Quantum Entanglement Test

One of the more bizarre implications of quantum theory is the so-called “spooky action at a distance” effect. If two quantum particles are entangled, measuring the state of one simultaneously defines the state of the other, regardless of the distance between them.

This behavior appears to defy the rule that nothing can travel faster than the speed of light: information regarding the state of particle X must reach particle Y instantaneously, before any measurement of particle Y could take place. As an alternative, Einstein proposed that “hidden variables” pre-program entangled particles to behave in correlated ways, and so there is no need for actual communication between the particles.

The Bell inequality allows experimentalists to differentiate between true “spooky action” and “hidden variable” behavior. Consider two boxes labeled A and B. Each can accept a binary input (0 or 1) and delivers a binary output (-1 or +1). The Bell inequality holds that if both input bits are random, and the boxes cannot communicate with each other, then the correlation between the two output bits will be low. If spooky action at a distance takes place, then preparing the two boxes with a pair of entangled electron spins will force a high correlation between the output of the two boxes.

Actually constructing a Bell inequality test in the lab is difficult. Though it must be possible to prepare the two boxes with an entangled pair of electrons, they must be far enough apart to exclude any possible communication between them. More precisely, the “locality” condition requires that a signal from box A, traveling at the speed of light, cannot communicate its input bit to box B before the output of box B can be measured. Moreover, all trials must be measured. If not all output bits are detected, then it is possible that the set of measured outputs shows a higher correlation than the set of all trials. Previous tests have left open a loophole — either locality or detection — that might allow an explanation other than “spooky action” for the measured behavior.

Recently, workers at Delft University of Technology claim (Nature, http://arxiv.org/pdf/1508.05949v1.pdf) to have designed a loophole-free Bell experiment. Boxes A and B contain diamond chips, each containing a single N-V defect center, the spin state of which is controlled with microwave pulses. Each spin is entangled with the emission time of a single photon, and the two photons are entangled with each other at a third location, C. The physical separation between locations A, B, and C is large enough to close the locality loophole, while the ability to measure the behavior of the NV defect centers at A and B independently of the successful measurement of entangled photons at C closes the detection loophole. Nonetheless, the outputs of boxes A and B were highly correlated, well in excess of the requirements of the Bell inequality.

As a result, the authors claim, the experiment provided the most rigorous test yet of “spooky action,” imposing the strongest restrictions to date on any potential “hidden variable” theory. One of the more immediate implications of this finding is confirmation of the potential for quantum encryption: a loophole-free Bell test can be used to confirm that a commercial device claiming to provide entangled particles for encryption is actually behaving as described.