Causal Networks and Freedom of Choice in Bell's Theorem

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.

Automated summary

Bell's theorem proves the incompatibility between quantum theory and local hidden-variable models, showing that classical causal models cannot explain quantum correlations, but measurement dependence can be quantitatively upper bounded within a network.