Cramer's Backward Causality Experiment

[sanman]

Anyone read about this?

http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx

Now brace yourself for the backward-causality part: Because Signal B followed a shorter route to its detector, the fiddling in Signal A could theoretically show up in Signal B before Cramer actually fiddles with Signal A. It would be as if Cramer's actions had an effect that worked backward in time.

I'm not sure I agree with that part. I'd say that the effect in signal B would at best show up simultaneously as Cramer tampers with signal A.

Creating that longer circuitous route for signal A could be a way to simulate non-locality. After all, if signal A has to travel a longer route, it's as if it ends up at some distant location. So I suppose, in a technical way, simultaneity across non-local distances is a facsimile of time travel, but it isn't really genuine time travel.

This is like when Stephen Hawking said, "I think we can look into the past, but we can't alter it."

The Cramer experiment would work along the lines of Hawking's statement. Agree or disagree? Comments?

(Gee, how come nobody thought to do this experiment before, given all the investigation into these issues?)

[Hurkyl]

IMHO, the most accurate statement is that the effect doesn't show up until after the experiment is performed; no effect is visible until you have access to both data sets and can analyze correlations.

[cesiumfrog]

I actually wrote to Cramer, after another thread (http://www.physicsforums.com/showthread.php?t=166364) linked to specific details he has given for the new experiment. I basically said that his apparatus looks exactly the same as the Delayed Choice Quantum Eraser (an experiment already done, which he did not reference). He replied to me, saying the DCQE is irrelevant because of the coincidence correlation requirement: a requirement which, importantly, his experiment removes. He emphasised that the new experiment is a synthesis of a previous experiment by the DCQE group and an experiment from a particular German PhD thesis (which, strangely, seems not to have led to any journal publications).

I replied back to ask about a specific detail* of his apparatus, but also noted that the "ghost fringes" in the experiments he referenced were only observed in coincidence** measurements. He didn't respond again.

Anyway, his apparatus is conceptually clear (even if it fails as I expect), so once published the experiment should have pedagogical value (especially when students misinterpret the DCQE). It's good news if it will be finished soon this year.

*If I knew what he was using as the optical combiner, I think I could show where the path-information goes, and hence that it doesn't disappear. His one reply sort-of gave the impression he thought the coincidence requirement basically came only from "noise" photon pairs that don't interact with the slits.
**A colleague thought Cramer may have mistakenly overlooked this aspect.

[sanman]

Anyway, do you think that "long circuitous route" can be used to simulate "non-local scales"?

For instance, if you have a beam of light travel straight out for a distance of 1-light-year, then you know that's obviously a non-local scale of distance. But if you have your beam of light travel the same amount of distance along some long circuitous route which merely ends up taking it only a few inches away from the original starting point, then is it still roughly the same thing?

It seems to me that the Cramer experiment could merely serve as a partial disproof. If you tamper with signal A, and then your signal B which has taken the long circuitous route exhibits the same tampering effect much later, then you have disproven that "spooky action" is instantaneous. But if signal B demonstrates a tampering effect immediately, that result may not be conclusive, because it might merely be due to the long circuitous route not truly faking the non-locality.

[ged94]

How does this differ from what was done in Double Slit Quantum Eraser (http://arxiv.org/PS_cache/quant-ph/pdf/0106/0106078v1.pdf) by Walborn? They used a coincidence circuit in that setup, but in principle it's not necessary, right?

I'm curious if Cramer's setup can collect enough photons, even with a 300 mW pump, to generate an interference pattern within the short period of time between s1 and s2 to demonstrate "backwards causality".

[cesiumfrog]

They used a coincidence circuit in that setup, but in principle it's not necessary, right? Not right.

(Please let me know if you think there is any published record of entangled particles producing interference patterns outside of coincidence.)

[ged94]

Not right.

(Please let me know if you think there is any published record of entangled particles producing interference patterns outside of coincidence.)

