Experimental violation of a Bell's inequality with efficient detection

Local realism is the idea that objects have definite properties whether or not they are measured, and that measurements of these properties are not af fected by events taking place sufficiently far away(1). Einstein, Podolsky and Rosen(2) used these reasonable assumptions to conclude that quantum mec hanics is incomplete. Starting in 1965, Bell and others constructed mathema tical inequalities whereby experimental tests could distinguish between qua ntum mechanics and local realistic theories(1,3-5). Many experiments(1,6-15 ) have since been done that are consistent with quantum mechanics and incon sistent with local realism. But these conclusions remain the subject of con siderable interest and debate, and experiments are still being refined to o vercome 'loopholes' that might allow a local realistic interpretation. Here we have measured correlations in the classical properties of massive entan gled particles (Be-9(+) ions): these correlations violate a form of Bell's inequality. Our measured value of the appropriate Bell's 'signal' is 2.25 /- 0.03, whereas a value of 2 is the maximum allowed by local realistic the ories of nature. In contrast to previous measurements with massive particle s, this violation of Bell's inequality was obtained by use of a complete se t of measurements. Moreover, the high detection efficiency of our apparatus eliminates the so-called 'detection' loophole.