August 31, 2015 report

# Researchers find a way to close both loopholes in testing entanglement with Bell's inequality

*arXiv*so that others may see it while it undergoes peer review prior to being published in a yet to be announced journal.

At issue is proving that quantum entanglement does not occur due to some strange unexplainable communication factor, or variable as Einstein suggested—a task that has proved exceptionally challenging—so much so that despite nearly a century of trying, no one, until now apparently, has been able to do it.

One of the ways to "prove" that entanglement does not occur due to some unknown factor that allows for communication to move between two entanglement particles, is to cause entanglement to come about between two particles that are far enough apart that any unknown force allowing them to communicate, would have to travel faster than light, which everyone agrees cannot happen. That was one of the loopholes described by John Bell, who famously came up with a way to prove mathematically that it should be possible to distinguish between quantum mechanics and so-called hidden variables. If such variables existed, he noted, measurements of certain results would have to be less than a critical value. If an experiment could be run that violated that inequality, that would "prove" that quantum mechanics has at least some non-local characteristics. Another loophole, it has been noted, occurs because single photons are difficult to measure—some get lost during transmission, particularly if sending them at a great enough distance to overcome the first loophole, making experimental results difficult to verify.

In this new experiment, led by Ronald Hanson, the researchers set about closing both loopholes, which would theoretically shut the door on local realism. They set up two stations for creating photons entangled with an electron spin, far enough apart to close the first loophole. The entangled photons were all sent to a common third location via fiber cable where they were entangled under just the right conditions and measured (and tested for measurement with their entangled mate back at the original site). Knowing that the process would be highly inefficient, they arranged for the whole experiment to be repeated, over and over—at the end of nine days they had just 245 successes, but that was enough to meet Bell's inequality rule, showing that there was no hidden variable allowing for communication between entangled pairs—"proving" that local realism does not always apply in the quantum world.

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**More information:**Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km, arXiv:1508.05949 [quant-ph] arxiv.org/abs/1508.05949

**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.

© 2015 Phys.org

**Citation**: Researchers find a way to close both loopholes in testing entanglement with Bell's inequality (2015, August 31) retrieved 15 June 2019 from https://phys.org/news/2015-08-loopholes-entanglement-bell-inequality.html

## User comments

shaveraNoumenonThis non-intuitive nature of QM is really just a physical realization of an epistemological fact pointed out by Immanuel Kant, imo,.... that in order for experience to be de facto possible, objects of experience must conform to our a-priori and mind dependant conceptual structure, rather than that our concepts conform to objects of experience.

Kant wrongly believed that this guarantees certainty wrt synthetic a priori judgments. Rather, what is found is that yes, our a-priori intellectual faculties determine the 'form' of experience and so the conditions for the intuitive understanding, .... but by its nature is delimited to the macro-realm where the mind evolved to be able to observe. At the micro realm, we end up with an artificial synthesis given the above mentioned conditions for experience (observation) to be possible.

Noumenon..............

The article should be careful to emphasize that the Bell inequalities disprove "local" hidden variables,... as the Bell inequalities do not in fact disprove non-local hidden variables.

NoumenonEnd of hyperbole.

antigoracleI would suggest trying to walk through the wall, getting to your favourite lottery vendor, to purchase that winning ticket. Then, at least, you would have saved yourself a lot of cash.

PS. remember me when you win.

antigoracleSo, what you are saying, is that we all share this common delusion that is the "real" world?

NoumenonOnly that 'phenomenal reality' necessarily has a mind dependent component,.... i.e. entanglement,... and that the act of observation disturbs the system... the uncertainty principal is more profound than just being the observer-effect,...etc

The lesson of the Copenhagen Interpretation is rather that one must give up intuitive understanding for predictive knowledge. And further that this knowledge is not one of 'independent reality', but rather of the underlying-reality-as-experienced,.... as wrapped in concepts,... as subject to mind dependent conditions for observation.

NoumenonWhat do you object to in the above post? I answered a question directed to me. Are you implying that I'm wrong about what I meant? I then stated the Copenhagen position,... whether you agree with it or not, it is still of what I posted. Of course, I willing to be corrected if I'm factually wrong....

antialias_physorgNon-local hidden variables would constitute information transmission faster than light. And from all that we know (and confirmed by every experiment so far) that is the case.

QM goes counter to our intuition, but as you rightly point out: intuition is an evolved trait that evolved based on stimuli of the macro world. There is no reason why these rules (intuition) we formed about such a limited scale/range should be applicable at all scales.

So why it may 'feel wrong'. However, if every experiment tells us that our intuition about this is wrong then maybe we should accept at some point that our intuition is wrong and just accept the results of experiment.

NoumenonYes, exactly, which is what I meant when I posted ".... one must give up intuitive understanding for predictive knowledge." The point of the post was in support of a positivist pov as opposed to a 'realist' pov.

The Bohm-deBroglie pilot wave theory makes use of non-local correlations in a way that is compatible with SR, QM, and experimental observation,....[ not that I care for that model,... I was only pointing out a fact about Bell theorem.]

antialias_physorgDamn...missing a 'not'. Every experiment done so far tells us that FTL information transmission isn't part of this universe.

NoumenonImo, as in the Copenhagen interpretation, QM IS complete as is possible. Those who are dissatisfied seem to expect reality to be compatible with their intuitions, so they invent metaphysical baggage to save their sense of intuitive understanding. Bohr to Einstein; " ... Stop telling [Reality] what to do with his dice. ..... "

NoumenonI understood what you meant.

dociledociledocileNoumenonI read your link. There is no communication FTL in that experiment,... that would imply energy transfer through space in a time to account for the correlation. No.

Imo, the issue is one of misinterpretation of the experiment,... in confusing mind dependent components of phenomenal-reality with objective reality (see above). In particular that space and time are not physical substances, but rather necessary 'forms of thought' given the nature of mind. IOW, the mind is 'designed' with those intuitions in order to form a synthesis of experience.

The underlying objective reality at the quantum scale may have no need for space or time, or they are emergent causual orderings only relevant at the macro-scale.

From your link,...."The experiment shows that in quantum mechanics at least, some things transcend space-time, says Terence Rudolph"

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