High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. The Bell-CHSH inequality $S\leq2$ is violated by bipartite entanglement and only maximally entangled states can achieve $S=2\sqrt{2}$, the Tsirelson bound. Spontaneous parametric down-conversion sources can produce entangled photons with correlations close to the Tsirelson bound. Sagnac configurations offer intrinsic stability, compact footprint and high collection efficiency, however, there is often a trade off between source brightness and entanglement visibility. Here, we present a Sagnac polarization-entangled source with $2\sqrt{2}-S=(5.65\pm0.57\times10^{-3})$, on-par with the highest $S$ parameters recorded, while generating and detecting $(4660\pm70)pairs/s/mW$, which is a substantially higher brightness than previously reported for Sagnac sources and around two orders of magnitude brighter than for traditional cone sources with the highest $S$ parameters. Our source records $0.9953\pm0.0003$ concurrence and $0.99743\pm0.00014$ fidelity to an ideal Bell state. By studying systematic errors in Sagnac sources, we identify that the precision of the collection focal point inside the crystal plays the largest role in reducing the $S$ parameter in our experiment. We provide a pathway that could enable the highest $S$ parameter recorded with a Sagnac source to-date while maintaining very high brightness.

中文翻译：

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用Sagnac偏振纠缠光子源接近Tsirelson界

高保真偏振纠缠光子是量子通信，分布纠缠和量子隐形传态的强大资源。双向纠缠违反了Bell-CHSH不等式$ S \ leq2 $，只有最大纠缠态才能达到Tsirelson界$ S = 2 \ sqrt {2} $。自发的参量下转换源可以产生纠缠光子，其相关性接近Tsirelson界。Sagnac配置可提供固有的稳定性，紧凑的占用空间和较高的收集效率，但是，通常需要在光源亮度和纠缠可见度之间进行权衡。在这里，我们提出了一个Sagnac偏振纠缠源，其源为$ 2 \ sqrt {2} -S =（5.65 \ pm0.57 \ times10 ^ {-3}）$，与记录的最高$ S $参数相同，同时生成并检测$（4660 \ pm70）pairs / s / mW $，这比Sagnac光源以前报道的亮度要高得多，比$ S $参数最高的传统圆锥光源要亮大约两个数量级。我们的资料来源记录了$ 0.9953 \ pm0.0003 $的并发度和$ 0.99743 \ pm0.00014 $到理想贝尔州的保真度。通过研究Sagnac源中的系统误差，我们确定了晶体内部收集焦点的精度在减少实验中的$ S $参数中起着最大的作用。我们提供了一种途径，可以使Sagnac信号源迄今记录的最高$ S $参数，同时保持非常高的亮度。达到理想贝尔状态的0003美元并发和$ 0.99743 \ pm0.00014 $保真度。通过研究Sagnac源中的系统误差，我们确定了晶体内部收集焦点的精度在减少实验中的$ S $参数中起着最大的作用。我们提供了一种途径，可以使Sagnac信号源迄今记录的最高$ S $参数，同时保持非常高的亮度。达到理想贝尔状态的0003美元并发和$ 0.99743 \ pm0.00014 $保真度。通过研究Sagnac源中的系统误差，我们确定了晶体内部收集焦点的精度在减少实验中的$ S $参数中起着最大的作用。我们提供了一种途径，可以使Sagnac信号源迄今记录的最高$ S $参数，同时保持非常高的亮度。