Département de Physique Appliquée, Université de Genève, Genève, Switzerland.
Laboratoire d'Information Quantique (LIQ), Université Libre de Bruxelles, Bruxelles, Belgium.
Nat Commun. 2020 May 13;11(1):2378. doi: 10.1038/s41467-020-16137-4.
The possibility of Bell inequality violations in quantum theory had a profound impact on our understanding of the correlations that can be shared by distant parties. Generalizing the concept of Bell nonlocality to networks leads to novel forms of correlations, the characterization of which is, however, challenging. Here, we investigate constraints on correlations in networks under the natural assumptions of no-signaling and independence of the sources. We consider the triangle network with binary outputs, and derive strong constraints on correlations even though the parties receive no input, i.e., each party performs a fixed measurement. We show that some of these constraints are tight, by constructing explicit local models (i.e. where sources distribute classical variables) that can saturate them. However, we also observe that other constraints can apparently not be saturated by local models, which opens the possibility of having nonlocal (but non-signaling) correlations in the triangle network with binary outputs.
量子理论中贝尔不等式违背的可能性对我们理解远距离各方之间可以共享的相关性产生了深远的影响。将贝尔非定域性的概念推广到网络中会导致新型的相关性，然而，对其特征的描述具有挑战性。在这里，我们在无信号和源独立性的自然假设下，研究网络中相关性的约束。我们考虑具有二进制输出的三角形网络，并推导出即使各方没有输入（即，每个方执行固定测量），相关性的强约束。我们通过构建可以使这些约束达到饱和的显式局部模型（即，源分发经典变量的地方）来表明，其中一些约束是严格的。然而，我们还观察到，其他约束显然不能由局部模型来饱和，这使得在具有二进制输出的三角形网络中存在非局部（但无信号）相关性成为可能。
