Test Of Bell's Inequalities Using Trapped Ions - Brief Article

Journal of Research of the National Institute of Standards and Technology, March-April, 2001

In experiments using trapped, laser-cooled ions, NIST researchers in Boulder have demonstrated a violation of Bell's inequalities. These mathematical inequalities provide a basis for experimental tests whose results can distinguish between quantum mechanics and local realistic theories. Many experiments have been performed that are consistent with quantum mechanics and inconsistent with local realism. Because the conclusions derived from these experiments have generated considerable debate, the experiments are still being refined in order to overcome "loopholes" that might affect the results. NIST's experiments are the first to demonstrate violation of Bell's inequalities with massive particles ([9][Be.sup. ]ions) ions) obtained by use of a complete set of measurements. In addition, the high detection efficiency of the experiments eliminates the detection loophole for the first time. All previous experiments have had detection efficiencies low enough to allow the possibility for the subensemble of detected ev ents to agree with quantum mechanics even though the entire ensemble satisfies Bell's inequalities.

The NIST experiments prepare a pair of two-level atomic ions in a repeatable configuration. Next a laser field is applied to the particles; the classical manipulation variables are the phases of this field at each ions position. Finally, upon application of a detection laser beam, the classical property measured is the number of scattered photons emanating from the particles. The Bell signal B was constructed using the results for four sets of phase parameters. Analyses of the photon count distributions indicate that the Bells signal was B = 2.25 [ or -] 0.03, a result that clearly exceeds two, the maximum value allowed by local realistic theories of nature.