Experimental violation of a Bell's inequality with efficient detection

Abstract

Local realism is the idea that objects have definite properties whether or not they are measured, and that measurements of these properties are not affected by events taking place sufficiently far away. Einstein, Podolsky and Rosen used these reasonable assumptions to conclude that quantum mechanics is incomplete. Starting in 1965, Bell and others constructed mathematical inequalities whereby experimental tests could distinguish between quantum mechanics and local realistic theories. Many experiments have since been done that are consistent with quantum mechanics and inconsistent with local realism. But these conclusions remain the subject of considerable interest and debate, and experiments are still being refined to overcome 'loopholes' that might allow a local realistic interpretation. Here we have measured correlations in the classical properties of massive entangled particles (9Be+ 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 theories of nature. In contrast to previous measurements with massive particles, this violation of Bell's inequality was obtained by use of a complete set of measurements. Moreover, the high detection efficiency of our apparatus eliminates the so-called 'detection' loophole.

The ultimate physical limits of privacy

“…Shocking as it may be, our world is not of this kind. Statistical inequalities, such as e = 1/4, derived on the assumption that the values of unmeasured physical quantities do exist and commonly referred to as Bell's inequalities 33 , have been violated in a number of painstaking experiments 33,34,35,36,37,38,39,40,41,42,43,44 . We shall not dwell on the philosophical implications of this experimental fact (volumes have been written on the subject), but simply point out that it should be embraced by all those who worry about secrecy because what does not exist cannot be eavesdropped, and so it is much easier to keep secrets in a non-classical world.…”

Bell violation using entangled photons without the fair-sampling assumption

“…Due to experimental limitations, fair sampling has been assumed in nearly every Bell experiment performed to date; a few exceptions include (5)(6)(7)(8). To date, it has never been possible to avoid this assumption with photons due to the absence of efficient sources and detectors.…”

Quantum entanglement: facts and fiction – how wrong was Einstein after all?

“…We also draw attention to the possible importance of temperature. While the long-range entanglement still remains to be proven for material particles, there are many successful experiments evidencing their quantum nature (Rowe et al 2001;Turchette et al 1998). …”

“…Wineland and his group (Rowe et al 2001;Turchette et al 1998) demonstrated violation of Bell's inequality with material particles, using 9 Be + ions prepared in Bell-like singlet and triplet states, driven by stimulated Raman processes from pairs of laser beams in one of two geometries, an important parallel to the photon experiments that we shall discuss later. In a topical review Leggett (2002) discusses limits of QM and specifically over what length scales QM has been tested -he points into some directions where QM might potentially break down, one being very small length scales (very high energies) and another, which could be also chemically and biophysically interesting, being short times and space-like separation in the sense of special relativity.…”