Nature top ten: An experimental test of non-local realism
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.
The article was selected for the "top ten list" of Nature in April: www.nature.com/nature/topten/index.html
Classical physics clings tightly to the notions of realism (where external reality exists independent of observation) and locality (where sufficiently distant objects cannot influence each other). Quantum experiments in which the properties of distant but entangled particles are linked seem hard to reconcile with such notions, making local realistic theories untenable.
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.
Simon Gröblacher, Tomasz Paterek, Rainer Kaltenbaek, Caslav Brukner, Marek Zukowski, Markus Aspelmeyer & Anton Zeilinger
An experimental test of non-local realism
Nature 446, 871-875 (19. April 2007)
related literature:
A. J. Leggett, “Nonlocal hidden-variable theories and quantum mechanics: An incompatibility theorem”. Found. Phys. 33, 1469–1493 (2003).
