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Physicists prove Einstein's 'spooky' quantum entanglement

Multiple research teams claim to have conclusively demonstrated quantum entanglement, which had been disdainfully dubbed "spooky action at a distance" by Albert Einstein.

Laser beam in a quantum entanglement experiment at the University of Vienna in 2004.

© VOLKER STEGER/Science Photo Library/Corbis

Few things in science get crazier than quantum mechanics, with related theories sometimes sounding more like paranormal activity than physics. So when such theories gain experimental proof it's a big day for physicists.

Quantum entanglement is a curious phenomenon that occurs when two particles remain connected, even over large distances, in such a way that actions performed on one particle have an effect on the other. For instance, one particle might be spun in a clockwise direction. The result on the second particle would be an equal anti-clockwise spin.

Three different research papers claim to have closed loopholes in 50-year-old experiments that demonstrate quantum entanglement, proving its existence more definitively than ever before.

"Things get really interesting when two electrons become entangled," said Ronald Hanson from the University of Delft. "They are perfectly correlated, when you observe one, the other one will always be opposite. That effect is instantaneous, even if the other electron is in a rocket at the other end of the galaxy."

Albert Einstein, Boris Podolsky and Nathan Rosen described this in a 1935 paper, concluding that either single-particle quantum entanglement was impossible, or that the quantum-mechanical definition of physical reality still needed some work.

In 1964, physicist John Bell proposed that quantum entanglement could be demonstrated by separating the particles at a great enough distance that any correlating effect on both particles could not possibly be caused by local environmental factors. These were called the Bell Test experiments.

NIST physicist Krister Shalm working with the experiment's photon source.


However, the Bell Test experiments had some significant loopholes. The biggest is the "fair sampling" loophole, where the sampled particles are not representative of all entangled particles. Second is the "communication" or "locality" loophole, where the particles could be communicating via some hidden means at light-speed. Thirdly, the "free choice" loophole occurs when the orientation of the particle detectors are not chosen freely or randomly. That lack of free choice could explain any observed correlations.

It is these three loopholes that University of Delft researchers, led by Hanson, US National Institute of Standards and Technology researchers, led by Krister Shalm, and the University of Vienna researchers, led by Anton Zeilinger, claim to have closed.

Hanson and his team used a pair of diamonds with a gap in each diamond's atomic matrix, which trapped a single electron. The diamonds were placed 1.3 kilometres apart. The team then randomly measured one of two properties. If the particles are entangled, they would correlate in a way that cannot be explained by hidden variables.

"The large distance between our detectors ensures that neither the detectors, nor the electrons can exchange information within the time it takes to do the measurement, and so closes the locality loophole," explained lead author PhD student Bas Hensen. "We also close the [fair sampling] loophole, because in this experiment we measure all our entangled pairs." The team's research was published in Nature.

The University of Vienna's experiment, published in PNAS, separated the particles even farther. Using a massive detector between the Canary Islands of La Palma and Tenerife, it separated the particles by 143 kilometres. This allowed the locality loophole to be closed with even greater certainty. They closed the free choice loophole with a random-number generator.

NIST's infographic explaining its experiment.


NIST claims that its results, submitted to the journal Physical Review Letters, are even more definitive. A photon source was placed in one room and two detectors in two others, all three points over 100 metres apart.

The source creates and sends a pair of photons via fibre-optic cable to the two detectors. A random number generator then chooses one of two settings for the detector. If the particle's spin matches the detector settings, the detector picks it up with over 90 percent accuracy.

Use of the random number generator closed the free choice loophole, while the system's accuracy closed the fair sampling loophole. The experiment's efficiency closed the locality loophole.

The team calculated that the chance of locality causing their measurements was only 1 in 170 million.

"You can't prove quantum mechanics, but hidden local action is incompatible with our experiment," Shalm said. "Our results agree with what quantum mechanics predicts about the spooky actions shared by entangled particles."