The Key Technique Of Remote Continuous Variable Quantum Key Distribution System

In recent years,people pay more and more attention to the security in transmission of information.Quantum key distribution(QKD),as an important branch of quantum information,uses quantum states as the carrier of secure key to transmit messages between communication parties.The security depends on the basic principles of quantum mechanics rather than the complexity of mathematical computation.The enciphered massage using the shared secure keys and one-time pad protocol cannot be eavesdropped in principle by any third party(Eve)even though powerful quantum computing is employed.Therefore,QKD has a wide application scope in military,government affairs and finance,at the same time it has become one of the research hotspot of quantum information in the world.The main technical routes of QKD include discrete variable quantum key distribution(DV-QKD)and continuous variable quantum key distribution(CV-QKD).DV-QKD has been made significant progress in the laboratory and practical application,but there have the defect in high cost and difficulties in single photon detection.Compared with the DV-QKD,the carrier of the information in CV-QKD protocol is not the polarization or phase of the single photon,but the quadrature component of optical field.At present,the closest scheme to practicality is the Gaussian modulation coherent state(GG02)protocol,because of the characteristics of preparation of the source coherent state,the higher theoretical secure key rate in the medium and short distance,the low-cost balanced homodyne detection and the good compatibility with the existing optical communication network.Since the GG02 protocol is introduced in 2002,its theoretical unconditional security has been demonstrated and the relevant experimental systems are also being extensively researched.CV-QKD also began to move from laboratory to practical application.For further improving system performance,we also need to take into account of the practical security caused by imperfect devices,such as: influence mechanism and solution of imperfect Gaussian modulation on CV-QKD system.Another problem is how the CVQKD system can work normally even though under complex environment,for example: the influence of the fluctuation of SOP caused by the single mode fiber in external environment on the CV-QKD system and high-speed polarization control technology.At present,the loss of the optical fiber seriously limits the transmission distance of QKD,an effective solution is to develop quantum repeater,and one of the important parts is that it is necessary to consider quantum-state-preserving frequency conversion(QFC)that can connect quantum memory devices with quantum communication devices.For the sake of solving above problems,this thesis develops relevant theoretical and experimental research.Innovations in this thesis are:1.The fluctuated effects of phase modulator's half-wave voltage,extinction voltage and half-wave voltage of amplitude modulator on excess noise and channel loss are researched theoretically and experimentally.Based on this,two effective schemes are proposed and demonstrated to calibrate the working parameters of the modulators and high precision Gaussian modulation coherent state preparation is realized.When the transmission fiber is 50 km,the excess noise of system is controlled below 0.02,which can work efficiently and stably for a long time.2.The influence fluctuation of SOP on excess noise and channel loss of the CV-QKD system is analyzed theoretically and its validity was verified experimentally.Furthermore,based on the foundation of FPGA hardware and an integral type optical detector,multistep simulated annealing algorithm and self-adaptive gradient algorithm were proposed to search the target SOP satisfying the demand of CV-QKD system and achieve high-speed polarization controlling under the pulsed light.For the case of single random polarization scrambling,the average time of entire polarization controlling program could be as low as sub-milliseconds(827 ?s).For the case of continuous polarization scrambling,the CV-QKD system can still operate normally even though the scrambling rate is 314 rad/s combining the approach that the raw keys are filtered in terms of the relative phase fluctuations between the signal and the local oscillator.The problem of fluctuations of SOP due to complex external environment is effectively solved.3.Quantum frequency up-conversion of continuous variable entangled states via sum-frequency-generation process is demonstrated experimentally.The two-color entangled state initially entangled at 806 nm and 1518 nm is converted to a new entangled state at 530 nm and 1518 nm with the amplitude quadrature difference squeezing of 1.7 d B and phase quadrature sum squeezing of 1.8 d B,which satisfy the inseparability criterion for CV quantum entanglement.The maximum classical up-conversion efficiency(photonnumber conversion efficiency)for the weak 806 nm input field to the 530 nm up-conversion field is observed to be 80%.The implementation enables the observation of entanglement between two light fields spanning approximately 1.5 octaves in optical frequency.The experiments verify the high efficiency transfer of quantum information between quantum memory devices consisting of alkaline atoms with operation wavelengths around 0.8 ?m and quantum communication devices with operation wavelengths around 1.5 ?m,making it a practical building block for quantum information processing and communication networks.It can also be employed to prepare complex multicolor entangled states where direct generation of such states are usually difficult or even impossible.