Weih's data: what ad hoc explanations do local and non-local models give?

[harrylin]

The main problem with coincidence-time loophole as I see it is that you have to claim that detector efficiency is much higher than the one deduced from coincidence/single rate.
But then detector efficiency can be increased. And there should be some serious flaws in detector efficiency calibration (I do not say that's impossible but then there should be at least some vague ideas how this can happen).

If I understand you correctly, you think that it may be possible to - at least partly - compensate for that loophole by clarifying how well or how poorly certain explanations match the facts as we know. Indeed, that's exactly my aim.

Weihs claimed a detection efficiency of 5%; I don't know how they established that, and I'm not sure how that affects the different explanations. At first sight it will have on De Raedt's model little other effect than to lower the predicted correlations for large coincidence windows, and that is at least qualitatively what their simulations need in order to better match the data. And it is also qualitatively what Weihs' model needs.

Thus I'm afraid that without some serious work on it, we won't be able to say which model would be a better match by adding that consideration. However, you suggest to have already found a weak point in one of the models; I hope that you will elaborate.

On a side note: I think that particle based models such as of De Raedt are fundamentally flawed; but that issue is perhaps of no importance for this discussion.

As Weihs' et al make the coincidence window larger, the proportion of unpaired observations goes down and the correlations are dampened. As they make it smaller, the number of pairs counted in the statistics gets smaller and the correlations fluctuate more and more wildly.

Exactly - however, as I indicated here above, it looks to me that such a qualitative description is insufficient.

Imagine two photons who are trying to cheat by pre-arranging what setting they want to see.

Sorry, I have no reason to imagine such weird explanations - and certainly not in physicsforums. Here I limit myself to the discussion of hypotheses or models that can be found in the peer reviewed literature.

[..] de Raedt et al. [..] continue to publish papers with no reference whatever to the immense literature on these issues.

Off-topic as that is, just this: there may be papers that are lacking, but the last paper that I referred to in this thread happens to reference a certain Gill twice on these issues. Please don't treat this thread as a soap box!

 

[gill1109]

Harry Lin: when I say "imagine two photons trying to cheat" I am asking you to imagine that you are trying to write a computer program which generates the correlations you are after. It is not meant to be an explanation for what happens in the real world. It is an explanation for how de Raedt and similar researchers are able to engineer violations of CHSH, etc. Strong positive correlations have to be made stronger still, strong negative correlations have to be made even more strong negative? How to fix this? By a selection bias. Filter out potential observations which won't contribute to the pattern you want to create. We know this can only be done by some kind of communication between the two wings of the experiment. The trick is to let the observer do this communication, by selecting events to be considered as pairs under conditions which depend on the local hidden variables carried by the two particles.

This is called "rejection sampling" in the simulation literature, "biased sampling" or "selection bias" in the statistical literature.

By the way, I am not claiming that de Raedt and others are purposely trying to trick you. These possibilities have been known and studied for more than 40 years by specialists in the field. I use the word "trick" figuratively.

Richard Gill

 

[harrylin]

[..] It is not meant to be an explanation for what happens in the real world.

Sorry: this thread is about explanations for what happens in the real world, and only such.

 

[gill1109]

The thread is about what "ad hoc" explanations are given by local and non-local models. If you want to understand why de Raedt's (local) model works, and what limits are there to models of that type, you have to understand how "coincidence selection" opens a loophole, you have to understand just how wide that loophole is. Once you understand the mechanism they are exploiting, you will realize how easy the task was which they set themselves.

Maybe this does not tell you much about the physics of the real world. No. That's my point. De Raedt's work does not tell us much about the physics of the real world. Their models are just tricks, tricks which are in fact built on ideas which have been known for a long time.

This also tells us what is important for future experiments. It will be a great day when someone does a decent loophole free experiment. If they are going to use observed detection times and a coincidence window to determine pairs then the important criterion is the percentage of all the events on one side of the experiment which is succesfully paired with an event on the the other side. This percentage has to be above a certain threshhold.

If timing is determined externally, then the threshold is different (lower).

This also tells us that the important "efficiency" measure is not the efficiency of the detectors, as determined conventionally, but some kind of efficiency of the entire experimental setup.

These things have been known by theoreticians for a long time but many experimentalists and also the general (physics) public seems quite unaware of them. That gives a nice niche for de Raedt et al to write paper after paper with results which superficially seem exciting or controversial, but actually are not exciting at all.

