The Experimental Study Of Quantum Correlated Photon Pairs In The Cold Atomic Ensemble

Quantum information processing is an interdiscipline composed of quantum mechanics and quantum teleportation.This discipline also involves the correlation theory of quantum computation and quantum private communication.Quantum correlated photon pairs is a key elements of quantum information processing.There are several ways to generate the correlated photon pairs.One way is to use the Spontaneous Raman Scattering(SRS)in the cold atomic ensemble,the other way is to take advantage of the non-linear process of crystal to generate the correlated photon pairs.The way of the cold atomic ensemble is a little complicated,but the linewidth of the photon pairs is narrow and can easy coupling to the atomic ensemble.While the photon pairs generated by non-linear process of crystal has wider linewidth,is unfeasible to directly stored in the atomic ensemble.In 2001,Dual et al proposed the DLCZ protocol(for Duan,Lukin,Cirac,Zoller).In the DLCZ protocol,we take use of the SRS of the atomic ensemble to generate single photons(Stokes photons)at the same time a collective spin excitation is stored.After a period of storage,the spin excitation in the atomic ensemble can be emitted by the reading process meanwhile generating the Anti-Stokes photons.The generated photon pairs are nonclassical correlated.The group of Lukin and Kimble generate the entangled photon pairs via the quantum correlated photon pairs.Using the SRS process,we can get the entanglement between the atom ensemble and the light field.By exchanging the entanglement between two or more separated atoms ensemble,then the quantum network will be carried out.In this paper,we used the Spontaneous Raman Scattering of the atomic ensemble to generate the quantum correlated photon pairs.In this experiment,we studied the influence of the parameter of writing and reading light field to the coincidence counts and the secondorder correlated function.By optimizing the experimental parameters to obtain the relation curve of generation rate and g(2)changing with the writing and reading light field.Based on the existing experiment to find the most suitable parameters for producing correlated photon pairs.And we also improved the programe of the experimental measurement and verified the single photon characteristics of photon pairs.In this paper,the detail researches are as follow:(1)At first,we introduced the fundamental of the atomic ensemble and the concept of Spontaneous Raman Scattering in atomic ensemble.We also sketched the application of quantum repeater in cold atomic ensemble.And then we introduced the way to describe the polarization state of the photon.(2)We described the correlated function and the application of the quantum correlated photon pairs.The single photon feature of the photon pairs was verified.At last,we represented the Bell inequation and the way to calculation the fidelity.(3)On the base of the DLCZ protocol,we generated the quantum correlated photon pairs by SRS in the 87 Rb cold atomic ensemble.The research showed the power of the writing light field and reading light,the detuning of writing light field can effect on the generation rate and g(2)of the correlated photon pairs.When the writing light field excitation probability is 1%,and the detuning of the writing light field at +20MHz,at the same time the reading light field is 30 mW,the generation rates reach 18 per second.After we improved the programe of detection,and increase the writing light field excitation probability,the generation rates has been further improved.And we also verified the single photon feature of the photon pairs,when the writing light field excitation probability is 1.55%,the ? is 0.245,showed a strong single photon characteristics.In order to ensure that after the polarization of photons collected via single mode optical fiber does not change,we have adopted to compensate its polarization by a set of ?4-?2-?4.Then we also assembled and debugged a mode cleaner,using the PDH method we locked the cavity.The transmission of cavity reached 50%,and well optimized the mode of the reading light field.Finally,we summarized the process of generating quantum correlated photon pairs and the measured single-photon-characteristics of Anti-stokes photons.These experimental results provide foundation for generating entangled photon pairs and realizing quantum repeater based on atomic ensemble.