Investigation Of Practical Security Of Continuous Variable Quantum Key Distribution

With the development of quantum physics and quantum information theory,and relying on the unique properties of quantum signal,quantum private communication is put forward.It has the feature of unconditional security and is attracting more and more attention from people of a variety of fields.Its appearance has changed the traditional form of encryption,which relies on the mathematics and logic.And it opens the new mentality and method for cryptography based on the physics theory.Quantum private communication is based on the quantum cryptography,and quantum key distribution(Quantum key distribution,QKD)is the most mature of quantum private communication.One can implement the QKD using continuous variable or discrete variable quantum state.In theory,the QKD system has unconditional security.However,due to the imperfections of devices,unconditional security of system will be threatened to some extent.Therefore,the study of the practical security of the QKD system is of great significance to the practical application.This thesis focuses on the continuous variable quantum key distribution(Continuous variable quantum key distribution,CVQKD)system and studies its practical security,aiming at improving the performance and practical security of system in the real environment.In this thesis,the main researches are listed as following:1.We investigate the practical security monitoring based on a general attack,i.e.,an arbitrated individual attack or collective attack on the CVQKD system by Eve in this thesis.Actually,Eve may attack the system in a general way,i.e.,any kinds of individual or collective attacks,by using the imperfections of the system.The low bound of intensity disturbance of the local oscillator signal for eavesdropper successfully concealing herself is obtained,considering all noises can be used by Eve in the practical environment.Furthermore,we obtain an optimal monitoring condition for the practical CVQKD system so that legitimate communicators can monitor the general attack in real-time.As examples,practical security of two special systems are investigated under the special attacks.2.We integrate machine learning to achieve automatic parameter prediction for practical CVQKD.In this thesis,a novel approach is developed by integrating support vector regression(Support vector regression,SVR)model to optimize the performance and practical security of the QKD system.First,a SVR model is learned to precisely predict the time-along evolutions of the physical parameters of signals.Second,such predicted results are employed as feedback to control the QKD system for achieving the optimal performance and practical security.Finally,our proposed approach is exemplified in the Gaussian modulated coherent state(Gaussian modulated coherent state,GMCS)QKD system.The results show that the scheme can achieve the expected effect and has several significant advantages.3.To improve the performance and security,in this thesis we present a CVQKD scheme based on continuous random basis choice.Firstly,we introduce the scheme in detail.And then we calculate the secret key rate under collective attacks when using reverse reconciliation and investigate the practical security under realistic quantum channel and detector.The results show that this way obviously improves the performance,such as the secure communication distance.Our scheme avoids comparing the measurement basis and discarding the key bits,and it can be easily implemented with current technology.Moreover,the imperfection of the basis choice can be well removed by the known phase compensation algorithm.