Causal networks and freedom of choice in bell's theorem

R Chaves, G Moreno, E Polino, D Poderini, I Agresti… - PRX quantum, 2021 - APS
Bell's theorem is typically understood as the proof that quantum theory is incompatible with
local-hidden-variable models. More generally, we can see the violation of a Bell inequality
as witnessing the impossibility of explaining quantum correlations with classical causal
models. The violation of a Bell inequality, however, does not exclude classical models
where some level of measurement dependence is allowed, that is, the choice made by
observers can be correlated with the source generating the systems to be measured. Here …
Bell’s theorem is typically understood as the proof that quantum theory is incompatible with local-hidden-variable models. More generally, we can see the violation of a Bell inequality as witnessing the impossibility of explaining quantum correlations with classical causal models. The violation of a Bell inequality, however, does not exclude classical models where some level of measurement dependence is allowed, that is, the choice made by observers can be correlated with the source generating the systems to be measured. Here, we show that the level of measurement dependence can be quantitatively upper bounded if we arrange the Bell test within a network. Furthermore, we also prove that these results can be adapted in order to derive nonlinear Bell inequalities for a large class of causal networks and to identify quantumly realizable correlations that violate them.
American Physical Society