Probability that the quantum world obeys local realism is less than one in a billion, experiment shows

July 20, 2017 by Lisa Zyga, Phys.org feature

The experiment was performed on the Ludwig Maximilian University campus in Munich, Germany. Trap 1 is located in the basement of the physics building, and trap 2 is in the basement of the economics building, 398 meters away. Credit: Rosenfeld et al. Published by the American Physical Society
(Phys.org)—Physicists have reported some of the strongest evidence yet that that the quantum world does not obey local realism by demonstrating new evidence for the existence of quantum entanglement. By performing an essentially loophole-free Bell test, they have shown that two atoms separated by a distance of a quarter of a mile share correlations that should be impossible under the hypothesis of local realism, and are most likely explained by quantum entanglement.

The new Bell test was performed by a group of researchers led by Harald Weinfurter at the Ludwig Maximilian University of Munich and the Max Planck Institute for Quantum Optics, both in Germany.

The probability that the observed correlations can be explained by due to some unknown "hidden variables" rather than entanglement is less than one in a billion, the physicists write in their paper published in Physical Review Letters. By accounting for all of their accumulated data, taken over the course of seven months, that probability drops even further, down to about one in ten quadrillion (the number 1 followed by 16 zeros). This means that the violates either locality (that distant objects cannot influence each other in less than a certain amount of time) or realism (that objects exist whether or not someone measures them), or possibly both.

Three Bell tests

The test reported here is the latest -free Bell test: one that simultaneously closes the two biggest loopholes, the locality loophole and the detection loophole. Closing both loopholes is vital for excluding any alternative explanations, such as the possibility that two entangled objects are secretly sharing information (locality loophole) or that the particles being detected are not representative of the whole sample but rather form a special subset that skews the data (detection loophole).

The first loophole-free Bell test, reported in 2015 by a team led by Ronald Hanson at the University of Delft, demonstrated entanglement between the electron spins of nitrogen-vacancy (NV) centers in diamond. Shortly after, other loophole-free Bell tests reported entanglement between photons. The Bell test reported here demonstrates entanglement between a third type of system: the spin states of atoms.

"In my opinion, the greatest significance of this work is the definite ruling out of local realism," coauthor Wenjamin Rosenfeld, at the Ludwig Maximilian University of Munich and the Max Planck Institute for Quantum Optics, told Phys.org. "It is good that similar experiments were performed with different systems (photons, NV centers) essentially at the same time, so all results together can be taken as truly conclusive. Now it is no more a matter of belief whether nature can or cannot be described in a local-realistic way, but a matter of fact. (However, the freedom-of-choice problem still needs to be solved.)"

Experimental setup

The new experiment involved trapping one rubidium atom in the basement of the physics building at the Ludwig Maximilian University of Munich and trapping a second rubidium atom in the basement of the economics building, about 400 meters away. An optical fiber connected the two measurement sites.

In their tests, the scientists excited the atoms, causing them to emit photons at precisely defined times. The photons then travelled through the optical fiber and interfered with each other. This quantum interference, in theory, causes the atoms to become entangled. To detect this entanglement, the researchers performed measurements on the photons, repeating the measurements over and over for tens of thousands of photon pairs. The results showed overwhelmingly that the distant photon pairs were indeed entangled.

Last loophole

One of the last remaining possible loopholes for most Bell tests concerns the choice of measurement made on the atoms. Since these measurements can be performed in multiple ways, it's important to confirm that the experimenter is free to choose which particular measurement to make, and that hidden variables are not influencing the choice of measurement and somehow allowing the atoms to synchronize their properties. This possibility is called the free-will or freedom of choice loophole.

To attempt to close this loophole, the researchers used a high-speed quantum that chooses measurement settings that are truly random—almost. The problem is that there is a very slight possibility that the random number generators could have communicated with each other or the rest of the experiment before the experiment began. This could allow the atoms to know the random numbers, and consequently the measurements to be performed, in advance, allowing them to synchronize their properties.

