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Test of the physical significance of Bell nonlocality

Carlos VieiraRavishankar RamanathanAd\'an Cabello
Feb 2024
The experimental violation of a Bell inequality implies that at least one of a set of assumptions fails in nature. However, existing tests are inconclusive about which of the assumptions is the one that fails. Here, we show that there are quantum correlations that cannot be simulated with hidden variables that allow the slightest free will (or, equivalently, that limit, even minimally, retrocausal influences) or restrict, even minimally, actions at a distance. This result goes beyond Bell's theorem and demolishes the arguably most attractive motivation for considering hidden-variable theories with measurement dependence or actions at distance, namely, that simulating quantum correlations typically requires a small amount of these resources. We show that there is a feasible experiment that can discard any hidden-variable theory allowing for arbitrarily small free will and having arbitrarily small limitations to actions at a distance. The experiment involves two observers, each of them choosing between two measurements with $2^N$ outcomes. The larger $N$ for which a specific Bell-like inequality is violated, the larger the set of excluded hidden-variable theories. In the limit of $N$ tending to infinity, the only alternatives to the absence of hidden variables are complete superdeterminism or complete parameter dependence. We also explore the implications of this result for quantum information.
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