Quantum physics: Death by experiment for local realism

A fundamental scientific assumption called local realism conflicts with certain predictions of quantum mechanics. Those predictions have now been verified, with none of the loopholes that have compromised earlier tests. See Letter p.682 The celebrated Bell inequality, a theorem published by John Bell in 1964, has long served as a basis for experimentally testing whether nature satisfies local realism. All experiments conducted to date have implied rejection of local-realist hypotheses. But because of experimental limitations all those tests suffered from loopholes — either the locality or the detection loophole. Here, Ronald Hanson and colleagues perform a Bell test that closes these loopholes. Their results are consistent with a violation of the inequality, although the authors reject local-realist hypotheses by two standard deviations only. The experimental setup allows for improvements in the statistics that may consolidate the result. In addition to its fundamental importance, a loophole-free Bell test is an important building block in quantum information processing.