• Photonics Research
  • Vol. 9, Issue 10, 10001998 (2021)
Yi-Chen Liu1、†, Dong-Jie Guo1、†, Ran Yang1, Chang-Wei Sun1, Jia-Chen Duan1, Yan-Xiao Gong1、2、*, Zhenda Xie1、3、*, and Shi-Ning Zhu1
Author Affiliations
  • 1National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
  • 2e-mail: gongyanxiao@nju.edu.cn
  • 3e-mail: xiezhenda@nju.edu.cn
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    Narrowband photonic entanglement is a crucial resource for long-distance quantum communication and quantum information processing, including quantum memories. We demonstrate the first polarization entanglement with 7.1 GHz inherent bandwidth by counterpropagating domain engineering, which is also confirmed by Hong–Ou–Mandel interference with 155-ps base-to-base dip width and (97.1±0.59)% high visibility. The entanglement is harnessed with 18.5-standard-deviations Bell inequality violation, and further characterized with state tomography of (95.71±0.61)% fidelity. Such narrowband entanglement sets a cornerstone for practical quantum information applications.


    Quantum entanglement is the basis of fundamental quantum mechanics studies and quantum information technologies [13]. So far, the most developed entangled sources are via the optical approach because of its low decoherence and high purity. However, the entangled photon source needs to be compatible with information-processing devices for practical applications, where the photon–electron interaction is normally required. One important example is the memory [46] for quantum information, which is not only essential for quantum computation [7,8], but also necessary to realize quantum repeaters for long-distance quantum communication [912]. It is the ultimate solution to overcome the inevitable photon loss over large communication distances and regain the channel security and data rate. In the above cases, the bandwidth of such photon–electron interaction is fundamentally limited by the energy level of the electrons. The recent breakthrough in the solid-state quantum memories has pushed this bandwidth limit to the order of gigahertz [1318], though such bandwidth is still too narrow for the conventional entangled photon sources based on spontaneous parametric downconversion (SPDC) [19,20]. Much effort has been devoted to shrinking the biphoton bandwidth, such as passive filtering [21,22] or cavity enhancement [23]. But it either reduces the brightness, or adds complexity and instability of the system. On the other hand, the counterpropagating phase-matching [24] geometry can inherently reduce the phase-matching bandwidth [2528] without cavity interactions. This geometry relies on the optical microstructure manufacture, and such counterpropagating domain engineering has been demonstrated for mirrorless optical parametric oscillation [29] and SPDC [3033].

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    Yi-Chen Liu, Dong-Jie Guo, Ran Yang, Chang-Wei Sun, Jia-Chen Duan, Yan-Xiao Gong, Zhenda Xie, Shi-Ning Zhu. Narrowband photonic quantum entanglement with counterpropagating domain engineering[J]. Photonics Research, 2021, 9(10): 10001998
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    Category: Quantum Optics
    Received: Oct. 20, 2020
    Accepted: Jul. 23, 2021
    Published Online: Sep. 15, 2021
    The Author Email: Yan-Xiao Gong (gongyanxiao@nju.edu.cn), Zhenda Xie (xiezhenda@nju.edu.cn)