In virtually all of these sorts of experiments bell inequality tests are performed to demonstrate that the photons are in fact entangled, and it's not possible to do that without coincidence counting. Also, using coincidence circuits is the easiest way to avoid noise. Especially when Type I downconversion is used, where the signal to noise ratio is exceptionally low. But I don't see how the coincidence counting itself somehow retroactively changes the Bell state after both photons have already been registered. That seems even spookier than what Cramer is trying to demonstrate.

I do think that the "backwards causality" leg of the experiment will probably fail, since IMO they are misinterpreting the delayed erasure phenomenon; ie the wavefunction collapses at the interference pattern and coincidence counting is necessary to "pick and choose" the corresponding idler photons. But if the wavefunction collapses with the detection of the idler photons first then the signal photons should form the appropriate dispersion pattern irrespective of coincidence counting. And this positively demonstrates the "nonlocal communication" leg of the experiment.

Without the coincidence circuit they'll need to subtract the noise out after the fact, and this will surely require more delicate and expensive equipment, not to mention longer collection periods. But I don't see why it shouldn't work.

Or am I missing something?

[cesiumfrog]

It seems you would agree with Cramer that coincidence counting is there just to reduce noise, whereas the mainstream community thinks coincidence counting is there to pick a fringe pattern out of a Gaussian distribution (leaving an anti-fringe pattern behind).

What you're missing is that regardless of how long the collection period is, and even with theoretically zero noise, there is no existing reason to expect the gross signal to show an interference pattern, ever. As far as we know, no such thing has ever been demonstrated, despite that many such experiments have already been performed.

[ged94]

I think we're talking about two different things here. With the 3 or 4 detector scheme outlined in Kim's "Delayed Choice Quantum Eraser" I agree that the fringes and antifringes are distinguished only when the which-way path has been determined after the fact. A classical communication channel is obviously necessary there. But with the setup I referenced above, the interference pattern forms no matter what the final state of the idler photon turns out to be. The only thing that matters is the simple fact that the idler has been detected -- accordingly, no additional information is needed to be exchanged classically to determine the dispersion pattern.

If coincidence counting were necessary in this particular design, the coincidence circuit itself would be determining the Bell state that had already collapsed. How would that be possible?

[cesiumfrog]

In the experiment that you mention (though it has not yet passed peer review, and noting it is preferable for several reasons to please link to the abstract (http://arxiv.org/abs/quant-ph/0106078) only), the which path information for the signal photon can in principle be determined from its initial polarisation along a special axis (or equivalently, from the polarisation of its entangled idler photon "p" along that axis). Consequently, there is no gross interference (regardless of whether the signal photon is measured before or after the idler) as shown in figures 3 and 7.

If a polariser is placed in the idler beam path, only half of the idler photons will be detected (but by rotating the polariser, you can choose which half). By appropriate choice of rotation, you will only detect idler photons for which the corresponding signal partner-photon's initial polarisation (along the important axis) can not be determined, and this half of the signal photons will produce fringes (fig 4 and 8). However, the opposite half of the idler photons (which are blocked by the polariser) would have corresponded to signal partner-photons that produced *antifringes* (for proof see figures 5 and 9). There is no way to distinguish these two sets of signal photons without using the measurement results from the idler photons (ie. that's why correlations/coincidence-counting *is* necessary). So either the idler photons are not measured for polarisation (and you get fig. 7), or the idler photons are measured for polarisation (and you still get fig. 7, which is just the sum of a fringe pattern superimposed with an anti-fringe pattern), either way you can't tell the difference by only looking at the signal photons.

In summary, I'm saying you've not understood your experiment, because it only lends further evidence to my argument. uh.. "Next!" :smile:

[peter0302]

Sorry to re-open this old thread but it's the most recent I could find on the subject.

What's the bottom line here? Referring to Cramer's (apparently dead) experiment:

http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

Will the photons passing through S1 always display an interference pattern to the naked eye / camera? The answer would seem to have to be yes, because despite the fact that members of the same pair will go through the same slit, which slit they go through is still governed by wave mechanics, and so as soon as more than one photon is used the interference pattern emerges.

