1.State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China 2.Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Fund Project:Project supported by the National Basic Research Program of China (Grant No. 2016YFA0301402), the National Natural Science Foundation of China (Grant Nos. 11475109, 11274211), the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 11604191), the Applied Basic Research Program of Shanxi Province, China (Grant No. 201601D202007), and the Shanxi Provincial Fund for “1331 Project” Key Subjects Construction, China (Grant No. 1331KSC).
Received Date:06 July 2018
Accepted Date:07 November 2018
Available Online:01 January 2019
Published Online:20 January 2019
Abstract:The photon-atom interface is a basic component of quantum repeater, quantum network, and linear optical quantum computing. Different approaches have been tested in the last decade to develop quantum interface, such as quantum dots, single atoms and ions, color centers and cold atomic ensemble. In the cold atomic ensemble, a normal way to produce photon-atom interface is the Duan-Lukin-Cirac-Zoller (DLCZ) protocol. Used in the DLCZ protocol is an atomic ensemble that can emit single photons while creating a single atomic excitation, which is stored in the ensemble. The atomic excitation can be converted into a photon due to the collective interference. The influences of the retrieval efficiency on the atom-photon entanglement source have been studied in various experiments. But no one has studied the retrieval efficiency threshold of entanglement generation. In our experiment we study the retrieval efficiency dependence on read power and OD. Setting the power of the repump light beam to be 12.2 mW, 5.0 mW, 2.0 mW, 0.5 mW and 0.3 mW, OD of the cold atom ensemble is measured to be 20, 17, 10, 2, and 1, respectively. As we expected, the retrieval efficiency increases with increasing OD value and read power, the curve shows that the retrieval efficiency increases sharply with increasing the OD value and read power, then after a while slowly increases with increasing the OD values and read power. Then we measure the Bell parameter with increasing the retrieval efficiency by increasing the read power. It shows that the Bell parameter sharply increases for retrieval efficiency values ranging from 0 to 3%, but changes very small for retrieval efficiency values ranging from 3% to 18.3%. The maximum Bell parameter is 2.6. We further analysis the result, finding that the Bell parameter can be expressed as $S = \dfrac{{{S_{{\rm{MAX}}}}r}}{{(1 + 2\chi )r + 2B}}$. Fitting parameters to the curve are $\chi$= 1%, B = 0.073%. To avoid of multi-excitation the write power kept low that $\chi$ at 1% level. Then we can find out from the function that the signal-to-noise ratio is bigger than 6∶1 the Bell parameter will reach 2. The theoretical analysis and experimental results fit very well. So the further reason that alter the Bell parameter is the signal-to-noise ratio. We should decrease the noise while increasing the retrieval efficiency. This paper will help with rise the quality of entanglement generation through photon-atom interface. Keywords:cold atomic ensemble/ spontaneous Raman scattering/ retrieval efficiency/ photon-atom entanglement
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3.1.光学厚度与读出效率的关系
实验上研究了光量子纠缠界面系统中光学厚度与读出效率的关系, 通过改变冷原子中再抽运光的功率大小, 改变原子系综的光学厚度. 实验中测得再抽运光功率为12.2, 5.0, 2.0, 0.5和0.3 mW时冷原子介质对应的光学厚度为20, 17, 10, 2和1. 测量了读出效率随OD的变化, 实验结果如图4所示.可以看出,随着OD的增大, 光与原子纠缠界面的读出效率逐渐增大, 由2.1%增加至18%. 当OD由10继续增加时, 光与原子纠缠源的读出效率继续增加但相对之前变化缓慢. 图 4 读出效率随光学厚度的变化 Figure4. The retrieval efficiency as the function of optical depth.
23.2.反斯托克斯光子读出效率随读光功率的变化关系 -->
3.2.反斯托克斯光子读出效率随读光功率的变化关系
测量了反斯托克斯光子读出效率随读光功率的变化, 实验结果如图5所示, 其中黄色点表示读出效率$\gamma $随读光功率的变化, 黑色点表示反斯托克斯光子计数$N_{\rm AS}$随读光功率的变化, 红色点表示反斯托克斯光子收集通道上的噪声计数$N_{\rm b}$随读光功率的变化, 其中$N_{\rm AS}$和$N_{\rm b}$均是在300万次实验条件下得到的测量计数. 随着读光功率的增加, 读出效率和$N_{\rm AS}$逐渐增大, 两者的变化趋势基本一致, 而背景噪声基本不变, 当读光功率大于1.5 mW之后, 读出效率没有明显增加, 趋于饱和. 图 5 读出效率及$\scriptstyle N_{\rm AS}$随读光功率变化 Figure5. The retrieval efficiency and $\scriptstyle N_{\rm AS}$ as the function of power of read light field.