# Did they do a Loopholes free Bell test?

1. Jun 15, 2015

### Nick666

Did they manage to do a loopholes free Bell test ? The best I got from google was an article from february that says no , they only did one where 2 out of 3 loopholes were eliminated in one test.

2. Jun 15, 2015

### f95toli

I don't think is has been done yet, but I know a number of groups are planning to do such an experiment in the near future (some might already have started). In at least one case I am aware of the modification to their setup should be more or less trivial (e..g moving part of their setup to the opposite side of campus) so it shouldn't really be that hard (famous last words...)

Hence, I wouldn't at all be surprised if something is published in the next few months.

3. Jun 16, 2015

### zonde

I know about two loopholes. These are closed in two separate experiments:
Violation of Bell's inequality under strict Einstein locality conditions
Bell violation with entangled photons, free of the fair-sampling assumption

The third might be "free will" loophole but I'm not sure if it can be exploited in any scientific model. Maybe the idea of closing "free will" loophole is to eliminate possibility of poor random number generator.

Closing communication and fair-sampling loopholes in one experiment might take some time as they have conflicting requirements. Closing fair-sampling loophole requires that photons are not lost so you want detector closer to source, but closing communication loophole requires considerable distance between source and detectors and that of course increases photon losses unless you can perform experiment in vacuum.

4. Jun 16, 2015

### Nick666

This says that in the same experiment they closed the locality and freedom-of-choice loopholes.

5. Jun 20, 2015

### Nick666

I thought there were gonna be like a flood of answers. Isnt the topic the hottest and most important thing in quantum physics right now ?

6. Jun 20, 2015

### JK423

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!

But some people do care about closing all the loopholes... but for other reasons...
In the field of quantum cryptography, a new sub-field has emerged the past ten years, the so-called Device-Independent Quantum Key Distribution. There it has been shown that a loophole free violation of a Bell-inequality is important so that a secure key is established between two parties even if the measurement devices of each party are not trusted themselves (e.g. may have been hacked). Therefore, loophole free violations do offer great technological advantages.

7. Jun 22, 2015

### morrobay

So why dont we move on to explanations for the violations: In posts 214 and 219 here:
@ vanhess71 shows an explanation ( non local correlations) for the 100% perfect correlations when detector settings are aligned.
From here is there an explanation for some of the Bell inequality violations when detector settings at A and B are not aligned ?

Last edited: Jun 22, 2015
8. Jun 22, 2015

### zonde

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.

9. Aug 30, 2015

### gill1109

10. Aug 30, 2015

### akhmeteli

I have not studied the paper in detail, but would like to make some comments based on what the authors write in their article.

First, it was noted in another thread that the probability $p$=0.019/0.039 is not very impressive.

Second, authors write: "Our observation of a loophole-free Bell inequality violation thus rules out all local theories that accept ... that the outputs are final once recorded in the electronics." On the other hand, as I wrote here a few times, unitary evolution of quantum mechanics is, strictly speaking, incompatible with final outcomes of measurement, as far as I understand (for example, due to Poincare recurrence). Therefore, the authors' experimental results can only rule out local realistic theories that predict deviations from unitary evolution. For example, the local realistic theories of my article http://link.springer.com/content/pdf/10.1140/epjc/s10052-013-2371-4.pdf (Eur. Phys. J. C (2013) 73:2371) have the same evolution as unitary evolution of some quantum field theories.

11. Aug 30, 2015

### billschnieder

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.

12. Aug 31, 2015

### gill1109

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.

Entanglement swapping is not "post-processing". The central location (C) preserves a record of which pairs of measurements (at A and B) are interesting to look at. Sure, you only then go and fish out those pairs after the experiment was done. At some point you have to look at the correlations between the experimental records generated at A, B and C. The timing is very delicate and has to be very careful. That's what the referees have to look at closely. But the "marks" saying which ones count were made *before* the corresponding measurements were done.

Last edited: Aug 31, 2015
13. Aug 31, 2015

### georgir

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.

14. Aug 31, 2015

### gill1109

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.

And C's measurement result, on the basis of which A and B data gets selected, is quite simply: a photon is detected, one in each channel.

15. Aug 31, 2015

### DrChinese

Although only repeating what Gill said above: entanglement swapping is "processing" but you shouldn't confuse it with "post-selection". Regardless of how frequently the right circumstances occur, each event ready occurrence both causes and heralds a Bell pair that is being created in another spacetime region.

And it would take chutzpah to claim this setup doesn't disprove local realism, when the entanglement is performed non-local to the A and B measurements (and the selection of measurement settings). The local realist, after all, would say that what happens at C can have no bearing on any measurement outcome at A or B. I.e. there is no such physical state as entanglement! So why would one group of A's and B's exhibit measurement correlation differently than another, based on what is done at C? A sample of entangled pairs of electrons gives a different correlation rate (perfect correlations in the ideal case) than an un-entangled sample.

16. Aug 31, 2015

### billschnieder

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.

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?

17. Aug 31, 2015

### zonde

Yes, microwave pulses are "settings".
But information from A and B for post-selection is only about initial state of A and B, not later generated measurement settings.
They say: "As can be seen in the spacetime diagram in Fig. 2a, we ensure that this event-ready signal is space-like separated from the random input bit generation at locations A and B."

18. Aug 31, 2015

### zonde

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).

19. Sep 1, 2015

### gill1109

Let me describe the traditional Bell-CHSH experiment and the entanglement-swapping version through computer analogies (known as the "Bell game")

Standard Bell game
==================

You control three computers A, B, C
You may write computer programmes for them.
Many times, the following happens:

Computer C sends messages to A and B
No more communication allowed between A, B and C
I toss two fair coins and deliver outcomes (H/T) to A and B
A and B output binary outcomes +/-1

Your aim: when A and B both receive "H" the outcomes should be the same (both +1 or both -1)
When either or both of A and B receive "T" the outcomes should be different (+1 and -1)

We call each of these exchanges a "trial". Each trial, you either win or lose
(you either achieve your per-trial aim or you don't).

The whole game: your overall aim is to "win" statistically significantly more than 75% of the trials (say: 80%, with N pretty large). Bell says it can't be done. (Well - just once in a while it could happen purely by chance, obviously, but you can't write those computer programs so that you systematically win).

Modifed Bell game
=================

You control three computers A, B, C
You may write computer programmes for them.
Many times, the following happens:

Computers A and B send messages to C
No more communication allowed between A, B and C
I toss two fair coins and deliver outcomes (H/T) to A and B
A and B output binary outcomes +/-1
C delivers a binary outcome "go"/"no-go"

Your aim: when C outputs "go" *and* A and B both receive "H" the outcomes should be the same (both +1 or both -1)
When C outputs "go" *and* either or both of A and B receive "T" the outcomes should be different (+1 and -1)

We call each of these exchanges a "trial". Each trial in which C says "go", you either win or lose
(you either acheive the aim or you don't).

The whole game: your aim is to "win" statistically significantly more than 75% of the trials for which C said "go"(say: 80%, with N pretty large). Bell says it can't be done. (Well - just once in a while it could happen purely by chance, obviously, but you can't write those computer programs so that you systematically win).

20. Sep 1, 2015

### georgir

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.