Subsequently, when the switch is set to "0", it becomes impossible to correlate the VLP photon to the HLP photon, similar to the signal photon in the DCQE choosing the "eraser." If the switch is set to "1", however, it would be possible to correlate the photons, and thus extract the particle-like pattern from the observed interference pattern, just as DCQE does, _if_ there was a coincidence circuit.

In fact, the only difference between this and DCQE is that there are actually two erasers - the first, setting the switch to "0", is akin to the DCQE signal photon striking the eraser; but the second erasure occurs as soon as the photons at S1 strike the CCD without being recorded. Thus, even when the switch is set to "1", it is impossible to extract the particle-like image from the camera image because there is no record of photons from which to make the correlations. And consequently the camera always sees an interference pattern.

Is this not the flaw?

The only way to make this experiment work would be to force the photons at S2 to pass through one slit or the other without breaking the entanglement - not possible AFAIK.

[peter0302]

So does anyone know the final word on this? Has Cramer officially abandoned the project?

Also, doesn't the interference pattern ALWAYS show up without coincidence counting? That would make a helluva lot more sense than A. Zellinger's statement that the pattern disappears when the entangled twin's slit is determined. Should be the other way around.

[RandallB]

That would make a helluva lot more sense than A. Zellinger's statement that the pattern disappears when the entangled twin's slit is determined. Should be the other way around.
Exactly what statement are you talking about?
Your describing a statement about a condition that uses correlation counts, and the pattern disappearing is just what is expected ala standard Quantum Erasure.

If you don’t quote or reference what your talking about how can anyone follow what your saying?

[peter0302]

Gee, no need to be snippy about it.

A. Zellinger - Experiment and the foundations of quantum physics. Part III. A statement which you had quite a lot to say about in another thread!

My point is that if you believe Zellinger, Cramer's detector/camera will always register a Gaussian pattern. But I'm not clear as to why.

[RandallB]

Gee, no need to be snippy about it.

A. Zellinger - Experiment and the foundations of quantum physics. Part III. A statement which you had quite a lot to say about in another thread! Don’t intend to be snippy,
just brief in asking you to make your posts more complete for the benefit of other readers, rather than assume everyone is current with what you are reading.
Those that wish to look won’t find anything under “Zellinger” if you mean Anton Zeilinger.

If you are referring to my thread http://physicsforums.com/showthread.php?t=144792 that was about a published statement by Anton Zeilinger assuming results without correlation test actually being made, only that they could be made even though they are not.

Your comment was about a statement on actual correlations being made “the pattern disappears when the entangled twin's slit is determined”.

If you don’t have the quote available now I wait till you can find it. You are in a much better position to find it the complete statement your talking about.

[colorSpace]

Gee, no need to be snippy about it.

A. Zellinger - Experiment and the foundations of quantum physics. Part III. A statement which you had quite a lot to say about in another thread!

My point is that if you believe Zellinger, Cramer's detector/camera will always register a Gaussian pattern. But I'm not clear as to why.

This (Cramer's experiment) seems to be pretty much the "double-double-slit" experiment which I was talking about in the other thread. A.Zeilinger describes it in the book "Einstein's Schleier" (currently available only in german, even though it was written in english, I think). I'll try once more to present his explanation, since I don't know whether it is available in english, anywhere. If you want to keep looking for alternative takes on this qustion, do let me know if you find any.

The reason such experiments will always register an at first random looking set of data, according to AZ, is because, as I've said in the other thread, for such an experiment the source of light requires a minimum size (in order to produce impulse-entangled particles where the impulse of one is precisely enough the opposite of the other, so that one can conclude which-way-info of one from the other particle). And this minimum size will always create overlapping patterns, in so far as one can say it creates patterns.

