High-efficiency photon source brings day of reckoning closer for a famous quantum test

Physicists working at the National Institute of Standards and Technology (NIST) and the Joint Quantum Institute (JQI) are edging ever closer to getting really random.

Their work—a source that provides the most efficient delivery of a particularly useful sort of paired photons yet reported—sounds prosaic enough, but it represents a new high-water mark in a long-term effort toward two very different and important goals, a definitive test of a key feature of quantum theory and improved security for Internet transactions.

The quantum experiment at the end of the rainbow is an iron-clad test of Bell's inequality. The Irish physicist John Stewart Bell first proposed it in 1964 to resolve conflicting interpretations of one of the stranger parts of quantum theory. Theory seems to say that two "entangled" objects such as photons must respond to certain measurements such as their direction of polarization in a way that implies that each knows instantaneously what happens to the other. Even if they're so far apart that that information would have to travel faster than the speed of light.

One possible explanation is the so-called "hidden variable" idea. Maybe this behavior is somehow wired into the photons at the moment they're created; the results of our experiments are coordinated behind the scenes by some unknown extra process. Bell, in a clever bit of mathematical logic, pointed out that were this so, evidence of the unknown variable would show up statistically in our measurements of such entangled pairs—Bell's inequality.
"Simply put," explains NIST physicist Alan Migdall, "we determine the values for each outcome according to this relation and add them up. It says we can't get—in this version—anything greater than two. Except, we do.