Research On The Non-classical Correlated Photon Source Based On A Cold Atomic Ensemble

Building the quantum repeater for entanglement swapping,transmission and purification is a key point to realize long distance communication.First step is achieving effective transformation of the entanglement from the photon carrier to other media in which coherent information could be preserved much longer,that is, quantum memory.For some advantages,most of the schemes are based on atomic ensembles.In this article,the design of the whole experimental setup for cooling and trapping the atoms is described,which consists of the ultra-high vacuum system,the light path and background magnetic field for the MOT,the monitor system for real time observation,especially,the frequency stabilization and frequency shift of the diode lasers,which is very important to obtain a stable cold atomic cloud.The fluorescence intensity and absorption rate is measured to know how many atoms are trapped in the cloud.The optical depth,which is of great importance of the subsequent experiments,is greatly improve by replacing the Rb source with a better one,increasing the intensity and diameter of the trapping beams and choosing proper frequency detuning of the light.Spatial mode coupling is important for the generation and collection of non-classical correlated photon pairs.But it is difficult to achieve mode coupling in actual experiments as the scattering light is very weak.So we establish a stimulated Raman scattering set-up via four-wave mixing process in a double A-type atomic ensemble.Subsequent experiments verify the relation between spatial mode coupling and the efficiency of the wave mixing.As the probing and coupling beams are applied on the atoms simultaneously,the electromagnetically induced transparency signal is observed due to the coherent population trapping effects.The mixing light is collected and measured while the third pumping field exists,both in the collinear and non-collinear situation.While the coupling field is removed and the frequency of the probing light is locked to off resonance.The weak anti-Stokes signal forward and the Stokes signal backward are collected by single photon diode detectors.The coincidence detecting of them indicates non-classical correlation and violation of the Cauchy-Schwartz inequality.The experimental parameters,such as the intensity and frequency of the input field,the polarization angles,are varied to see their influence on the correlation, which is compared with the theoretic predictions.The writing and reading light are modulated to pulses by temporal sequence signal.The read pulse is delayed for several hundred nanoseconds.The normalized cross-correlation function of Stokes and anti-Stokes signal is measured and we find non-classical correlation still exists between them,that is,we establish the non-classical correlation between the anti-Stokes photon and the stimulated collective spin excitation mode in the cold atoms.Some other theoretic work is involved is this article.A scheme of probabilistic and robust preparation of a GHZ-type state via atomic ensembles and linear optics is proposed.There are some innovations in this article.Non-classical correlated Raman scattering photon pairs are generated in a noncollinear configuration and the influence of the experimental parameters such as the frequency detuning of the writing light on the cross-correlation function is investigated,both in experimental analysis and theoretic calculations.If the optical depth of the atomic cloud and the transmission efficiency of the generated photons cannot be improved a lot,a suitable choice of the parameters is of great importance to the non-classical property of the correlation source.