Not really. I mean, Bell inequalities are being violated routinely these days. (Almost) Nobody believes that if all loopholes are closed the experimental outcome will be different, everybody still expects to see the same Bell violation. If we did close all the loopholes, and we saw to our surprise that now we don't get any Bell violation, that would mean that quantum theory is wrong since the latter predicts a violation. But nobody believes that the theory is wrong, at least not at the low energies at which Bell experiments are conducted. Therefore, nobody cares!Nick666 said:I thought there were going to be like a flood of answers. Isnt the topic the hottest and most important thing in quantum physics right now ?
JK423 said:Not really. I mean, Bell inequalities are being violated routinely these days. (Almost) Nobody believes that if all loopholes are closed the experimental outcome will be different, everybody still expects to see the same Bell violation.
Bell inequalities specify the limit of correlations that vanhess71 type models can reach when detector settings at A and B are not aligned. To state it more directly vanhess71 type model can not violate Bell inequalites.morrobay said:So why don't we move on to explanations for the violations: In posts 214 and 219 here:
https://www.physicsforums.com/threads/von-neumann-qm-rules-equivalent-to-bohm.816876/page-11 @ vanhess71 shows an explanation for the 100% perfect correlations when detector settings are aligned. So from here can there be a progression to understanding the violations when detector settings at A and B are not aligned ?
I have not studied the paper in detail, but would like to make some comments based on what the authors write in their article.gill1109 said:http://www.nature.com/news/quantum-spookiness-passes-toughest-test-yet-1.18255 (paper posted on arXiv, not passed peer review yet)
The settings are chosen by a quantum RNG http://arxiv.org/pdf/1506.02712v1.pdf So it's an "independent" piece of quantum optics / electronics. Personally I would have preferred a state of the art pseudo RNG. I don't know if they can be fast enough. It would be fine by me even that pseudo random settings are generated in advance and read from a file as needed. The point is to make it ludicrous (contrary to Occam's razor) that by some kind of conspiratorial and unknown physics, Alice's spin measurement somehow already "knows" Bob's setting.billschnieder said:I hope the referees push them to be more clear in their descriptions. There is a lot hidden between the lines. I've already identified in the other closed thread. For example, are the "settings" different from the randomly chosen microwave pulses which generate the entangled photons?
Another description of the experiment, see diagram at https://www.newscientist.com/articl...roved-real-in-first-loophole-free-experiment/). In usual CHSH setups, Alice and Bob each have 2 settings [1,2] which they randomly switch between. In this experiment, that appears to be the microwave pulses. These pulses excite the crystals to produce photons which are entangled with the electrons. Both photons are then sent to station C, where they are post-selected in order to find an ensemble for which the electrons at A and B could be considered as entangled (This is what entanglement swapping is all about, the two electrons being previously unentangled). Some time after the photons have left to be "filtered" at C, but before any signal can travel from C back to A and B, the state of the electrons at A and B are "read-out". Only those results at A and B which correspond to successful filtration at C are kept. Everything else is discarded. This corresponds to a success rate of 6.4e-9.
My suspicion is that the post-processing or "entanglement swapping" will be the key to unlock this experiment.
Apologies, I see this is somewhat off tangent from the thread topic itself but I just have to object here. Entanglement swapping is completely post-processing. The correlations between A and B are noticeable and interesting only if you post-select samples where C decided to do a measurement, and then even take into account his measurement result to see if it means correlation or anti-correlation.gill1109 said:Entanglement swapping is not "post-processing".
I agree that the calculation of correlations is done *post experiment* but the calculated correlations already exist from the moment that the outcomes at A, B and C exist. And indeed the timing is very important so you have to rule out not only that A's settings could have influenced B's outcomes, but also that C's "seal of approval" could have influence A and B's settings. Which the authors do, in their paper.georgir said:Apologies, I see this is somewhat off tangent from the thread topic itself but I just have to object here. Entanglement swapping is completely post-processing. The correlations between A and B are noticeable and interesting only if you post-select samples where C decided to do a measurement, and then even take into account his measurement result to see if it means correlation or anti-correlation.
billschnieder said:My suspicion is that the post-processing or "entanglement swapping" will be the key to unlock this experiment.
