用Sagnac偏振纠缠光子源逼近Tsirelson束缚
Approaching the Tsirelson bound with a Sagnac source of polarization-entangled photons
摘要：
高保真偏振纠缠光子是实现量子通信、分布纠缠和量子隐形传态的重要资源。两体纠缠违背了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配置提供了固有的稳定性，紧凑的占地面积和高收集效率，然而，往往是源亮度和纠缠可见性之间的权衡。在这里，我们提出了一个Sagnac偏振纠缠源与$2\sqrt{2}-S=(5.65\pm0.57)\times10^{-3}$，与记录的最高值，同时产生和检测$(4660\pm70)$对/s/mW，这是一个显着更高的亮度比以前报道的Sagnac源和大约两个数量级的亮度比传统的锥源与最高的$S$参数。我们的源记录了$0.9953\pm0.0003$的并发性和$0.99743\pm0.00014$对理想贝尔状态的保真度。通过研究Sagnac源的系统误差，我们发现晶体内的收集焦点的精度在我们的实验中对减小$S$参数起着最大的作用。我们提供了一种途径，可以使最高的$S$参数记录与Sagnac源的日期，同时保持非常高的亮度。
英文摘要：
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 values 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 $S$ parameter. 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.
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