdropper's attack focused on local oscillator,and the scheme based on measurement-device-independent?MDI?protocol can effectively remove all side channel attacks performed by eavesdropper related to practical detectors.Nevertheless,these protocols usually have shortcomings such as short transmission distance,insufficient consideration of practical security performance and complex experimental implementation.The proposal of these high-performance CVQKD protocols is of great significance to improve the performance of CVQKD system,the practical security performance analysis of CVQKD system and the improvement of existing CVQKD protocols.This dissertation focuses on the theoretical scheme of high-performance CVQKD protocols,which mainly includes the following innovative research contents:?1?The scheme with the use of noiseless linear amplifier?NLA?to improve the performance of plug-and-play dual-phase-modulated coherent-state CVQKD protocol is proposed.The key rate calculation and simulation show that although the NLA is a probabilistic amplifier,it can still increase the maximal channel loss by 20 log₁₀ gd B with its successful operation,it can improve the system's tolerance to excess noise and the robustness as well.?2?The practical security performance of one-way CVQKD protocol based on NLA is analyzed in detail.Not only the imperfect homodyne detector and the imperfect amplification process in the system are considered,but also the secret key rate figures are analyzed under the finite-size effects framework.Among them,the influence of finitesize effects are closely related to the total data block between the two legitimate parties.Through analysis and calculation,it can be concluded that the amount of total data block exchanged between the two parties will also become a key parameter affecting the performance of the system in the practical implementation of the scheme.Increasing the total data block exchanged between the two legitimate parties can help the practical performance of the system approach the performance in the asymptotic case.?3?The CV-MDI QKD based on passive state preparation is proposed for the first time.This protocol can generate two correlated modes by using a common thermal source through a beam splitter,then the two parties can distribute secret keys according to the basic steps of the general CV-MDI QKD protocol,which can elegantly reduce the complexity of the system as well remove the loopholes from the modulators with inadequate extinction ratio.We also consider the case under the finite-size regime.Moreover,the simulation results show that the increase in the average photon number of the thermal source can help improve the performance of the system and suppress the excess noise induced by passive state preparation effectively.