"To understand what Hanson and his colleagues did, we have to go back to the 1930s, when physicists were struggling to come to terms with the strange predictions of the nascent science of quantum mechanics. The theory suggested that particles could become entangled, so that measuring one would instantly influence the measurement of the other, even if they were far apart. What's more, it also suggested that, prior to being measured, a particle's properties only exist in a fuzzy cloud of probabilities.

 

"Nonsense, said Einstein, who famously proclaimed that God does not play dice with the universe and called entanglement 'spooky action at a distance'. He and others favoured the principle of local realism, which broadly says that only nearby objects can influence each other and that the universe is 'real' -- observing it doesn't bring it into existence by crystallising vague probabilities. They argued that hidden variables at some deeper layer of reality could explain quantum theory's apparent weirdness. On the other side, physicists like Niels Bohr insisted that we accept the new quantum reality, because it explained problems that classical theories of light and energy couldn't handle.

 

"In the 1960s, the debate shifted to Bohr's side. John Bell, a physicist at CERN, realised there was a limit to how connected two particles' properties could be if local realism was to be believed. He formulated this insight into a mathematical expression called an inequality. If tests showed that the connection between particles exceeded the limit he set, local realism was toast. 'This is the magic of Bell's inequality,' says Johannes Kofler, a member of Zeilinger's team. 'It brought an almost purely philosophical thing, where no one knew how to decide between two positions, down to a thing you could experimentally test.'

 

"And test they did. Experiments have been violating Bell's inequality for decades, and the majority of physicists now believe Einstein's views on local realism were wrong. But doubts remained. All prior tests were subject to potential loopholes, leaving a gap that could allow Einstein's camp to come surging back. 'The notion of local realism is so ingrained into our daily thinking, even as physicists, that it is very important to definitely close all the loopholes,' says Zeilinger.

 

"A Bell test begins with a source of photons, which spits out two at a time and sends them in different directions to two detectors, operated by a hypothetical pair conventionally known as Alice and Bob. The pair have chosen the settings on their detectors independently so that only photons with certain properties can get through. If the photons are entangled, they can influence each other and repeated tests will show a stronger pattern between Alice and Bob's measurements than local realism would allow. But what if Alice and Bob are passing unseen signals -- perhaps through Einstein's deeper hidden layer of reality -- that allow the detectors to communicate? Then you couldn't be sure that the particles are truly influencing each other in their instant, spooky way. This is known as the locality loophole, and it can be closed by moving the detectors far enough apart that there isn't enough time for a signal to cross over before the measurement is complete. Previously, Zeilinger and others did just that, including shooting photons between two Canary Islands 144 kilometres apart.

 

"Close one loophole, though, and another opens. The Bell test relies on building up a statistical picture with repeated experiments, so it doesn't work if your equipment doesn't pick up enough photons. The problem gets worse the further you separate the detectors, seeing as photons can get lost on the way. So moving the detectors apart to close the locality loophole begins to widen the detection one. 'There's a trade-off between these two things,' says Kofler. That meant hard-core local realists always had a loophole to explain away previous experiments -- until now.

 

"'Our experiment realizes the first Bell test that simultaneously addressed both the detection loophole and the locality loophole,' writes Hanson's team in a paper detailing the study. In this set-up, Alice and Bob sit in two laboratories 1.3 kilometres apart, far enough to close the locality loophole. Each laboratory has a diamond containing an electron with a property called spin. The team hits the diamonds with randomly produced microwave pulses. This makes them each emit a photon that is entangled with the electron's spin. These photons are sent to a third location, C, where a device clocks their arrival time. If photons arrive from Alice and Bob at exactly the same time, the two electron spins become entangled with each other. So the electrons are now entangled across the distance of the two labs -- just what we need for a Bell test. What's more, the detectors observing their spin are of high enough quality to close the detector loophole. But the downside is that few pairs of photons arrive at C together -- just a few per hour. The team took 245 measurements, so it was a long wait. The result was clear: they detected more highly correlated spins than local realism would allow (arxiv.org/abs/1508.05949v1). The weird world of quantum mechanics is our world. 'If they've succeeded, then without any doubt they've done a remarkable experiment,' says Sandu Popescu of the University of Bristol, UK. But he points out that most people expected this result -- 'I can't say everybody was holding their breath to see what happens.'

 

"What's important is that these kinds of experiments drive the development of new technology like quantum cryptography, he says. Networks that use quantum properties to guarantee secrecy are already springing up across the globe, but the loopholes are potential bugs in the laws of physics that might have allowed hackers through. 'Bell tests are a security guarantee,' says Kofler. You could say Hanson's team just patched the universe." [Quoted from here; accessed 12/09/2015. Several paragraphs merged; three links added. Quotation marks altered to conform with the conventions adopted at this site.]