Experimental Test of Nonlocal Causality

TL;DR

This paper tests nonlocal causal models in quantum mechanics by using experimental data to bound causal influences and evaluate a new Bell-type inequality, revealing quantum mechanics' incompatibility with certain nonlocal causal explanations.

Contribution

It introduces a novel Bell-type inequality for nonlocal causal models and experimentally tests its bounds, challenging a broad class of nonlocal causal explanations of quantum correlations.

Findings

Quantum mechanics cannot be explained by a broad class of nonlocal causal models.

The experiment bounds the strength of causal influence between entangled particles.

Results show incompatibility of quantum mechanics with certain nonlocal causal assumptions.

Abstract

Explaining observations in terms of causes and effects is central to all of empirical science. Correlations between entangled quantum particles, however, seem to defy such an explanation. To recover a causal picture in this case, some of the fundamental assumptions of causal explanations have to give way. Here we consider a broad class of models where one of these assumptions, Bell's local causality, is relaxed by allowing a direct influence from one measurement outcome to the other. We use interventional and observational data from a photonic experiment to bound the strength of this causal influence in a two-party Bell scenario and test a novel Bell-type inequality for the considered models. Our results demonstrate the incompatibility of quantum mechanics with an important class of nonlocal causal models, which includes Bell's original model as a special case. Recovering a classical causal picture of quantum correlations thus requires an even more counter-intuitive modification of our classical notion of cause and effect.