## A Very Clever Experimental Test of a Bell Inequality

Travelling and very busy most of today so not much time to post. I did, however, however get time to peruse a very nice paper I saw on the arXiv with the following abstract:

For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. In his seminal work, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory. Bell showed that in any local realist theory the correlations between distant measurements satisfy an inequality and, moreover, that this inequality can be violated according to quantum theory. This provided a recipe for experimental tests of the fundamental principles underlying the laws of nature. In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments to date required additional assumptions to obtain a contradiction with local realism, resulting in loopholes. Here we report on a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell’s inequality. We employ an event-ready scheme that enables the generation of high-fidelity entanglement between distant electron spins. Efficient spin readout avoids the fair sampling assumption (detection loophole), while the use of fast random basis selection and readout combined with a spatial separation of 1.3 km ensure the required locality conditions. We perform 245 trials testing the CHSH-Bell inequality S≤2 and find S=2.42±0.20. A null hypothesis test yields a probability of p=0.039 that a local-realist model for space-like separated sites produces data with a violation at least as large as observed, even when allowing for memory in the devices. This result rules out large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification.

While there’s nothing particularly surprising about the result – the nonlocality of quantum physics is pretty well established – this is a particularly neat experiment so I encourage you to read the paper!

Perhaps some day someone will carry out this, even neater, experiment!

PS Anyone know where I can apply to for a randomness certificate?

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August 26, 2015 at 6:45 pm

These things are well worth doing in order to close loopholes, but nobody seriously doubts nowadays that the predictions of QM are going to fail in such experimental situations. I think that the GHZ and Hardy demonstrations of “Bell effects” are more striking (although it is less obvious that nonlocality is the culprit); see refs 6 and 7 in

https://telescoper.wordpress.com/2015/08/03/guest-post-hidden-variables-just-a-little-shy/

August 26, 2015 at 11:33 pm

“…the nonlocality of quantum physics is pretty well established…”

Well, even that paper notes there are -two- assumptions tested by these things, and locality is only one of them.

August 27, 2015 at 7:25 am

There are three hats in the ring: Bell’s inequality, quantum mechanics, and experimental observations. Nonlocality is inferred by seeing whether observations violate Bell’s inequality. They do, but those experiments involve one or two further assumptions – so congratulations to these experimentalists in eliminating one of those.

If, however, you regard this experimental work as a test of quantum mechanics in a hitherto-untested part of parameter space, you get the result that QM passes the test once more; and by now, after 80 years, you have gained strong confidence in it. Now, the predictions of QM unequivocally violate Bell’s inequality – and therefore imply nonlocality.

That is the appropriate way to view these results.

August 27, 2015 at 10:55 am

Well, there’s locality, realism, and quantum mechanics. I agree nobody is surprised about quantum mechanics, but (as far I can tell from skimming the paper) we’re still assuming both locality and realism to get an answer. I don’t know which of those is wrong, and I don’t think there’s any good way to tell, but one of our other rather successful theories relies on locality and not realism.

August 27, 2015 at 10:56 am

The paper defines realism: “physical properties are defined prior to and independent of observation”. So, an electron has its position even if we don’t look, for example.

August 27, 2015 at 6:00 pm

Thanks for defining it before I launched into a rant about how it’s not testable! Kochen-Specker showed that some physical properties of noncommuting observables cannot exist simultaneously – a stronger result than Heisenberg’s that they can’t be measured simultaneously. But they used QM in deriving this result. So, axioms that seem distinct in these results are in fact tied together. Tricky stuff…

August 27, 2015 at 9:15 pm

I’ll have to look that up! Is it related to QM being inconsistent with counter-factual definiteness? But I can’t recall if -that- result requires locality. To some extent a lot of this ends up restating Bell’s Theorem (or its implications).

August 27, 2015 at 9:22 pm

Bell is not the same as Kochen-Specker although David Mermin showed how they are related. See the references in my hidden variables essay at

https://telescoper.wordpress.com/2015/08/03/guest-post-hidden-variables-just-a-little-shy/

I have my own exposition of Kochen-Specker which i’ll email to Peter with a request to email on to you.

August 27, 2015 at 8:04 pm

The proof of this inconsistency with the principles of reality, if accurate, would immediately discard the very potentiality of Quantum Theory’s existence. It’s amusing but common and comprehensible that this type of anti-reality pseudo-science has gained so much prominence over the years, as we are indeed living in a dominantly philosophically-pseudo world culture.

August 27, 2015 at 9:20 pm

I am sympathetic to your rant, but there is more than one definition of the word “realism” in all this. The definition I think you mean is not one that is experimentally testable, but rather motivates science, and I agree with you. Brendan is using it to mean something a bit different above.