You will find that it is indeed post-processing by selection of sub-ensembles. You take four ensembles A,B,C,D of particles, with every particle in A entangled with a sibling in B, and every particle in C entangled with a sibling in D. No entanglement between AB and CD pairs. You measure B and C together and based on the joint result of B and C, you select a subset of A and D that would now be entangled with each other. It is obviously post-selection.DrChinese said:Although only repeating what Gill said above: entanglement swapping is "processing" but you shouldn't confuse it with "post-selection".
The swapping is done non-local to A and B, but it uses information from A and B to do the post-selection. A key question is whether in this experiment, the microwave pulses are "settings" or not?when the entanglement is performed non-local to the A and B measurements (and the selection of measurement settings).
Yes, microwave pulses are "settings".billschnieder said:The swapping is done non-local to A and B, but it uses information from A and B to do the post-selection. A key question is whether in this experiment, the microwave pulses are "settings" or not?
I agree with this. Using quantum RNG is sort of begging the question fallacy (we assume quantum RNG behaves as QT says in order to test what QT says).gill1109 said:The settings are chosen by a quantum RNG http://arxiv.org/pdf/1506.02712v1.pdf So it's an "independent" piece of quantum optics / electronics. Personally I would have preferred a state of the art pseudo RNG. I don't know if they can be fast enough. It would be fine by me even that pseudo random settings are generated in advance and read from a file as needed. The point is to make it ludicrous (contrary to Occam's razor) that by some kind of conspiratorial and unknown physics, Alice's spin measurement somehow already "knows" Bob's setting.
I think these two games are a good description of what "local realism" can allow is happening, both in a traditional Bell-CHSH experiment (particles leave the source C and go to two locations A and B) and in the new generation of experiment using entanglement swapping (particles leave locations A and B and meet at C where they are measured and where a selection occurs of "favourable" situations). The whole point is that under local realism we can't expect a better than 75% success rate. In either game. However replace my computers A, B and C with quantum devices and quantum communication and in theory we could have an 85% success rate. (The Delft experiment had an 80% success rate).georgir said:Gill, are you trying to formulate a "Dr. Chinese challenge" for the swapping experiment?
I may be misunderstanding either you or the entanglement swapping experiment itself, but I don't think your "games" are good description of what happens. Remember, even though A and B do not have anything shared between them, they do each have an entangled pair shared with C.
Please learn about the CHSH inequality and read Bell (1981) "Bertlmann's socks". Alice and Bob each choose between two measurement directions. Alice chooses between 0 and 90 degrees; Bob between 45 and 135. IMHO it's just as easy to understand as stories where Alice and Bob each have three measurement directions.georgir said:I am not quite sure how to interpret your game. What do the messages and coins represent?
It is my impression that in order to get a Bell test you need to consider at least 3 different measurement bases.
In most understandable explanations I've seen, you use i.e. 0 deg and +/- 30 deg and have to have 25% correlation between 0 and +/-30 but 75% between +/-30, etc.
gill1109 said:Please learn about the CHSH inequality and read Bell (1981) "Bertlmann's socks". Alice and Bob each choose between two measurement directions. Alice chooses between 0 and 90 degrees; Bob between 45 and 135. IMHO it's just as easy to understand as stories where Alice and Bob each have three measurement directions.
Remember the original Bell (1964) inequality also built in an assumption of perfect anti-correlation when Alice and Bob used the same measurement settings. If you would test that assumption experimentally, you would find that it's not exactly true. So the original Bell inequality was rapidly discarded in favour of CHSH, as far as real experiments are concerned. For tutorial introductions, there are advantages and disadvantages to both versions...
The idea of a Bell game goes back a long time. Already in the 80's people understood that Bell's inequality is driven by elementary logic and probability, not about calculus. (NB Bell's original inequality is a special case of CHSH, which results by setting one of the four correlations equal to +/-1).
