Key Technology Study Of High-speed Continuous-variable Quantum Key Distribution System

Quantum private communication is a burgeoning subject,it combine the advantage of the quantum mechanics and classics cryptology and can obtain the true security.Quantum key distribution?QKD?is an important element of quantum private communication.From the viewpoint of the practicality,quantum key distribution has huge value in the field of military,finance and information security.So,study of QKD is very strategic.Continuous-variable quantum key distribution?CV-QKD?,as a significant implementation of quantum private communication,has many advantages,such as high secret bit rate,low cost,convenient deployment and anti-interference,etc.in the short distance.Especially it could be combined with existing classical optical network by CWDM.Therefore,CV-QKD has huge economic values and become the main development trend of future quantum cryptography.In this dissertation,we study the quantum cryptography schemes and their securities,the stability of the system,the practical security of the CV-QKD and the influences of the faulty device.The main innovative results are listed as following:1.We study the synchronization of the CV-QKD and design a simple and highefficiency synchronization method according to the quantum channel.We expound the rationality of the method,bring in relevant parameters,make the simulation and finally analyze the performance of the synchronization.We verify the proposed method on high-speed CV-QKD system with 25 MHz repeatability clock.We analyze the parameters in the proposed method by the experimental data and the influence to successful ratio of synchronization,give the optimal value of the parameters in laboratory.Experimental results show that our method has good effect of synchronization and is helpful to high-speed and stabilized CV-QKD system.2.We study the correlative character of the random number and propose a new quantum random number generation scheme which is implemented by measuring quantum noise of the amplified vacuum state.What's more,we study the relations among the gain of the light amplifier,the length of quantization,total entropy,quantum noise entropy and final generation rate of random number.The results show that the length of quantization has a knee point according to the specific gain of the light amplifier.Quantum noise entropy will increase linely under the knee point while increase indistinctive and become more complicated when exceed it.The results also show the gain of the light amplifier has a knee point according to the specific length of quantization.Similarly,Quantum noise entropy will increase indistinctive when exceed the knee point.So,we can choose the proper gain of the light amplifier and length of quantization,then we can get much more quantum random number,and simultaneously the system is simple and suited to realize in engineering.3.We study the practical security of the CV-QKD system and the relations between the secure key rate and the frequency of repeatability clock under the condition of the finite sampling bandwidth effects.The results show that under the conditon of finite sampling bandwidth effects,paramater estimation will deviate while Eve shift the pusle and this effect reduces the lower bound of secret key rate,which may leave security loopholes for Eve to attack the system.Furthermore,the secret bit rate will converge to a saturation value but not always increase with the system repetition rate due to the finite sampling bandwidth effect.To resist the influences of such imperfections,we propose an available countermeasure,called the dual sampling detection scheme,to solve this problem.The proposed method applies another ADC to measure a part of LO and sampling the output.Results show that the finite sampling bandwidth effects can be effectively removed and this approach is available in practical pulsed CVQKD system.In addition,the secret key rate linearly increases with the system repetition rate f_rep with our proposed approach.4.We study the stability of the practical high-speed CV-QKD.According to the environment and influences of the faulty device,we analyze the polarization drift and phase drift detailedly and propose the corresponding solution.We design and optimize the whole experiment and complete the experiment of the high-speed CV-QKD system in the laboratory.The results show that our solution is effective and CV-QKD system can work well up to 12 hours and get final secure key rate of 52 Kbps with 25 MHz repeatability clock on 50 Km fiber link.