The physicists explain that the only way to completely close this loophole is to use an extraterrestrial random number generator, such as the inherently random photon emission from stars located millions of light-years away. The vast distance between the stars and an Earth-based experiment would make it practically impossible for any covert communication to occur, since it would mean that such communication would have had to take place before the light left the stars, millions of years ago. Several physics labs are currently developing extraterrestrial random number generators for this purpose.

Secure communication

Since is likely to be an important resource in future secure quantum technologies, closing these loopholes helps to increase the security of future applications at the most fundamental level. The researchers expect that the methods used in this study will also contribute to new developments in quantum information systems and quantum repeater networks, which are used for communicating quantum information over long distances. They plan to further investigate this application in the future.

"Apart from further fundamental questions considering the freedom-of-choice problematic, there is a lot one can work on here," Rosenfeld said. "On the one side one can try to push the system further (especially the fidelity of the entangled state) to be able to perform so-called 'device-independent' protocols. These would allow to obtain a secure cryptographic key even from devices which are potentially not trusted (provided by a third party). Here, Bell's inequality provides the possibility to , whether the devices were somehow prepared in advance to produce a key which is known to an adversary. Moreover, the techniques for generating entanglement between distant objects are important for networks enabling secure communication over long distances."

Explore further: Stars as random number generators could test foundations of physics

More information: Wenjamin Rosenfeld et al. "Event-Ready Bell Test Using Entangled Atoms Simultaneously Closing Detection and Locality Loopholes." Physical Review Letters. DOI: 10.1103/PhysRevLett.119.010402

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19 comments

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sirdumpalot
5 / 5 (3) Jul 20, 2017
yay for the progress of human experiment, now to adjust our philosophies based on this evidence!
Da Schneib
4.3 / 5 (6) Jul 20, 2017
This is only the latest Bell experiment to go well beyond 5 sigma. It's great they closed all the loopholes in this one, but we did that a decade ago and there are still folks arguing about it. It's getting close to time to start labeling them cranks if they can't accept experimental results.
ddaye
5 / 5 (1) Jul 20, 2017
yay for the progress of human experiment, now to adjust our philosophies based on this evidence!

That's what John Cleese would call "the tricky bit." Seems the US at least is backing away philosophically from the evidence we developed between today and back through the bubonic plague.
inorg_lsc
not rated yet Jul 21, 2017
Idealism may explain the correlations. What if in the "real world" ( a hyperspace outside {x,y,z,t}), in which all particles "really" exist, both entangled atoms are represented by the same "entity", which projects itself into our physical world as two separated atoms. Whichever property changes in either of these atoms will then correlate with any observed changes in another. How exactly that happens is not yet known.
physman
not rated yet Jul 21, 2017
How does this affect retrocausality as an interpretation? There was a good article about this the other day: https://phys.org/...ure.html
Dingbone
Jul 21, 2017
This comment has been removed by a moderator.
Dingbone
Jul 21, 2017
This comment has been removed by a moderator.
sirdumpalot
not rated yet Jul 21, 2017
Idealism may explain the correlations. What if in the "real world" ( a hyperspace outside {x,y,z,t}), in which all particles "really" exist, both entangled atoms are represented by the same "entity", which projects itself into our physical world as two separated atoms. Whichever property changes in either of these atoms will then correlate with any observed changes in another. How exactly that happens is not yet known.


Or, given Copernican, Galillean, special and general relativity - why not relational QM?
someone11235813
not rated yet Jul 21, 2017
I think this is an exciting development and I'm not wanting to be obstreperous but regarding this bit..."since it would mean that such communication would have had to take place before the light left the stars, millions of years ago..." is it reasonable to say that if the entire universe is entangled could it still be possible that these photons emitted billions of years ago could still somehow 'know' about the experiment?

physman
not rated yet Jul 21, 2017
@someone11235813 I think that is entirely possible, but if these kind of scenarios are at play then you reach a point where you can't "prove" anything.