The messages represent particles, or if you prefer, physical transmission of information. The coins represent random choices of measurement settings. Each computers represents a source of some particles or a measurement device (destination of particles). The computer programs running on the computers represent pieces of a local hidden variables theory. So the messages might just contain the values of all hidden variables in the theory we are simulating.georgir said:I am not quite sure how to interpret your game. What do the messages and coins represent?
It is my impression that in order to get a Bell test you need to consider at least 3 different measurement bases.
In most understandable explanations I've seen, you use i.e. 0 deg and +/- 30 deg and have to have 25% difference between 0 and +/-30 but 75% between +/-30, etc.
georgir said:Ok, I hate the feeling that I'm missing something obvious. But apparently I am. I can't see how using only 0, 45 and 90 can work to formulate a Bell test. 0 and 90 are always perfectly anti-correlated, 45 or 135 is always 1/2 correlated to either of them... any choice between those pairs of settings seems pointless to me as it is not affecting the 1/2 correlation. Anyway, I'll try to find more time and read up on "CHSH" in the future.
The go/no-go part is a way to implement the strict timing requirements. We want to do measurements at the two locations A and B, very rapidly, far apart. We want to engineer that there are two parts of a two-component quantum system localized in these times and places, when we do those measurements. How to arrange that? It rutns out to be very difficult to get quantum systems at distant locations A and B in the good entangled state at the same *prespecified* time "to order". ie at a time chosen and fixed in advance. The entanglement-swapping method is just one of many tricks which finesses this difficulty.georgir said:Ok, half those angles makes sense. Should've seen that... And I see now how QM wins your game in cos(22.5deg)^2 or 85% of the times. This is cool.
EDIT: I still don't understand what's the go/no go part though.
georgir said:EDIT: I still don't understand what's the go/no go part though.
QM predicts a violation of the CHSH-inequality when the observed pairs are entangled. Ensuring that pairs are entangled is notoriously difficult, and particularly when the distance between them increase. This experiment has the nice property that one can generate signals that tells us whether the particles are entangled or not (the go/no go), just before the settings are randomly chosen and the results read out (or to be more precise, the signal is recorded outside the lightcone of the read out). Which means that we can discard all the uninteresting unentangled pairs, which would otherwise just add random noise to the correlations.georgir said:EDIT: I still don't understand what's the go/no go part though.
And the photons are produced by the microwave pulses hitting the crystals https://d1o50x50snmhul.cloudfront.net/wp-content/uploads/2015/08/09_12908151-800x897.jpg?zonde said:Yes, microwave pulses are "settings".
And the post-selection is based on the photons?But information from A and B for post-selection is only about initial state of A and B, not later generated measurement settings.
No, the photons used for signalling that "the event is ready" is not produced by the microwave puls that reads out the spin. You should read the paper, and in particular the long texts accompanying the figures.billschnieder said:And the photons are produced by the microwave pulses hitting the crystals https://d1o50x50snmhul.cloudfront.net/wp-content/uploads/2015/08/09_12908151-800x897.jpg?And the post-selection is based on the photons?
billschnieder said:You will find that it is indeed post-processing by selection of sub-ensembles. You take four ensembles A,B,C,D of particles, with every particle in A entangled with a sibling in B, and every particle in C entangled with a sibling in D. No entanglement between AB and CD pairs. You measure B and C together and based on the joint result of B and C, you select a subset of A and D that would now be entangled with each other. It is obviously post-selection.
billschnieder said:You will find that it is indeed post-processing by selection of sub-ensembles. You take four ensembles A,B,C,D of particles, with every particle in A entangled with a sibling in B, and every particle in C entangled with a sibling in D. No entanglement between AB and CD pairs. You measure B and C together and based on the joint result of B and C, you select a subset of A and D that would now be entangled with each other. It is obviously post-selection.
Heinera said:This experiment has the nice property that one can generate signals that tells us whether the particles are entangled or not (the go/no go), just before the settings are randomly chosen and the results read out (or to be more precise, the signal is recorded outside the lightcone of the read out).