Our day-to-day reality is deterministic; things happen in specific locations and in a well-defined order. But all that nice, orderly behavior seems to go out the window in the quantum world, a fact that has prompted scientists to try to find ways of squeezing determinism back in. But now, one of the leading ideas for doing so—one that has been on shaky ground for years—may have been definitively ruled out.

The history of this issue dates back to when Einstein derided quantum mechanics for allowing "spooky action at a distance." In a paper where he famously argued his objections formally, he described how entanglement, where the states of two particles are inextricably linked, violates what we call "local realism." If we measure the state of one entangled particle, then we know the state of the other, even if it's halfway across the Universe.

Einstein felt this was ludicrous, but, as people started to devise experiments to test the ideas, it turned out to be correct. As far as we can tell, entanglement either means that reality isn't localized, or measurement triggers some event that is either instantaneous regardless of distance, or moves faster than the speed of light.

Lots of people weren't particularly happy about this, and one of the responses are what have been termed "hidden variable models." These posit that entanglement involves manipulating a property of the particles that we haven't yet defined and are therefore unable to measure. What looks like quantum magic is simply our ignorance.

Testing for ignorance

How does one devise a test for ignorance? The key insight here came from John Stuart Bell. Taking a break from his day job of designing accelerators for CERN, Bell mathematically showed that it was possible to distinguish between quantum mechanics and hidden variables. If hidden variables existed, the results of certain measurements would be less than a critical value. If the measurements violated this inequality, then we'd know that quantum mechanics has to be non-local.

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Over the years, a number of tests have probed Bell's inequality, with all of them coming down against hidden variables. But these experiments left a few loopholes open. The first of these was simply a matter of distance. Testing Bell's inequality requires making two measurements on entangled particles. If you don't make these measurements nearly simultaneously, then it allows something that can travel at the speed of light to carry information between the particles. Local realism is preserved.

This was possible to work around by entangling photons and sending them in opposite directions. With enough distance, even not-quite-instantaneous measurements can still be done faster than the speed of light would allow information to travel between the instruments making the measurements.

While slamming the door shut on one loophole, these experiments opened up a second. Individual photons are hard to measure. Some get lost while running through the optical cabling, and others get lost by the hardware doing the measuring. So, all you really measure is a sample of the total number of photons used in the experiment.

If that sample is typical of the entire population of photons, then this experiment is fine. But it's formally possible that the experimental measurements would exclude a specific subset of photons—the ones that would bring things into agreement with Bell's inequality. (Yes, this sounds weird. No, it doesn't sound any weirder than lots of other things about quantum mechanics.)

Closing loopholes

This last loophole appears to have been closed. Rather than working with photons, a group of European researchers worked with electron spins in nitrogen vacancies of diamonds. These are long-lived and easy to measure, so they handle the sampling problem. But diamonds are notoriously hard to move faster than the speed of light, so it's hard to entangle two of them and separate them by a substantial distance.

To get around this, the researchers started with the diamonds over a kilometer apart in two different labs at the Delft University of Technology. They entangled each electron spin with a photon, then sent the photons over fiber optic cable to a third lab somewhere in between. There, the photons were entangled, which in turn caused the electrons in the diamonds to be entangled as well.

The whole process was horribly inefficient, with a success rate of 6.4 × 10−9, mostly because photons kept getting lost in the fiber optic cables. But, over the course of nine days, the setup managed to successfully entangle the nitrogen vacancies 245 times.

The authors calculate that the 1.3km distance between the diamonds allows them 4.27 microseconds before light could get from one diamond to the other. To be cautious, they only read the state of the nitrogen vacancies for 3.7 microseconds to allow for some experimental error.

Based on their measurements, the key value in Bell's inequality, which is expected to be less than two for hidden variable behavior, is 2.42. Therefore, hidden variables can be rejected with high statistical confidence.

This is not a result that people weren't expecting at this point in time, given that all the loophole-ridden experiments had been suggesting the same thing. And the paper hasn't even been through peer review yet. But, because tests of Bell's inequality are so well understood and the loopholes so well-defined, the work is already receiving high praise. Nature News quotes one physicist as predicting the work will get one of the paper's authors a Nobel within the next few years.

So, if you're one of those people who was hoping that reality might be a bit more, well, real, all we can say is that you're out of luck.

The arXiv. Abstract number: 1508.05949 (About the arXiv).