 

[harrylin]

The thread is about what "ad hoc" explanations are given by local and non-local models. If you want to understand why de Raedt's (local) model works, and what limits are there to models of that type, you have to understand how "coincidence selection" opens a loophole [..] This also tells us what is important for future experiments. [..]

I suppose that everyone participating in this discussion, understands that; so far the easy part! The challenge is here to compare Weihs' data with the two competing models -which include hypotheses/excuses about the physics of the real world.

1. I'm interested in how the models compare for several coincidence windows; it may be sufficient to use the data as presented by De Raedt in his latest paper.

2. De Raedt compares QM with another aspect of the data, from which he concludes that QM does not match the observations. Again it will be interesting to know how successfully that issue can be "explained away", and if his own model fares better or worse.

3. Zonde also did a comparison, but regretfully I did not grasp the issue he looked into. I hope that he will elaborate.

 

[gill1109]

Harrylin: since QM (quantum mechanics) is more general than LR (local realism) how could de Raedt conclude that QM does not fit?

 

[harrylin]

Harrylin: since QM (quantum mechanics) is more general than LR (local realism) how could de Raedt conclude that QM does not fit?

I have not yet studied their paper, and apparently neither did you. However, I don't really understand your question. A single aspect or result of experimental data (even one that had at first not been examined) can in principle disprove a theory.

 

[gill1109]

Harrylin: I studied a number of their earlier papers and came to the conclusion that they were devoid of scientific interest. The authors are blissfully unaware of the literature on the foundations of QM. They are good at writing computer programs which do things which we already know can in principle be done by computer programs.

You did not understand my question? Let me say it again in different words. Everything that LR allows, can be mimicked by QM. But the converse is not true. Hence if an experiment disproves QM, it disproves LR.

So de Raedt et al. might be able to conclude from statistical analysis of experimental outcomes that a particular model from QM does not fit, but they cannot conclude in general that QM does not fit.

 

[harrylin]

I suppose that everyone participating in this discussion, understands that; so far the easy part! The challenge is here to compare Weihs' data with the two competing models -which include hypotheses/excuses about the physics of the real world.

1. I'm interested in how the models compare for several coincidence windows; it may be sufficient to use the data as presented by De Raedt in his latest paper.

2. De Raedt compares QM with another aspect of the data, from which he concludes that QM does not match the observations. Again it will be interesting to know how successfully that issue can be "explained away", and if his own model fares better or worse.

Addendum: I now wonder if I was mistaken, and that in his last paper De Raedt actually does the very kind of analysis that I was after, but in a different way from what I had in mind. If so, the points 1 and 2 here above merge into a single one.

I think that two causes for the correlations to reduce with increased time window are agreed upon by everyone:

1. detector efficiency
2. average time between light pulses (photons)

In addition, it will be good to clearly state the two competing explanations for high correlations with small time windows (allowing for slightly variant formulations):

a. (common non-local model) : influence at a distance, such that the first detection event sets the spin or polarisation for the other one. This influence is inferred from QM predictions.
b. (De Raedt's local model) : influence of spin or polarisation on detection time delay. This influence is inferred from QM predictions as well as from independent experiments.

If the above contains a mistake I'll be happy if someone tells me.

 

[gill1109]

Harrylin: In case of explanation (b), then I wonder why experimenters never see a bigger deviation from the CHSH bound of 2 than the QM bound of 2 sqrt 2. De Raedt's local model in which detection time delay is related to hidden variables would easily allow a violation of CHSH up to the logical maximal value of 4, given the overwhelming proportion of discarded unpaired events in the present experiments.

I disagree with your interpretation of explanation (a). If you do not assume the reality and time-space location of outcomes of measurements which were not performed anyway, there is no influence at a distance. The so-called influence at a distance only affects things we can't see anyway. It influences the outcomes of measurements which were not performed. Easy, it seems to me, to deny the "reality" of things which are actually only constructs of our own mind, anyway.

QM does not violate the principle that there should be no action at a distance. You can't use QM correlations for signaling.

Richard Gill
http://www.math.leidenuniv.n/~gill

 

[harrylin]

[..] Let me say it again in different words. Everything that LR allows, can be mimicked by QM. [..]

That's obviously wrong - and not relevant for this discussion. If you want to discuss that idea, please start a topic on it.

 

[gill1109]

It's obviously right, and well known too. But I have no need to discuss it.

 

[harrylin]

Harrylin: In case of explanation (b), then I wonder why experimenters never see a bigger deviation from the CHSH bound of 2 than the QM bound of 2 sqrt 2. [...]

I see that one of the models according to b) yields exactly the QM bound; as you studied it, perhaps you can find the answer.

