Generalized $n$-locality inequalities in star-network configuration and their optimal quantum violations

Abstract

Standard multiparty Bell experiments involve a single source shared by a set of observers. In contrast, network Bell experiments feature multiple independent sources, and each of them may distribute physical systems to a set of observers who perform randomly chosen measurements. The $n$-locality scenario in star-network configuration involves $n$ number of edge observers (Alices), a central observer (Bob), and $n$ number of independent sources having no prior correlation. Each Alice shares an independent state with the central observer Bob. Usually, in network Bell experiments, one considers that each party measures only two observables. In this work, we propose a non-trivial generalization of $n$-locality scenario in star-network configuration, where each Alice performs some integer $m$ number of binary-outcome measurements, and the central party Bob performs $2^{m-1}$ binary-outcome measurements. We derive a family of generalized $n$-locality inequalities for any arbitrary $m$. Using {blue}{an elegant} sum-of-squares approach, we derive the optimal quantum violation of the aforementioned inequalities can be attained when each and every Alice measures $m$ number of mutually anticommuting observables. For $m=2$ and $3$, one obtains the optimal quantum value {blue}{for qubit system local to each Alice, and it is sufficient to consider the sharing of} a two-qubit entangled state between each Alice and Bob. We further demonstrate that the optimal quantum violation of $n$-locality inequality for any arbitrary $m$ can be obtained when every Alice shares $\lfloor m/2\rfloor$ copies of two-qubit maximally entangled state with the central party Bob. We also argue that for $m>3$, a single copy of a two-qubit entangled state may not be enough to exhibit the violation of $n$-locality inequality but multiple copies of it can activate the quantum violation.