title: Experimental test of nonlocal causality.
abstract: Explaining observations in terms of causes and effects is central to empirical science. However, correlations between entangled quantum particles seem to defy such an explanation. This implies that some of the fundamental assumptions of causal explanations have to give way. We consider a relaxation of one of these assumptions, Bell's local causality, by allowing outcome dependence: a direct causal influence between the outcomes of measurements of remote parties. We use interventional data from a photonic experiment to bound the strength of this causal influence in a two-party Bell scenario, and observational data from a Bell-type inequality test for the considered models. Our results demonstrate the incompatibility of quantum mechanics with a broad class of nonlocal causal models, which includes Bell-local models as a special case. Recovering a classical causal picture of quantum correlations thus requires an even more radical modification of our classical notion of cause and effect.
authors.affiliation: Centre for Engineered Quantum Systems, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.; Centre for Quantum Computation and Communication Technology, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.
authors.forename: Martin
authors.initials: M
authors.lastname: Ringbauer
authors.affiliation: Centre for Engineered Quantum Systems, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.; Centre for Quantum Computation and Communication Technology, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.
authors.forename: Christina
authors.initials: C
authors.lastname: Giarmatzi
authors.affiliation: Institute of Physics and Freiburg Center for Data Analysis and Modeling, University of Freiburg, 79104 Freiburg, Germany.; Institute for Theoretical Physics, University of Cologne, 50937 Cologne, Germany.; International Institute of Physics, Universidade Federal do Rio Grande do Norte, 59070-405 Natal-RN, Brazil.
authors.forename: Rafael
authors.initials: R
authors.lastname: Chaves
authors.affiliation: Centre for Engineered Quantum Systems, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.
authors.forename: Fabio
authors.identifier: 0000-0002-6547-6005
authors.initials: F
authors.lastname: Costa
authors.affiliation: Centre for Engineered Quantum Systems, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.; Centre for Quantum Computation and Communication Technology, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.
authors.forename: Andrew G
authors.identifier: 0000-0001-9639-5200
authors.initials: AG
authors.lastname: White
authors.affiliation: Centre for Engineered Quantum Systems, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.; Centre for Quantum Computation and Communication Technology, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.; School of Engineering and Physical Sciences, Scottish Universities Physics Alliance, Heriot-Watt University, Edinburgh EH14 4AS, UK.
authors.forename: Alessandro
authors.identifier: 0000-0002-5089-3148
authors.initials: A
authors.lastname: Fedrizzi
mesh_terms: D008962:Models, Theoretical; D055096:Optical Devices; D017785:Photons; D011789:Quantum Theory; D012586:Science
keywords: Bell inequality; Quantum foundations; causal models; causality; local causality; local realism; nonlocal causality; nonlocality; quantum mechanics