I disagree with your interpretation of explanation (a). [..] The so-called influence at a distance only affects [..] the outcomes of measurements which were not performed. [..] QM does not violate the principle that there should be no action at a distance.

Interpretation and explanation are almost synonyms. Interpretation a) is certainly a common explanation (with not detectable action but influence at a distance), and I suspect that your variant has no effect on the data analysis here. Nevertheless, explanation a) only refers to obtained measurement data, so I'm afraid that I don't understand your alternative explanation, let's call it a2). Perhaps you can give a peer-reviewed reference to that explanation?

PS: in the context of Bell, "non-local" means influence at a distance

 

[gill1109]

Peer-reviewed reference? How about J. Bell In his "Bertlman's socks" paper? It's one of at least four positions to take.

 

[harrylin]

Peer-reviewed reference? How about J. Bell In his "Bertlman's socks" paper? It's one of at least four positions to take.

Also good. Bell's No.1 refers to "local" explanations of which I mentioned one as explanation b), and no.3 is the common "non-local" explanation that I mentioned as a).
http://cdsweb.cern.ch/record/142461?ln=en
Bell's no.2 is a kind of conspiracy theory and no.4 denies reality below a certain level; I think that both explanations can pass any test with flying colours, so that they are not falsifiable and useless for this discussion.

About this discussion: it seems to be drifting away into generalities. Please from now on refer to specific explanations for Weihs' data, as for example Zonde did in post #2.

 

[gill1109]

I disagree that number 4 denies reality below a certain level. Remember that what is called "reality" here is a construct in a mathematical model, a construct of things "behind" the actual reality, supposed to explain them in some mechanistic way. Actually, "local realism" should be called "local idealism". The alternative is to accept the message of QM that nature is irreducibly stochastic. See for instance
arXiv:quant-ph/0508016
General properties of Nonsignaling Theories
Ll. Masanes, A. Acin, N. Gisin
Phys. Rev. A. 73, 012112 (2006)

 

[harrylin]

I disagree that number 4 denies reality below a certain level.[..]

To be precise, you disagree that 'there is no reality below some "classical level"', means to deny reality below a certain level! But never mind:

The alternative is to accept the message of QM that nature is irreducibly stochastic.

While local models can be stochastic, Masanes' paper is about nonlocal models but let's call it explanation c). In my next post I'll give a quantitative description belonging to all considered explanations for testing with Weihs' data and I will assume that by design, c) predicts exactly the same as a). If not, please specify.

 

[gill1109]

Harrylin, you must read Masanes et al. more carefully. Their definition of "non-local" is not the same as yours. By "non-local" they merely mean "violating a Bell inequality".

 

[billschnieder]

Harrylin, you must read Masanes et al. more carefully. Their definition of "non-local" is not the same as yours. By "non-local" they merely mean "violating a Bell inequality".

That is surely a very specious definition of "non-locality" almost not worthy of any attention. Why not simply stick to "violating a Bell inequality", why use the term "non-locality" at all, rather than any other such as "imaginary" or "non-sense"?

 

[harrylin]

OK, I think that it is possible to be a little more specific.

Two causes for the correlations to reduce with increased time window are probably agreed upon by everyone:

1. detector efficiency; Weihs writes 5%, based on what?
2. average time between light pulses (photons); De Raedt writes 30 μs, based on what?

In addition, it will be good to clearly state the main competing explanations for high correlations with small time windows. Using Bell's meaning of words (but allowing for [STRIKE]slightly[/STRIKE] greatly variant formulations):

a. (common "non-local" QM models): An unspecified influence at a distance, such that the first detection event instantly determines the spin or polarisation for the other one, no matter how far away ("collapse of the wave function"). As this influence is inferred from QM predictions, the predicted result is exactly that of QM for 100% detector efficiency and large enough time between pulses to exclude false identification of pairs.

For example, one proposed model (if I understand it correctly) is that of an instantaneous influence propagating from the first photon to the second one, such that the second photon obtains a polarisation at 90 degrees angle compared to that of the detected polarisation of the first one.

b. common "local-realist" models: no "spooky" influence at a distance, nor wild science fiction.
Strangely enough I know only one such model, of De Raedt et al: influence of polarisation on detection time delay. This influence is inferred from QM predictions as well as from independent experiments, based on an assumed "local-realist" universe. The best match with QM was obtained for T(x)=T0|sin2x|⁴ (?) - I don't know what these symbols mean and is that is their latest empirical equation? I think that I have seen ^3 elsewhere...

