Research On Security Analysis And Performance Improvement Of Continuous-Variable Quantum Key Distribution In Practical Environment

Quantum key distribution(QKD)is one of the most important technologies in the field of quantum cryptography.With the help of the no-cloning principle in quantum physics,two legitimate communication parties can realize the key distribution with information-theoretically unconditional security.In general,according to the space dimension of the quantum states the keys are encoded on,there are two types of QKD,the discrete-variable(DV)and continuous-variable(CV)ones.DV-QKD was come up with earlier,where the key information is always encoded on the relative phase and polarization of the single photon,and the Hilbert space of the quantum state is finite-dimensional.Though CV-QKD was born above one decade later,where the key information is encoded on the infinite dimensional states and the information carrier is the quadrature of the optical field,it attracts widespread attention since its birth,especially the coherent state protocol.This is because CV-QKD can be realized by exploiting the mature standard telecom components commonly used in the classical optical coherent communication,which not only significantly lowers the requirement for quantum state preparation and detection techniques but also saves the cost of system implementation,showing the potential of obtaining large-scale practical applications.Security is the cornerstone of QKD and the first issue to be guaranteed.Although the theoretical security analysis of CV-QKD develops rapidly,it still faces the mismatch between the actual system and the theoretical model on the road of practical application,which will lead to the emergence of security loopholes and the risk of being attacked.Therefore,some countermeasures should be taken.On the one hand,this problem can be solved by designing a protocol that does not depend on the credibility of the device,but the security proof may not be complete.On the other hand,employing monitoring for a CV-QKD system is a feasible solution to this problem thanks to its convenient implementation.On the premise of ensuring security,the practical CV-QKD system aims to achieve high performance in a simplified system structure,including longer secure transmission distance and higher secure key rate.In view of the above two problems faced by the CV-QKD system in the actual environment,this thesis focus on the following works:1.In view of the security problem of the source in the practical one-way CV-QKD system,we analyze and point out the possible security loop-holes caused by the traditional source monitoring scheme,and propose a novel scheme.Source monitoring plays an crucial role for the practical CV-QKD sys-tem as it is the first barrier to ensure the practical security.However,through analysis we found that due to the existence of the laser intensity fluctuation,one of the resources of the source noise,the monitoring result will deviate from the actual one in the conventional source monitoring scheme.By deriving the pa-rameter estimation procedure the secret key rate is shown to be overestimated,inducing security loopholes potentially exploited by the eavesdropper to con-ceal his attack.To solve this problem,we propose a source monitoring scheme by employing the one-time shot-noise unit calibration method,which can mon-itor the impact of the relative intensity noise on the final secret key rate in real time.2.In view of the security problem of the sources in CV measurement-device-independent(MDI)QKD system,according to the resource allocation of the two source sides Alice and Bob,we propose three practical source monitor-ing schemes for CV-MDI QKD.CV-MDI QKD can resist all the attacks against the detection side,but it faces the security problem of the source sides,so the actual security needs to be guaranteed through the monitoring of the source.In order to accurately reflect the actual output of the source,we adopt the source monitoring scheme using one-time shot-noise unit calibration method,and pro-pose three kinds of sources monitoring solution.The CV-MDIQKD system can achieve better performance when using any of the source monitoring schemes.More importantly,the effect of the relative intensity noise of the laser can be monitored in real time without causing new security problems.3.To improve the performance of the C V-QKD,we propose a polarization-multiplexed architecture C V-QKD architecture with a proof-of-principle exper-iment.Currently,almost all the CV-QKD system only make use of a single polarization channel for encoding the key information with another orthogonal polarization channel is in idle,which is a waste of precious channel resources.To further improve the channel spectrum efficiency and increase the secret key rate,we adopting the polarization multiplexing technique into CV-QKD where the phase rotation and polarization mixing can be simultaneously compensated with a dual-reference compensation scheme.The compensation scheme can be realized through post-processing without adding hardware structure,which can simplify the complexity of the system implementation.Moreover,the feasi-bility of the scheme is preliminarily verified by the principle verification ex-periment.The secret key rate is expected to be 50 kbps over 25 km of fiber through the security analysis of the four-state modulation protocol.In addition to improving the secret key rate,the polarization-multiplexd system eliminates the need for polarization control,and can take advantages of the mature opti-cal modules in the optical coherent communication systems,further reducing the cost of the system implementation,which shows a promising prospects of practical application.