Generation Of Entangled Source Of Light And Atom Based On ⁸⁷Rb Cold Atom Ensemble

Quantum information science tries to use the ability of quantum field to accomplish the impossible task in classical field.Although sufficient conditions have been formulated for the physical resources required to achieve quantum computing and communication,there is an deeper understanding of the power of quantum measurement combined with conditional evolution for accomplishing diverse tasks in quantum information science.In this respect,an important development is the DLZC scheme proposed by Duan,Lukin,Cirac and Zoller,which is used to realize long-distance quantum communication and entanglement distribution on quantum networks.The theoretical scheme of DLCZ is a probability scheme based on single photon detection of atomic entanglement.In single photon detection,the photon source is essentially indistinguishable,and entanglement is established between long distances through quantum repeater.The DLCZ scheme has built-in quantum storage function and entanglement purification of quantum state after completion of quantum storage and entanglement swapping,which can be fully realized in the current experimental range,and achieve the same goal as the previous more complex scheme.After the proposal of the scheme,the quantum relay scheme based on the atomic ensemble has received extensive attention.Using the spontaneous Raman scattering process,the entanglement source of light and atom is generated in the cold atomic ensemble and the spin wave is stored in the atomic ensemble.Then,the spin wave stored in the cold atom is read and converted into a single photon for detection,realize the entanglement between two photons.This paper mainly introduces the related concepts of quantum information,the preparation of 87Rb cold atom ensemble and the preparation of light and atom entanglement source based on 87Rb cold atom ensemble.In the preparation of the entangled source,the readout efficiency is an important factor affecting the entanglement quality.We measured the change of readout efficiency with storage time and power and the change of bell parameter S with storage time under two kinds of reading frequency.Finally,in order to overcome the repeat rate in the quantum repeat scheme,we propose a temporally multiplexing quantum repeat scheme.The main contents of this paper are as follows:(1)Firstly,the concept of quantum repeat scheme and the implementation of multiplexing quantum repeat scheme are introduced.Then,How to prepare 87Rb cold atomic ensemble by magneto-optical trap technology is introduced,We take the cold atomic ensemble as quantum node?Finally,how to prepare the entangled source of light and atom and how to judge the quality of entangled source are introduced.(2)Based on the cold atom ensemble,using the spontaneous Raman scattering process,through two pairs of writing and reading light,a source of entanglement between light and atom is produced.Based on the entanglement source,the stored spin wave is read out by selecting two different transition frequencies of the reading light at the same writing frequency.The readout efficiency and the bell parameter S value are measured at two different reading frequencies.The readout efficiency is improved significantly with the decrease of bell parameter S value.The interaction between the two readout transition frequencies and atoms is analyzed qualitatively and quantitatively.The theoretical and experimental results are in good agreement.(3)In order to improve the repeat rate in the repeat scheme,we propose a temporally multiplexing quantum repeat scheme.Through the acousto-optic network system,we apply 19 write pulses which come from different directions.A Stokes photon is generated by spontaneous Raman scattering process,while the spin wave is stored in the atom.the FPGA outputs a feed forward signal,a read plus switch on controlled by the signal,which converts the spin wave into an anti-Stokes photon that measured coincidence count with stokes.Compared with single mode entanglement source,The generation probability of Stokes and anti Stokes photons has increased nearly 19 times.Bell parameter S value of the multiplexed entanglement source is 2.30±0.02,which is significantly lower than that of the single mode entanglement source.This is caused by the noise introduced by the unnecessary spin waves stored in the atoms when apply 19 write pulses on atom.(4)The research work of this paper is summarized,and puts forward the next research plan.1.In order to reduce the influence of noise in the temporally multiplexing quantum repeat scheme,we propose an idea in the following experiments,which is to add a medium precision two mirror cavity to the atomic ensemble;2.On the basis of realizing 19 channels of temporally multiplexing quantum repeat scheme,combined with the space multiplexing entanglement sources mentioned in the first chapter,we intend to prepare a space-temporally multiplexing entanglement source to realize multiple modes of high-efficiency optical and atomic entanglement sources...