It's kinda like how you can't disprove "last thursdayism": the theory that universe was created exactly the way it looks now last Thursday. This means that any of the evidence pointing to an older age was also created last Thursday.
someone11235813
not rated yet Jul 21, 2017
@physman, when you say 'created' I'm assuming you're talking about the random fluctuation thing which doesn't really make any sense for reasons I don't want to delve into, if you mean created in the magic god sense then I don't think that's a fair comparison, since what I have suggested doesn't seem much different than Penrose's billion year delayed choice experiment. And to be fair, the very concept of this sort of precision Bell test would have Einstein tumbling, mumbling and grumbling in his grave.
billpress11
not rated yet Jul 21, 2017
I wish this article explained how they interfered with each other, any ideas? I am assuming they are coming at each other from opposite directions since they are coming from different locations.

Quote from article: "In their tests, the scientists excited the atoms, causing them to emit photons at precisely defined times. The photons then travelled through the optical fiber and interfered with each other. This quantum interference, in theory, causes the atoms to become entangled. To detect this entanglement, the researchers performed measurements on the photons, repeating the measurements over and over for tens of thousands of photon pairs. The results showed overwhelmingly that the distant photon pairs were indeed entangled."

theon
not rated yet Jul 22, 2017
didn't close the contextuality loophole. Fine experiment, over the hill interpretation
Soundgardener
not rated yet Jul 24, 2017
Scientists creating invisible demon spirits that only exist as 'probability clouds' until someone "believes in them" enough for them to come into the world (Peter Pan should sue!), sending them flying all over campus to get them EXCITED enough for the final ritual where they pairbond and MUTUALLY INTERFERE WITH EACH OTHER in order to create a magical link that ignores time completely (why, so they can read each other's minds to get a constant update on how awkward the relationship has become since that crazy night).

See this is EXACTLY why people think Cern is a portal to free Satan from the Black Cube time machine on Saturn ;)
Kron
5 / 5 (1) Jul 24, 2017
Now it is no more a matter of belief whether nature can or cannot be described in a local-realistic way, but a matter of fact. (However, the freedom-of-choice problem still needs to be solved.)"

The freedom-of-choice problem is unsolvable. Randomness does not exist in a pure deterministic world.

The "Last Loophole" sub-heading is just meaningless drivel.

physicists explain that the only way to completely close this loophole is to use an extraterrestrial random number generator.

This closes no loop-hole. Terrestrial or Extra, the problem of determinism (Superdeterminism in QM) remains the same. In a determinate world no such thing as randomness exists, so random number generators don't function, period.
sirdumpalot
not rated yet Jul 25, 2017
@Kron - and still one shouts: https://arxiv.org...9002.pdf and https://arxiv.org...4064.pdf ... determinism is only an issue if you posit properties of objects independent of observation (the non-scientific notion of Form that Parmenides and Plato drilled into Western though still curses thinking, even in the 21st century)
Experience is neither deterministic nor in-deterministic - there is a regularity within all observed experience that is predictable, best using the scientific method of observation and measurement. But how does one observe or measure 'all'? What QM observer can be outside of the sum of all QM interactions?
Physicists have lost philosophy in the 20th - a foolish act given that the scientific method starts with something much closer to philosophy in the stating of a hypothesis. Nagarjuna's 'Fundamental Wisdom of the Middle Way' is a must for anyone who wishes for a rock solid philosophical argument for the relational interpretation
sirdumpalot
not rated yet Jul 25, 2017
https://ia800400....0Way.pdf

From part two (part one is a dialectic of 200AD India, hard to break without some hints!)
Hyperfuzzy
not rated yet Jul 25, 2017
So QM is not real. Correct?
sirdumpalot
not rated yet Jul 25, 2017
Not Essentially. Not with a world of Form, independent of observation, hidden behind the layers of actuality that is dependent on both subject and object, the observed and the observer.. heck even the quantum zeno effect offers a feedback loop for the observer to affect the observed - what else do you need to see the whole thing as relational?

Logically, it's the only thing that works with 'energy (information) can neither be created nor destroyed, it can only change forms' - a sea of objects defined as 1s must have come from 0, and you have your God argument.

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