And in addition:

c. Other explanations as mentioned by Bell involve for example a kind of conspiracy theory or one that assumes no reality below some "classical level", and there are more of those that are not necessarily truly "non-local". Their common feature is that their prediction is identical to that QM by design, so that at least for QM tests they cannot be distinguished from a). See also: https://www.physicsforums.com/showthread.php?t=590592

Perhaps some people can give precisions and/or clarifications of the explanations that they fancy.

For further discussion it will be particular interesting to compare De Raedt et al's model against QM (= all the other proposed models, I suppose) with Weihs' data. For that we should know more precisely:

- the basis for the 5%
- the basis for the 30 μs
- the exact formula that De Raedt et al used for their most successful simulations (see the thread on De Raedt's simulations)
- the possible delay time range according to the literature, insofar as this has been observed
- ?

Harald

 

[Gordon Watson]

That is surely a very specious definition of "non-locality" almost not worthy of any attention. Why not simply stick to "violating a Bell inequality", why use the term "non-locality" at all, rather than any other such as "imaginary" or "non-sense"?

Bill: Happily, in addition to Einstein-locality: something else that we agree about! Nicely put! GW

 

[gill1109]

Bill Schnieder, Gordon Watson: I don't like the terminology either. But: "what's in a name"? It is the *results* of Masanes et al. which are interesting. As for the terminology: this has nowadays become fairly standard in quantum information. Like it or not, it's what people say. It's become a technical term with an established technical meaning.

 

[Gordon Watson]

Bill Schnieder, Gordon Watson: I don't like the terminology either. But: "what's in a name"? It is the *results* of Masanes et al. which are interesting. As for the terminology: this has nowadays become fairly standard in quantum information. Like it or not, it's what people say. It's become a technical term with an established technical meaning.

Hi Richard, fair enough, and thanks for sharing your dislike. But when "non-locality" is so often "what people say" ... and when it is then so often taken or understood to negate or refute the well-established Einstein-locality ... one of my favourite classical principles ... and when the established technical meaning is not so clearly spelt out, as Bill suggests ... confusion tends to reign, imho. Witness the many on-going discussions on the subject. With thanks again, Gordon.

 

[harrylin]

[..] it will be good to clearly state the main competing explanations for high correlations with small time windows. [..]

common "local-realist" models: no "spooky" influence at a distance, nor wild science fiction.
Strangely enough I know only one such model, of De Raedt et al: influence of polarisation on detection time delay. This influence is inferred from QM predictions as well as from independent experiments, based on an assumed "local-realist" universe. [..]
For further discussion it will be particular interesting to compare De Raedt et al's model against QM (= all the other proposed models, I suppose) with Weihs' data. For that we should know more precisely:

- the basis for the 5%
- the basis for the 30 μs
- the exact formula that De Raedt et al used for their most successful simulations (see the thread on De Raedt's simulations)
- the possible delay time range according to the literature, insofar as this has been observed
- ?

Harald

I now came a little bit further with the possible variation in delay time as function of polarization according to the literature.

My first suspect in Weih's experiment was birefringence in his photonic crystal. However that was a dead-end road, for he writes in Arxiv9810080v1:

"we pump a BBO-crystal with 400 mW of 351 nm light
from an Argon-ion-laser. A telescope was used to nar-
row the UV-pump beam [12], in order to enhance the
coupling of the 702 nm photons into the two single-
mode glass fibers. On the way to the fibers, the pho-
tons passed a half-wave plate and the compensator crys-
tals necessary to compensate for in-crystal birefringence"

The next obvious suspect is the electro-optic modulator that he used, as these are commonly made of similar materials. About that he comments in that same paper which was published in Phys. Rev. Letters:

"Each of the observers switched the direction of local
polarization analysis with a transverse electro-optic modulator.
It’s optic axes was set at 45◦ with respect to the
subsequent polarizer. Applying a voltage causes a rotation
of the polarization of light passing through the modulator
by a certain angle proportional to the voltage [13].
For the measurements the modulators were switched fast
between a rotation of 0° and 45°. [..]
The total of the delays occurring in the electronics and
optics of our random number generator, sampling circuit,
amplifier, electro-optic modulator and avalanche photodiodes
was measured to be 75 ns. [..]

