Ontological Models Predictively Inequivalent to Quantum Theory

Recently, it has been argued that no extension of quantum theory can have improved predictive power under a strong assumption of free choice of the experimental settings and validity of quantum mechanics. Here, under a different free choice assumption, we describe a model which violates this statement for almost all states of a bipartite two-level system, in a possibly experimentally testable way. From consistency with quantum mechanics and the nonsignaling principle, we derive a bound on the local averages for the family of deterministic ontological theories that our model belongs to.

Figure 1

Constraint for ${\delta }_{\psi }(a)$ as a function of entanglement parametrized by $\theta$ (dashed line) when $\widehat{A}(a)={\sigma }_z$. The solid line represents ${\delta }_{\psi }(a)$ for the generalization of the Bell’s model. The bound can be saturated in the special case of the local observable here considered by redefining the outcomes supports.