Quantum correlations, often observed as violations of Bell inequalities1,2,3,4,5, are critical to our understanding of the quantum world, with far-reaching technological6,7,8,9 and fundamental impact. Many tests of Bell inequalities have studied pairs of correlated particles. However, interest in multi-particle quantum correlations is driving the experimental frontier to test larger systems. All violations to date require supplementary assumptions that open results to loopholes, the closing of which is one of the most important challenges in quantum science. Seminal experiments have closed some loopholes10,11,12,13,14,15,16, but no experiment has closed locality loopholes with three or more particles. Here, we close both the locality and freedom-of-choice loopholes by distributing three-photon Greenberger–Horne–Zeilinger entangled states17 to independent observers. We measured a violation of Mermin's inequality18 with parameter 2.77 ± 0.08, violating its classical bound by nine standard deviations. These results are a milestone in multi-party quantum communication19 and a significant advancement of the foundations of quantum mechanics20.
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The authors thank D. Hamel and T. Bergmann for technical discussions, J. Dengis and C. Holloway for assistance in the laboratory, M. Ditty, F. Hamdullahpur, D. Huber, D. Parent, R. Zalagenas, M. Lazaridis and G. Dixon for their support in gaining access to the roof of RAC, P. Fulcher for safety training, A. Conrad, AGFA and C. Stewart for allowing access to private property, B. Zinger, G. Doyle, D. Copeland, R. Reger, B. Mill, T. Galloway and Z. Wang for carpentry and electronics, S. Payne at Leysop Ltd for assistance with Pockels cells, M. Morelli at UBNT.ca for technical advice on wireless networks, Roncare and UW plant operations for snow ploughing, and M. Seibel for towing. The authors acknowledge financial support from the National Sciences and Engineering Research Council (NSERC), Canada Research Chairs (CRC), Canada Foundation for Innovation (CFI), European Research Council (ERC, Project “EnSeNa”), Industry Canada, Canadian Institute for Advanced Research (CIFAR), Ontario Centres of Excellence (OCE) and QuantumWorks. R.P. acknowledges support from the Fonds zur Förderung der wissenschaftlichen Forschung (FWF, J2960-N20), Ministry of Research and Innovation Canada (MRI), the Vienna International Post-Doctoral (VIPS) Program of the Austrian Federal Ministry of Science and Research and the City of Vienna, as well as the European Commission (Marie Curie, FP7-PEOPLE-2011-IIF).
The authors declare no competing financial interests.
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Erven, C., Meyer-Scott, E., Fisher, K. et al. Experimental three-photon quantum nonlocality under strict locality conditions. Nature Photon 8, 292–296 (2014). https://doi.org/10.1038/nphoton.2014.50
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