Definitions of multipartite nonlocality

In a multipartite setting, it is possible to distinguish quantum states that are genuinely $n$-way entangled from those that are separable with respect to some bipartition. Similarly, the nonlocal correlations that can arise from measurements on entangled states can be classified into those that are genuinely $n$-way nonlocal, and those that are local with respect to some bipartition. Svetlichny introduced an inequality intended as a test for genuine tripartite nonlocality. This work introduces two alternative definitions of $n$-way nonlocality, which we argue are better motivated both from the point of view of the study of nature, and from the point of view of quantum information theory. We show that these definitions are strictly weaker than Svetlichny's, and introduce a series of suitable Bell-type inequalities for the detection of three-way nonlocality. Numerical evidence suggests that all three-way entangled pure quantum states can produce three-way nonlocal correlations.

Figure 1

Let $X,Y,a,b\in \{0,1\}$. The particle pair labeled 1 is independent from the pair labeled 2. The joint state ${\lambda }_1$ is such that if $a_1=Y_1$, then $P_{{\lambda }_1}\left(a_1{b}_1\right|X_1{Y}_1)=0$, whereas ${\lambda }_2$ is such that if $b_2\ne X_2$, then $P_{{\lambda }_2}\left(a_2{b}_2\right|X_2{Y}_2)=0$. Consistent predictions are impossible if measurement choices are as shown.