Continuous-variable Quantum Key Distribution: Optical System Design And Improved Schemes

Quantum private communication, as an emerging discipline, has received peo-ple’s great attention since it came out. By combining the classical cryptography andthe quantum physics, it explains the concept of security from the physical layer andbrings revolution in cryptography and communication. From the viewpoint of futuretrends, quantum private communication has great application perspective and com-mercial values, on both the military matters and business areas. Therefore, it is veryvaluable to research on quantum private communication.Continuous-variable quantum key distribution, as a signifcant implementation ofquantumprivatecommunication, hasmanyadvantages, suchashighsecretbitrate, lowcost and convenient deployment, etc. Especially it could be combined with existingclassical optical network, which permits the quality of quantum signal. Therefore,continuous-variable quantum communication is the main development trend of futurequantum cryptography.In this thesis, we study the design of optical system and study the improved CV-QKD scheme. The main researches are listed as following:1. Wedesignandfnishthetotalopticalcircuitsofafber-basedcontinuous-variablequantum key distribution system. We describe the design principle of the opti-cal system in detail, including the structure of the components and the controlof polarization. The system uses a mixed structure of single-mode fbers andpolarization maintenance fbers. It has been packaged in compact modules andhas good stability. Then we make experiments to test the key parameters in theCV-QKD system, and evaluate the performance of the total system.2. We study the stability of the practical CV-QKD system. First we analyze thefactors which afect the long-time stability of CV-QKD system, including the stability of the CW laser source and the drift of the bias voltage. Then we focuson the phenomenon of polarization drift in fber-based CV-QKD system. Weanalyze its reasons, and propose a novel polarization compensating scheme. Fi-nally we make experiments to validate the scheme. Experimental results showthat this compensating scheme indeed has an excellent performance.3. We study a generalized protocol of discrete modulation for continuous-variablequantum key distribution with coherent states in a Gaussian lossy and noisychannel and investigate its performance against collective attacks. We fnd thatdiscrete modulation protocols with more than eight states do not perform betterthan the eight-state protocol. Then, we study the improvement of this protocolby using a nondeterministic noiseless linear amplifer (NLA) on Bob’s detec-tion stage. Results indicate that a NLA can signifcantly extend the maximumtransmission distance.4. We propose a novel scheme for CV-QKD using the subcarrier multiplexing tech-nique. This scheme allows to distribute channels independent Gaussian mod-ulated CV-QKD in parallel with one laser source and several phase modula-tors. We analyze the infuence of nonlinear signal mixing and the security in theasymptoticlimit. Resultsindicatethatbyusingthismultiplexingtechnique, eachchannel will be introduced a non-Gaussian extra source noise, resulting slightlyshort of the maximum transmission distance, while the total secret key rate canbe considerably increased.