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 $(97.1\pm 0.59)\%$ high visibility. The entanglement is harnessed with 18.5-standard-deviations Bell inequality violation, and further characterized with state tomography of $(95.71\pm 0.61)\%$ fidelity. Such narrowband entanglement sets a cornerstone for practical quantum information applications.

中文翻译：

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具有反向传播域工程的窄带光子量子纠缠

窄带光子纠缠是长距离量子通信和量子信息处理（包括量子存储器）的重要资源。我们通过反向传播域工程证明了具有 7.1 GHz 固有带宽的第一次极化纠缠，这也通过具有 155 ps 基到基倾角宽度的 Hong-Ou-Mandel 干扰和$(97.1\pm 0.59)\%$较高的知名度。纠缠被利用 18.5 标准差的贝尔不等式违反，并进一步用状态断层扫描表征$(95.71\pm 0.61)\%$保真度。这种窄带纠缠为实际量子信息应用奠定了基石。