Conversion Between Light And Atomic Coherence In Cold Atom Ensemble

The conversion between the light and atomic coherence is one of the basic problems in studying the interaction between light and atom.Quantum state transformation between light and atom can be used to realize entanglement of macroscopic atomic ensemble,quantum teleportation,quantum storage,and quantum repeater.Therefore,it plays a very important role in quantum information processing and quantum network.The number of atoms in the atomic ensemble can be greatly improved through the laser cooling and trapping technology,and the ground state coherence performance of the atoms can be effectively manipulated to maximize the mode matching with the input light field mode.The atomic ensemble can achieve more than 90% of the atom-light conversion efficiency and even optical storage efficiency.The part of the field that is not stored in the coherence of the atomic state,known as leakage,is generally considered to be the loss of the quantum system and cannot be exploited.However,as long as the coherence of the field is not destroyed,the information it carries can be reused.The linear transformation between the light and atomic coherence is essentially a linear beam splitter model,and the atom ensemble itself is highly manipulable.Therefore,the transformation between light and atom can be used to realize the manipulable quantum interface.Based on the concept of dark state polarition in electromagnetically induced transparency effect,the concept of virtual optical-atomic beam splitter is proposed,and the conversion between light and atomic coherence is studied in the cold atom ensemble.In addition,it is found that the inevitable loss in the conversion process will lead to non-Hermitian conversion between light and atomic coherence.By adjusting the laser frequency and the optical depth of the atomic ensemble,we found that the light-atomic beam splitter can be transformed from Hermitian to non-Hermitian.Therefore,we have implemented a Hermicity- adjustable beam splitter,which has more freedom of operation than conventional solid-state beam splitter.This paper includes the following three works:1.The DLCZ scheme is used to realize the light-atom coherence conversion to generate correlated photon pairs.In a two-dimensional magneto-optical trap,the correlation photon pair and heralded single photon are generated through reading and writing,and the variation of two-photon wave packet is studied under different experimental conditions.In the reading process of DLCZ scheme,the conversion of atomic spin wave is observed by changing the light intensity of reading light,and the beam splitting process of atomic spin wave is simulated.The reading efficiency is proportional to the reading intensity and pulse width.2.In the three-level electromagnetically induced transparency(EIT)system,coherent beam splitting between optical field and atomic ground state coherence is realized through EIT storage procedure.EIT storage efficiency is also different under various control light conditions,and the ratio of unstored light and atomic spin wave will change.When controlling light from 0.5 m W to 12 m W,the ratio of light and atomic coherence is effectively adjusted.At 3 m W,the ratio of light and atom is 50:50.Therefore,in DLCZ and EIT storage processes,the conversion efficiency between light and atoms can be controlled by changing the laser intensity of pumping light,so as to achieve a beam splitter with adjustable beam ratio.3.Combine the light and atomic spin wave on the virtual beam splitter,and cause the simultaneous conversion and interference of the light and atomic spin wave by applying the control light pulse.By adjusting the single photon detuning of electromagnetically induced transparency and atomic optical depth can control the loss in the process of splitting,which affect the interference result: when the loss can be ignored,light and atomic spin wave transformation is Hermitian,interference fringe complement with each other;when the loss increases,the conversion between light and atoms present non-Hermitian features.Finally,a Hermicity-adjustable light-atom beam splitter is realized.