The quantum entanglement of spatially separated objects embodies the essence of what's profoundly weird about quantum theory. Albert Einstein called it spukhafte Fernwirkung (spooky action at a distance). Independent measurements on the two separated objects manifest a degree of correlation that no amount of collusion before the separation could possibly have prearranged. And yet, since the early 1970s experimenters have been creating entangled states that do indeed exhibit the predicted correlation that common sense—as quantified in John Bell's famous inequality—excludes as impossible. 1 (See the article by David Mermin in Physics Today, April 1985, page 38.)
Pioneering tests of Bell's inequality with entangled photons separated by macroscopic distances were carried out by Stuart Freedman and John Clauser in the 1970s and by Alain Aspect and coworkers in the 1980s. And at distances on the order of microns, entanglement has been studied in pairs of atoms with a view...
