Experimental Study On Urban Environment Free-space Continuous Variable Quantum Key Distribution

Quantum cryptography,proposed by Wiesner[1]in 1960's,is a cross subject in-cluding physics and cryptography,among which the quantum key distribution(QKD)is the most developed and widely used branch.The first QKD protocol was proposed by Bennett and Brassard in 1984(BB84)[2],which encodes messages via the polarization of a single photon.However,due to the defection and non-ideal performances,and limited by the ability of information encoding,the discrete modulation protocols such as BB84 suffer from disadvantages like relative low detection efficiency and key rate.While coherent states continuous variable QKD protocol was proposed by Grosshans and Grangier in 2002(GG02)[3],which modulates information continuously on two non-communicated quadratures and exploits homodyne detection with silicon based photo diodes to overcome such disadvantages mentioned above and furthermore,it could be convenient to use implementations highly developed in classical communications.In recent years,continuous variable QKD protocols based on fiber channel are studied and applied widely,while in contrast in the free space.Compared to the for-mer,free space channel is not sensitive to polarization and phase,and has lower losses.However,it could be affected by atmospheric turbulence,leading to instability of light intensity and high excess noise,which has substantial influence on the final key rate.Thus free space continuous variable QKD is currently still in the stage of principle ex-periment while some theoretical analysis model on atmospheric turbulence has been reported.In this thesis,we accomplished a unidimensional continuous variable QKD[4-5]ex-periment in urban free space environment of about 500 meters.The transmittance and excess noise and their variance level were given.However,limited by the measurement precision of setup and atmospheric turbulence,the results are unstable.For better es-timation accuracy of channel parameters,we proposed a method to monitor the local oscillator intensity during each pulse period by using auxiliary pulses and by which the variation trends of transmittance and excess noise were given.We wish these works may pave the road for further studies of free space continuous variable quantum key distribution.