Then depending on whether both particles are allowed to act wave-like, or not, there will be interference pattern hidden in the random looking data set (or not), but available for analysis only with information from both measurement sites. This way it comes to the same situation as usually with entanglement: There is an instant effect, but it can't be used to send arbitrary messages since the measurement results are randomized, and the effect can only be discovered with the help of classically transmitted additional data which allows discovering the relationships between the two random looking data sets (when there is any).

You might want to look into "quantum teleportation" (and perhaps "entanglement swapping"), since they go slightly further than "plain" entanglement, in transmitting qubits and such, and it becomes somewhat more obvious that there is 'something' being transmitted, which otherwise becomes more apparent only if one has an experiment using variable measurement angles, or such.

[RandallB]

This (Cramer's experiment) seems to be pretty much the "double-double-slit" experiment which I was talking about in the other thread. NO its not.
Cramer only uses a single double-slit in the path to the “Camera” area.

The “Image Slits” are locations where he intends to capture photons for his switching system. He is not subjecting those photons to a real double slit.

[colorSpace]

Actually, given the nature of the discussions surrounding this topic, I should have been more cautious when saying the double-double-slit experiment would be like Cramer's.

I can only say that the intent textually described in the first link in the first post in this thread sounds very much like what I understand to be the purpose of A.Zeilinger's double-double-slit experiment (which has actually been performed).

[colorSpace]

NO its not.
Cramer only uses a single double-slit in the path to the “Camera” area.

The “Image Slits” are locations where he intends to capture photons for his switching system. He is not subjecting those photons to a real double slit.

Yes, I just posted a response saying this (before reading yours).

Otherwise my longer previous post still applies, IMHO.

[peter0302]

Ok - so - will the camera always see a Gaussian pattern or not!?!?! :) Now I'm even more confused. :/

[RandallB]

.... this thread sounds very much like what I understand to be the purpose of A.Zeilinger's double-double-slit experiment (which has actually been performed). “Actually been performed” ? Where and by who?
Do you have a reference or link detailing the results of such a double-double-slit experiment where both “entangled” beams go through their own set of real double slits?
Did they give detailed results for non-correlated results for at least one side (that should answer Peters question) along with correlation results between the patterns produced in the two observation areas.

My main complain about Zeilinger was how he extrapolated from a single double slit experiment (a student Thesis Paper by Dopher) using correlations to speculate about how in his opinion a double-double-slit should work. And from those speculations he then presumed to make definitive conclusions without any direct experimental evidence to support his claims.

I’ve not seen where Zeilinger has attempted such an experiment.
Let us know where you found this experiment actually performed by someone.

[colorSpace]

I’ve not seen where Zeilinger has attempted such an experiment.

My source is his book "Einstein's Schleier" (AFAIK not available in english). I'll look up to see whether it mentions a specific implementation on a specific date. The description is rather detailed, but I don't remember a date or such, right now. It sounds like an experiment simple to perform, if one has the correct source of light. So I'd assume that it should be easily reproducible.

[peter0302]

In his Foundations article he says the reaosn the pattern is always seen in the one photon if you don't observe the other in a destructive way is because, otherwise, it's possible in principle for someone else to construct which-slit using the photon you didn't observe. You can't just non-observe it; you have to destroy it. Then and only then is there an interference pattern.

It's an interesting explanation, and it jives with the Copenhagen Interpretation, but I don't know of any experimental evidence confirming this.

Inicdentally I wrote to Cramer asking him what the deal with the experiment was and he hasn't replied...

[colorSpace]

My source is his book "Einstein's Schleier" (AFAIK not available in english). I'll look up to see whether it mentions a specific implementation on a specific date. The description is rather detailed, but I don't remember a date or such, right now. It sounds like an experiment simple to perform, if one has the correct source of light. So I'd assume that it should be easily reproducible.

No, sorry, the book itself actually talks explicitly about 'thought experiments', in this chapter. I'm not sure whether I got the impression that there was 'real' experiment somewhere else, or whether that was simply a false impression on my part.

[Edit:] (Which surprises me since the book repeatedly states that the experiment would be simple.)

