Experimental Study Of Preparation Of Entangled Photon Pairs In Cold Atomic Ensemble

As the core of the quantum communication resources,the preparation of entangled state has been closely focused.By the absorption of light quantum information is inevitable in the optical fiber,long-distance quantum key distribution is still facing a serious problem.In order to solve this problem,firstly the theory of quantum repeater scheme was put forward in 1998.In 2001,a few schlors including LuMing proposed a practical quantum repeater experiment scheme based on linear optic and atomic ensemble,namely the famous DLCZ?short for Duan,Lukin,Cirac,Zoller?scheme.At present,the preparation of entanglement source of the high quality has become one of the main research contents in the field of quantum physics.In the experimental process,often the generation of correlated photon pairs is seen as the basis of obtaining entangled photon pairs.Now,the common method for preparating correlated photon pairs is spontaneous parametric down-conversion.The experiment device for the scheme is simple and easy to operate while line width of photons is always in the THz magnitude,which can't match with quantum memory line.In 2008,a SPDC scheme through a enhanced cavity from Jian-wei Pan group had realized MHz magnitude of line width of photon pairs.At the same time,we observe that it is difficult to meet the higher technical requirements.In order to solve the problem of matching line width,several research groups began using the spontaneous Raman scattering?SRS?process to complete the non-classical correlation between photons.The scheme was achieved by the Kimble team of the United States in the experiment as early as in 2003.Ultilizing spontaneous Raman scattering process,this paper implements the preparation of correlated photon pairs in cold atom ensemble,and on this basis,two road entangled photons sources are produced.In the end,by applying the right projection measurement,three photon GHZ state is obtained.The main contents of this paper are as follows:?1?It introduces the development of cold atoms and quantum entanglement and the basic principle of DLCZ scheme.?2?A pair of stokes and anti stokes photons are produced in cold ⁸⁷Rb atoms through applying spontaneous Raman scattering?SRS?process,and the related experiments have been carried out for studying the characteristics of photon pairs.By analysing mechanism and selecting the appropriate experimental parameters,such as read,write,detuning to study the generated rate of correlated photon pairs as the change of storage time,and the second order intensity correlation function as the change of the storage time.In addition,the anti-stokes photon announced type single photon statistical properties of alpha change with storage time and write light excitation rate of relationship.?3?Taking advantage of experimental preparation of correlated photon pairs above,the photon pairs from two different space modes are generated and collected.Caused by the change of the storage time,the value of second-order correlated function and photonic coincidence counts are measured.By changing the stokes photon measuring Angle,we also obtained polarization correlation function of photon pairs.In addition,by giving different projections to the stokes and anti stokes photons,it measured Bell parameter values S of correlated photons in two different space modes.And then,the value of S in separated space pattern as different storage time is measured.At the same time,different projection measurement was implemented basing on orthogonal basis vector of H-V,D-A and R-L,the experiment did obtain value of state fidelity on the moment of storage time 30 ns,and further measure the state fidelity value when storage time changed.In the end,applying appropriate projection measurement to one of the four photons state,three photon GHZ state is observed,which proved divergence between local realism by using the entanglement criterion and Mermin inequality.?4?The generation of photon pairs under different conditions on the process of SRS is summarized while the experimental process of the correlation and entangled measurement result is obtained which pave the way for further improvement of entanglement source and the realization of quantum repeater.