Experiment Confirms Quantum Weirdness


Breakthrough! The campus of Delft University of Technology where this landmark experiment was conducted showing the locations of equipment housing two spacialy separated electrons 1.3km apart (on either side of the image) together with an intermediate measurement station. Courtesy: photography by Slagboom en Peters BV


Science does not get much stranger than the whacky world of quantum mechanics – our theory of very small sub-atomic particles which seems to defy common sense in so many ways.

Physicists have long known that quantum mechanics is at odds with our everyday view of the world but they have never been able to prove whether this really is the case or whether the theory is somehow flawed. Until now.

Results of an experiment confirming the weirdness of quantum mechanics are reported today (Wednesday 21 October 2015) in the prestigious scientific journal Nature. The experiment proves that common sense has no part to play in the quantum world.

Lead researcher Ronald Hanson, of Delft University of Technology in the Netherlands put it to me this way: “The fundamental science of this experiment shows that the spooky action of quantum mechanics is real and there is no other way of explaining it”.

Quantum mechanics is one of the two great achievements of twentieth century physics; the other being relativity. Relativity says that nothing can travel faster than the speed of light. And this is consistent with the everyday idea that physicists call locality which means that a local action can not affect anything else until a signal travelling at the speed of light has had time to get there; but quantum mechanics appears to break this rule. If two sub-atomic particles are connected through a phenomenon known as quantum entanglement then making an experimental measurement on one particle instantaneously affects the other, no matter how far apart they are.

No reasonable definition of reality could be expected to permit this

Furthermore, we like to think that measurements of what is happening in the quantum arena will correspond with physical properties in the everyday world; that there is some underlying meaningful basis to the apparent randomness and uncertainty of quantum mechanics. This concept is known as realism. Relativity like most of physics appears to be consistent with realism but quantum mechanics is not. We assume that any everyday object will have certain characteristics and features that are particular to it; for example, if we look at a ripe banana, we know that what we will see is a yellow piece of fruit. Quantum mechanics is not like that: the characteristics of quantum particles such as electrons and photons are not fixed in an all-bananas-are-yellow way; instead, the characteristics we see in these particles seem to depend on how we look at them.

These apparent problems with quantum mechanics have been known about from the beginning. The great physicist Albert Einstein was not happy with this state of affairs and thought there must be an error with the theory. Indeed, in 1935 he co-authored a famous scientific paper pointing out the problems with quantum mechanics which stated that “No reasonable definition of reality could be expected to permit this”.

Possibly. But no one said reality had to be reasonable. Nor that Einstein had to be right. The trouble was that scientists were unable to demonstrate whether this strange quantum behaviour was due to a flaw in their theory or whether it was a real consequence of an idea that was essentially correct.

Then in 1964, the physicist John Bell proved that no theory of nature that obeys the twin requirements of locality and realism – what physicists call local realism – can reproduce all the predictions of quantum theory. Crucially, he went on to show how we could test whether the predictions of quantum mechanics were truly incompatible with what we might expect to see in the everyday world.

Bell demonstrated that experimental results would need to fall within a certain statistical range – known as Bell’s inequality – for local realism to hold true at the quantum level; a situation which would imply that quantum mechanics was flawed. However, if the experimental results fall outside of this statistical range then the crazy theory of quantum mechanics would be shown to reject local realism; and we would know for sure that quantum mechanics really does not conform to our common sense notions.

Landmark experiment

So far, all the experiments performed by physicists have supported the idea that quantum mechanics is indeed whacky and rejects local realism. But experimental shortcomings have left caveats and loopholes. Physicists have not quite managed to nail the problem.

Now an experiment has been performed by Hanson and colleagues that is free from these loopholes. They have conducted a Bell test that confirms to a statistically significant extent that quantum mechanics rejects local realism. In short, they have shown that quantum theory really is as weird as we thought.

To do this they had to get around the so called locality loophole and create quantum entangled pairs of particles that are sufficiently far apart so that not even light-speed signals between them could affect the experimental result. They also had to ensure that they plugged the so called detection loophole; this meant that they could not afford to ignore particles that get lost in the system. Previous experiments had closed one or the other of the loopholes but Hanson’s is the first to address both.

By closing these two loopholes in the Bell test experiment, Hanson and colleagues have also shown that it is theoretically possible to use quantum entanglement to create an unbreakable encryption system for electronic communication. “This is potentially more exciting than proving Einstein wrong,” quips Hanson. “It is a proof of principle for secure communications,” he explains.

But the real significance of this experiment is that it provides confirmation that the quantum mechanical world really does not conform with local realism – that our theory is correct.

Make no mistake, if this result is replicated by others then this loophole-free Bell test will prove to be one of the landmark experiments in the history of physics.

Quantum weirdness is real.


This movie explains the concept of locality and quantum entanglement and the current experiment. Courtesy: Text: Michael van Baal. Graphics: Scixel

This movie explains how the Bell test works with a couple in love in the fictitious Bell restaurant (animation). Courtesy: Text: Michael van Baal. Graphics: Scixel


Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres by B. Hensen, H. Bernien, A. E. Dre ́au, A. Reiserer, N. Kalb, M. S. Blok, J. Ruitenberg, R. F. L. Vermeulen, R. N. Schouten, C. Abella ́n, W. Amaya, V. Pruneri, M. W. Mitchell, M. Markham, D. J. Twitchen, D. Elkouss, S. Wehner, T. H. Taminiau & R. Hanson published in Nature 21 October 2015 doi:10.1038/nature15759

Read the abstract and get the paper from here.

See also

Here is the paper that Einstein co-authored that said “No reasonable definition of reality could be expected to permit this”  (Einstein, A., Podolsky, B. & Rosen, N. Can quantum-mechanical description of physical reality be considered complete? Physical Review 47, 777–780 (1935)).

Here is John Bell’s classic Bell inequality paper (Bell, J. S. On the Einstein–Podolsky–Rosen paradox. Physics 1, 195–200 (1964)).

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