Sae Woo Nam’s team at NIST is honored for a landmark experiment which, in conjunction with related results elsewhere, put to rest a debate about the nature of quantum reality that has persisted for eight decades. The dispute, first raised by Albert Einstein and colleagues, involves a condition called “entanglement.” The theory of quantum mechanics posits that two entangled particles can be so strongly correlated that a measurement made on one will determine the state of the other – even though the properties are inherently uncertain until the measurement is made, and even if the particles are separated so far that there is no way that one could influence the other without traveling faster than the speed of light. Einstein could not accept that idea of “spooky action at a distance,” and argued that the particles must somehow, at the moment of entanglement, take on a set of “local” properties that determine subsequent measurements.
In the 1960s, Irish physicist John Stewart Bell devised a test to establish the existence of entanglement. And for decades, numerous such “Bell tests” produced results consistent with quantum theory. But none provided conclusive evidence because each was subject to one or more loopholes that allowed a possible alternative interpretation of the findings. But in 2015, Nam’s group – along with separate results from teams in the Netherlands and Austria --succeeded in conducting the first “loophole-free” Bell tests, thus bringing the long-standing question about entanglement to an unequivocal conclusion.