High Performance Raman Quantum Memory In Rubidium Atomic Ensembles

Quantum state transfer and memory is widely used in quantum information and quantum computation,especially for quantum networks.As an important media in researches,Atomic ensembles and atom-like media has been widely used in quantum optics in the past.Nowadays,atomic ensembles play an important role in quantum memory since it is easy to control and has high coupling coefficient.Atomic ensembles,especially high optical density atomic ensembles can be used for not only preparation of quantum state but also for the state transfer and memory.Therefore the interaction between high optical density atomic ensembles and laser light is always the focus of the researches in quantum optics.Last decades,researches on the high efficiency quantum state transfer and quantum memory have been fully developed with several protocols such as:Electrical Induced Transparency(EIT),far-off resonant Raman memory,Gradient Photon Echo Memory(GEM)and Faraday effect.Among these protocols,far-off Raman memory brings great concentration since its high speed,high efficient and the capacity of multimode storage.Lots of experimental and theoretical researches focus on the Raman scattering in atomic ensembles has:been done with few achievement.Compare with other protocols,Raman memory has been predict in theory that it can achieve high efficiency quantum memory which is still not done in experiment.Against this backdrop,we study the efficiently quantum state transfer process between 87 Rb atomic ensemble and light.In this article we study the condition of achieving high efficiency and demonstrate high performance Raman memory.The detail research point is as follow:1.Since Raman Scattering can create a correlation between atomic spin wave and light,we experimentally research storage the phase information into atomic ensembles and then retrieve it out.In our experiment,a high order correlation function measurement method has been developed and used for retrieve the phase information from the random phase in spontaneous Raman Scattering.Spontaneous Raman Scattering has been used for encoding the phase information and store in the atomic spin wave.The high order correlation function help to retrieve the phase information from atomic spin wave by decoding from the random phase caused by spontaneous Raman Scattering.2.Based on the experiment result above,we use the correlation between atomic spin wave and light in Raman scattering to research the retrieval efficiency in retrieve process.We firstly study the influence of multi spatial mode effect of the spontaneous Raman Scattering when retrieving the spin wave information.It has been proved in theoretically and experimentally that the spatial mode will affect the retrieval efficiency in Raman scattering.3.Then we use coherent feedback effect to clean the spatial mode of spontaneous Raman scattering and achieved as high as 90%retrieval efficiency.With the help the coherent feedback,we achieve a better correlation between light and atomic spin wave.By measuring of correlation of scattering light and retrieval light we find that the correlation has been purify by the coherent feedback process.Moreover,the coherent feedback help reduced the noise of Raman scattering comparing to the normal type.4.Based on the efficiency retrieval process,we study the far-off resonant quantum memory in experiment.An optimal control pulse with optimal temporal waveform is used in the experiment to achieve a high efficiency of Raman memory.By the optimal control pulse,we also achieved a high fidelity even when the signal only contains a few photons.Such a high performance can be used for real quantum state storage such as single photon storage.5.Then,the coherent feedback process has been used to improve the efficiency and get a high fidelity.We have prove that even several photons can also achieve a high efficiency and fidelity.Moreover,the condition to achieve such high performance in feedback process is much lower than the normal one.This can be used for realization in quantum network.Such research in experiment and theory help to improve the performance of quantum state transfer process and the realization in quantum network and quantum computation.