[13]Precisely speaking, the modulator introduces a phase
shift between the linearly polarized components parallel
and perpendicular to its optic axis (at 45°). Together
with two quarter-wave plates (at 0° or 90°) before and
after the modulator this results in a polarization rotation
in real space as usually seen in circularly birefringent
media. The latter quarter-wave plate can be abandoned
here because it is parallel to the axis of the subsequent
polarizer and thus introduces only a phase which cannot
be measured anyway. The quarter-wave plate in front of
the modulator is substituted by our fiber and the initial
polarization controllers."

To my regret, I do not understand this. What type of electro-optic modulator did he use, and how did he account for its birefringence?
Can someone else perhaps explain this to me?

 

[audioloop]

why not use only one pair of entangled electrons ?
just that.

 

[harrylin]

I now came a little bit further with the possible variation in delay time as function of polarization according to the literature.

My first suspect in Weih's experiment was birefringence in his photonic crystal. However that was a dead-end road [..] The next obvious suspect is the electro-optic modulator that he used, as these are commonly made of similar materials. [..] What type of electro-optic modulator did he use, and how did he account for its birefringence?
Can someone else perhaps explain this to me?

As there were no comments I will clarify the above: apparently he did not consider the spread in time delays due to the EOM's birefringence. However, that should be done (and could be done!) in order to interpret the experimental findings.

 

[DrChinese]

Peter Morgan has been working with the raw Weihs data for several years. He has done some deep analysis of it, and today posted some results in the arxiv:

A graphical presentation of signal delays in the datasets of Weihs et al
Peter Morgan (2012)

http://arxiv.org/abs/1207.5775

"A graphical presentation of the timing of avalanche photodiode events in the datasets from the experiment of Weihs et al. [Phys. Rev. Lett. 81, 5039 (1998)] makes manifest the existence of two types of signal delay: (1) The introduction of rapid switching of the input to a pair of transverse electro-optical modulators causes a delay of approximately 20 nanoseconds for a proportion of coincident avalanche photodiode events; this effect has been previously noted, but a different cause is suggested by the data as considered here. (2) There are delays that depend on in which avalanche photodiode an event occurs; this effect has also been previously noted even though it is only strongly apparent when the relative time difference between avalanche photodiode events is near the stated 0.5 nanosecond accuracy of the timestamps (but it is identifiable because of 75 picosecond resolution). The cause of the second effect is a difference between signal delays for the four avalanche photodiodes, for which correction can be made by straightforward local adjustments (with almost no effect on the degree of violation of Bell-CHSH inequalities)."

 

[Peter Morgan]

Peter Morgan has been working with the raw Weihs data for several years. He has done some deep analysis of it

Hi DrC. "For several years" only in elapsed time. In committed time perhaps a few months. I have to disagree with "deep", though I think the paper I uploaded yesterday is kinda cute for its data visualization, which is different from anything I've seen other people do.

I was prompted to post this paper (the computation for which I did about two years ago) to the arXiv by Alejandro Hnilo, whose research group in Argentina has just finished a Bell-type experiment in which they use a pulsed laser. The distances are relatively short, but they record the timings of the laser pulses as well as the timings of Alice's and Bob's measurement events. They have posted their dataset privately, and it may become publicly available in time; they're currently working on their analysis.

 

[harrylin]

[..] A graphical presentation of signal delays in the datasets of Weihs et al Peter Morgan (2012)

http://arxiv.org/abs/1207.5775

Thanks that looks useful!

[..] I think the paper I uploaded yesterday is kinda cute for its data visualization, which is different from anything I've seen other people do.

I was prompted to post this paper (the computation for which I did about two years ago) to the arXiv by Alejandro Hnilo, whose research group in Argentina has just finished a Bell-type experiment in which they use a pulsed laser. The distances are relatively short, but they record the timings of the laser pulses as well as the timings of Alice's and Bob's measurement events. They have posted their dataset privately, and it may become publicly available in time; they're currently working on their analysis.

By any chance, did you also look into the time delay differences due to the EOM's birefringence?

 

[Peter Morgan]

By any chance, did you also look into the time delay differences due to the EOM's birefringence?

I basically did not much more than what you see in the paper on the arXiv. What looks pretty clear is that a local adjustment can be made to the timings that eliminates the timing features at the nanosecond scale that I at first identified in the longdist35 dataset. I didn't look quantitatively at what might be discovered by looking at multiple datasets (which can become a lot of work, so one wants a relatively strong feeling that it might be worthwhile).

I'm not sure whether the Weihs data contains enough information to characterize what parts of the various timing delays are caused by the electro-optical modulator.

BTW: let's get Gregor Weihs' name right. It's not Weih, nor Weih's. Of course I should feel especially sensitive about this, because there's one place in my arXiv paper where I use Wiehs.