Bell inequalities represent one of the cornerstones of quantum foundations, and a fundamental tool for quantum technologies. Although a lot of effort was put in exploring and generalizing them, because of the wave function collapse it was deemed impossible to estimate the entire Bell parameter from one entangled pair, since this would involve measuring incompatible observables on the same quantum state. Conversely, here it is reported the first implementation of a new generation of Bell inequality tests, able to extract a Bell parameter value from each entangled pair and, at the same time, preserve the pair entanglement instead of destroying it. This is obtained by exploiting sequences of weak measurements, allowing incompatible observable measurements on a quantum state without collapsing its wave function. On the fundamental side, by removing the need to choose between different measurement bases our approach stretches the concept of counterfactual definiteness, since it allows measuring the entangled pair in all the bases needed for the Bell inequality test, intrinsically eliminating the issues connected with the otherwise not-chosen bases. On the practical side, after our Bell parameter measurement the entanglement within the pair remains (basically) unaltered, hence exploitable for other quantum-technology-related or foundational purposes.

中文翻译：

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单个纠缠对上贝尔参数的纠缠保持测量


贝尔不等式是量子基础的基石之一，也是量子技术的基本工具。尽管人们在探索和推广它们方面付出了很多努力，但由于波函数塌缩，人们认为不可能从一对纠缠估计估计整个贝尔参数，因为这将涉及测量同一量子态上不兼容的可观测量。相反，这里报道了新一代贝尔不等式测试的首次实现，能够从每个纠缠对中提取贝尔参数值，同时保留对纠缠而不是破坏它。这是通过利用弱测量序列来实现的，允许在量子态上进行不兼容的可观测测量，而不会破坏其波函数。从根本上来说，通过消除在不同测量基础之间进行选择的需要，我们的方法扩展了反事实确定性的概念，因为它允许测量贝尔不等式检验所需的所有基础中的纠缠对，从本质上消除了与其他测量相关的问题未选择的基地。在实际方面，在我们的贝尔参数测量之后，对中的纠缠保持（基本上）不变，因此可用于其他量子技术相关或基础目的。