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Quantum entanglement, loophole free

Unprecedented experiments test Bell’s theorem without the need for extra assumptions.

Quantum mechanics posits that a measurement on one of two entangled systems instantly changes the wavefunction of the other, no matter how distant. Might that counterintuitive effect be avoided through a supplementary theory of local hidden variables, in which a measurement’s outcome depends only on events in its past light cone? In 1964 John Bell showed that the question is not merely philosophical: The two types of theory can be distinguished through a series of measurements on separated systems. Experimental Bell tests have come down on the side of quantum mechanics, but until recently, they’ve all failed to close one or more important loopholes. The so-called locality loophole is open when the measurements are too slow, so a hidden light-speed signal emanating from one might affect the outcome of the other; the detection loophole is open when the measurements are too inefficient, so the detected trials could display entanglement-like correlations even when the set of all trials does not. Now three groups have demonstrated experiments that close both those loopholes simultaneously. First, Ronald Hanson and colleagues (Delft University of Technology) performed a Bell test on two diamond-defect spins located in labs 1.3 km apart. They prepared the spins using an entanglement-swapping scheme, as sketched in the figure: entangling each spin with a photon, then jointly measuring the photons at a central location. Each spin was then measured in a basis chosen by a random number generator (RNG). More recently, groups led by Sae Woo Nam (NIST) and Anton Zeilinger (University of Vienna) used a more conventional setup: generating entangled photons at a central source and then measuring them at detectors tens of meters away. The loophole-free results are important, not so much as proof that quantum entanglement is real, but as a stepping stone toward perfectly secure quantum cryptography: Methods for fooling Bell tests on purpose by exploiting the loopholes are similar to those for hacking quantum cryptosystems (see Physics Today, December 2011, page 20). (B. Hensen et al., Nature 526, 682, 2015; L. K. Shalm et al., http://arxiv.org/abs/1511.03189; M. Giustina et al., http://arxiv.org/abs/1511.03190.)

Quantum entanglement, loophole free - figure 1

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