麻豆传媒

It鈥檚 all as strange as it seems (Image: Julia Guiches/Picturetank)

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IT鈥橲 official: the universe is weird. Our everyday experience tells us that distant objects cannot influence each other, and don鈥檛 disappear just because no one is looking. Even Albert Einstein was dead against such ideas because they clashed so badly with our view of the real world.

鈥淓ven Einstein was dead against quantum ideas because they clashed with our view of the real world鈥�

But it turns out we are wrong 鈥� the quantum nature of reality means, on some level, these things can and do happen. A groundbreaking experiment puts the final nail in the coffin of our ordinary view of the universe, settling an argument that has raged for nearly a century.

Teams of physicists around the globe have been racing to perfect this experiment for decades. Now, a group led by Ronald Hanson at Delft University of Technology in the Netherlands has finally cracked it. 鈥淚t鈥檚 a very nice and beautiful experiment, and one can only congratulate the group for that,鈥� says Anton Zeilinger, head of one of the rival teams at the University of Vienna, Austria. 鈥淰ery well done.鈥�

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鈥檚 more, it also suggested that, prior to being measured, a particle鈥檚 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 鈥渟pooky action at a distance鈥�. 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 鈥渞eal鈥� 鈥� observing it doesn鈥檛 bring it into existence by crystallising vague probabilities. They argued that hidden variables at some deeper layer of reality could explain quantum theory鈥檚 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鈥檛 handle.

In the 1960s, the debate shifted to Bohr鈥檚 side. John Bell, a physicist at CERN, realised there was a limit to how connected two particles鈥� 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.

鈥淭his is the magic of Bell鈥檚 inequality,鈥� says Johannes Kofler, a member of Zeilinger鈥檚 team. 鈥淚t brought an almost purely philosophical thing, where no one knew how to decide between two positions, down to a thing you could experimentally test.鈥�

And test they did. Experiments have been violating Bell鈥檚 inequality for decades, and the majority of physicists now believe Einstein鈥檚 views on local realism were wrong. But doubts remained. All prior tests were subject to potential loopholes, leaving a gap that could allow Einstein鈥檚 camp to come surging back.

鈥淭he 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,鈥� 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鈥檚 measurements than local realism would allow.

But what if Alice and Bob are passing unseen signals 鈥� perhaps through Einstein鈥檚 deeper hidden layer of reality 鈥� that allow the detectors to communicate? Then you couldn鈥檛 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鈥檛 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鈥檛 work if your equipment doesn鈥檛 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.

鈥淭here鈥檚 a trade-off between these two things,鈥� says Kofler. That meant hard-core local realists always had a loophole to explain away previous experiments 鈥� until now.

鈥淥ur experiment realizes the first Bell test that simultaneously addressed both the detection loophole and the locality loophole,鈥� writes Hanson鈥檚 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鈥檚 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 鈥� just what we need for a Bell test. What鈥檚 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 鈥� 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 (). The weird world of quantum mechanics is our world.

鈥淭he results of the experiment were clear: the weird world of quantum mechanics is our world鈥�

鈥淚f they鈥檝e succeeded, then without any doubt they鈥檝e done a remarkable experiment,鈥� says Sandu Popescu of the University of Bristol, UK. But he points out that most people expected this result 鈥� 鈥淚 can鈥檛 say everybody was holding their breath to see what happens.鈥�

What鈥檚 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. 鈥淏ell tests are a security guarantee,鈥� says Kofler. You could say Hanson鈥檚 team just patched the universe.

There is one loophole left for local realists to cling to, but no experiment can ever rule it out. What if there is some kind of link between the random microwave generators and the detectors? Then Alice and Bob may think they are free to choose the settings on their equipment, but hidden variables could interfere with the choice and thwart the Bell test.

Hanson鈥檚 team notes this is a possibility, but assume it isn鈥檛 the case. Zeilinger鈥檚 experiment attempts to deal with this freedom of choice loophole by separating the random number generators and detectors, while others have proposed using photons from distant quasars to produce random numbers, resulting in billions of years of separation.

None of this helps in the long run 鈥� if the fate of the universe is predetermined, with the flutter of every photon set in stone, no one would ever have a choice about anything. 鈥淭he freedom of choice loophole will never be closed fully,鈥� says Kofler.

What would Einstein have made of this result? Unfortunately he died before Bell proposed his inequality, but he would likely be enamoured with the lengths people have gone to prove him wrong. 鈥淚 would give a lot to know what his reaction would be,鈥� says Zeilinger. 鈥淚 think he would be very impressed.鈥�

This article appeared in print under the headline 鈥淨uantum weirdness is reality鈥