Violations of locality and free choice are equivalent resources in Bell experiments

TL;DR

This paper demonstrates that in Bell experiments, violations of locality and free choice are equally resource-intensive, with their costs directly related to the degree of Bell inequality violations, making causal explanations for such violations interchangeable.

Contribution

It introduces a measure of resource cost for violating locality and free choice, showing their equivalence in explaining Bell inequality violations based on experimental statistics.

Findings

Locality and free choice violations require equal resources to explain observed correlations.

The measure of these violations is directly related to Bell inequality violations.

Results are applicable to any nonsignaling statistics in Bell experiments.

Abstract

Bell inequalities rest on three fundamental assumptions: realism, locality, and free choice, which lead to nontrivial constraints on correlations in very simple experiments. If we retain realism, then violation of the inequalities implies that at least one of the remaining two assumptions must fail, which can have profound consequences for the causal explanation of the experiment. We investigate the extent to which a given assumption needs to be relaxed for the other to hold at all costs, based on the observation that a violation need not occur on every experimental trial, even when describing correlations violating Bell inequalities. How often this needs to be the case determines the degree of, respectively, locality or free choice in the observed experimental behavior. Despite their disparate character, we show that both assumptions are equally costly. Namely, the resources required to explain the experimental statistics (measured by the frequency of causal interventions of either sort) are exactly the same. Furthermore, we compute such defined measures of locality and free choice for any nonsignaling statistics in a Bell experiment with binary settings, showing that it is directly related to the amount of violation of the so-called Clauser-Horne-Shimony-Holt inequalities. This result is theory independent as it refers directly to the experimental statistics. Additionally, we show how the local fraction results for quantum-mechanical frameworks with infinite number of settings translate into analogous statements for the measure of free choice we introduce. Thus, concerning statistics, causal explanations resorting to either locality or free choice violations are fully interchangeable.