The world is as weird as we feared. A new experiment confirms yet again the existence of correlations between distant entangled quantum particles – and this time we have measured the phenomenon so precisely, there is only a minuscule chance it is a fluke.
This is sometimes called “spooky action at a distance”, a phrase used by Albert Einstein in the 1930s when he protested the possibility of two entangled particles exhibiting correlated behaviours across extreme distances, appearing to act instantaneously, violating limits like the speed of light.
Thirty years later, physicist John Bell devised a way to test this odd possibility, a test that has since been used in many experiments. The conclusions these came to have all been the same: that what physicists call “local realism” doesn’t hold. The implication of this is that quantum objects don’t exist when they are unobserved and they can influence each other even when very far apart.
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Another 30 years after Bell, at the Perimeter Institute in Canada formulated an alternative test, which was mathematically simpler, but proved harder to actually do in experiments.
at the University of Science and Technology of China and his colleagues have now conducted Hardy’s test without any so-called loopholes for the first time. Loopholes would be ways in which local realism could still be true, despite experiments finding apparent spooky action at a distance. They found that the probability that our world obeys local realism is the infinitesimally small 1 in 10-16348.
“Our experiment challenges classical physical intuition,” says Zhang.
To put the nail in the coffin of local realism, the researchers used a device that simultaneously emitted two entangled particles of light, or photons, and sent them off in opposite directions. After travelling about 90 metres, each photon hit a separate detector that measured its polarisation, a property that describes how a particle of light is oriented. These detectors had several different ways to assess polarisation, but the method each used was decided as randomly as possible to rule out the possibility of any non-quantum correlation between the two.
After about 6 hours, more than 4 billion particle pairs had been emitted. The researchers tallied up all the measurements to obtain what is known as Hardy’s value. A negative value equates with a world that is locally real, a positive one with the opposite.
The experiment produced a positive Hardy’s value with a statistical significance of 5 sigma, the gold standard in physics for demonstrating that a result isn’t a statistical fluke.
“I found this work very convincing. They have successfully performed a loophole-free experiment,” says Hardy. He says similar past experiments included several loopholes that diluted the strength of the results – for example, in some experiments some of the photons go undetected or the detections are imprecise or erroneous, which can invalidate the test. Zhang says that he and his team improved on both the performance of the detectors and the mathematical analysis of their data.
Ultimately, the detectors had 82.2 per cent efficiency, which at the University of Seville in Spain says is “kind of a miracle” and a result of years of work on making photon detectors as efficient as possible.
“The experiment is undoubtedly very impressive. Closing multiple experimental loopholes simultaneously is always challenging,” says at the Federal Institute of Technology in Zurich, Switzerland.
However, none of the researchers were surprised by the final results as the evidence in favour of nature being non-local has been mounting for years. For Cabello, the technical achievements of the work are very valuable, as they could lead to new discoveries about non-locality and help answer the question of why nature seems to be this way.
Hardy says the fact that local realism fails even in the most cutting-edge experiments is an indicator of something missing from how we understand the world of quantum particles like photons. “We need to find a deeper theory,” he says.
In the meantime, the new experiment could directly benefit technologies that use quantum entanglement as a resource, like incredibly secure quantum encrypted communication methods, says Zhang.
Physical Review Letters
