Most working scientists hold fast to the concept of ‘realism’—a viewpoint according to which an external reality exists independent of observation. But quantum physics has shattered some of our cornerstone beliefs. According to Bell’s theorem, any theory that is based on the joint assumption of realism and locality (meaning that local events cannot be affected by actions in space-like separated regions) is at variance with certain quantum predictions. Experiments with entangled pairs of particles have amply confirmed these quantum predictions, thus rendering local realistic theories untenable. Maintaining realism as a fundamental concept would therefore necessitate the introduction of ‘spooky’ actions that defy locality. Here we show by both theory and experiment that a broad and rather reasonable class of such non-local realistic theories is incompatible with experimentally observable quantum correlations. In the experiment, we measure previously untested correlations between two entangled photons, and show that these correlations violate an inequality proposed by Leggett for non-local realistic theories. Our result suggests that giving up the concept of locality is not sufficient to be consistent with quantum experiments, unless certain intuitive features of realism are abandoned.
We are grateful to A. J. Leggett for stimulating this work and for discussions. We also thank D. Greenberger, M. A. Horne, T. Jennewein, J. Kofler, S. Malin and A. Shimony for their comments. T.P. is grateful for the hospitality of the IQOQI, Vienna. M.A. thanks L. Gohlike for his hospitality at the Seven Pines VIII. We acknowledge support from the Austrian Science Fund (FWF), the European Commission, the Austrian Exchange Service (ÖAD), the Foundation for Polish Science (FNP), the Polish Ministry of Higher Education and Science, the City of Vienna and the Foundational Questions Institute (FQXi).
This file contains Supplementary Equations. The authors give an explicit example of a nonlocal hidden-variable model that fulfils all assumptions of the considered class. It is shown that the model can explain existing Bell experiments. In the second part the authors provide a detailed derivation of the inequality for the class of theories.