[RandallB]

No, sorry, the book itself actually talks explicitly about 'thought experiments', in this chapter. I'm not sure whether I got the impression that there was 'real' experiment somewhere else, or whether that was simply a false impression on my part.

[Edit:] (Which surprises me since the book repeatedly states that the experiment would be simple.) No, sorry, the book itself actually talks explicitly about 'thought experiments', in this chapter. I'm not sure whether I got the impression that there was 'real' experiment somewhere else, or whether that was simply a false impression on my part.

[Edit:] (Which surprises me since the book repeatedly states that the experiment would be simple.) I figured something was wrong. For my own use I don’t find I can rely on Zeilinger information without carefully researching it for supporting detail, which I usually find missing. Much of that problem might be due to German to English translations. So, even though a lot of folks on this forum hold Zeilinger opinions in high regard - I do not, mostly because so many are lead to that same "false impression" by his writings.

At the same time, many that hold Zeilinger in high regard consider Cramer (the subject of this Thread) next to being a crackpot for his TI retro causality ideas. To me Cramer seems very much the real scientist willing to put his ideas to what could be a falsifiable scientific test. From Cramer’s own writing he is clear there is only a small chance his experiment might succeed.
Personal I’m convinced, as are most, there is no chance he can succeed; but that is based on my interpretation of existing results of other experiments that do not directly address his questions. At least Cramer is at least willing to back up his ideas by proposing and even attempting to run a real experiment.

Cramer Update

I was able to speak to Cramer for a few minutes yesterday and got an update on his experiment.
The experiment does continue but must be fit around other commitments – like his real job, relocation it to another lab in the building, travel comments etc.

The prep testing last Summer convinced him that the “Camera” he had obtained was not good enough and is changing to an array of detectors.

He expects receiving as few as ten photons in the “Camera” area with none in the “dark areas” will be enough for a statistical confirmation of an interference pattern, but only if “noise” can be first tested as at a low enough level.

He is confident the size of the down conversion (type 2) crystal and distance used well avoid any “Walkoff” issues. But producing an adequate volume of test photons is yet to be confirmed using the special cut SPDC. It is to produce the H V photon pairs in a single beam together. I didn’t even know there was an option other than the double cone (figure 8) distribution you find in most Type II Down Conversion experiments. That explains a lot as to how and why the prism will remove the remaining pump beam and the polar prism separates the H & V beams into separate testable areas. I had always assumed both type of PDC produced “cones” and had not been able to figure out how those prisms would work.

So, the experiment is still there, has a lot of work to do in confirming the set up is valid, but is not on a fast track as he works through the details mostly on his own with some other help as needed.
No projected time frame for when to expect results.

[peter0302]

I agree with your sentiments re: Zelinger. He seems like a brilliant and well-respected scientist but he says things in his articles that just don't have experimental support. Kudos to Cramer for actually putting his money where his mouth is and testing his ideas!

My money is on there always being an interference pattern.

[colorSpace]

I figured something was wrong. For my own use I don’t find I can rely on Zeilinger information without carefully researching it for supporting detail, which I usually find missing. Much of that problem might be due to German to English translations. So, even though a lot of folks on this forum hold Zeilinger opinions in high regard - I do not, mostly because so many are lead to that same "false impression" by his writings.

At the same time, many that hold Zeilinger in high regard consider Cramer (the subject of this Thread) next to being a crackpot for his TI retro causality ideas. To me Cramer seems very much the real scientist willing to put his ideas to what could be a falsifiable scientific test. From Cramer’s own writing he is clear there is only a small chance his experiment might succeed.
Personal I’m convinced, as are most, there is no chance he can succeed; but that is based on my interpretation of existing results of other experiments that do not directly address his questions. At least Cramer is at least willing to back up his ideas by proposing and even attempting to run a real experiment.

I haven't read many articles by Zeilinger, but mostly his two books and reports of his experiments. But the chapter on the double-double slit experiment is actually very clear on being a thought experiment, and my impression that there was a real experiment might have been due to the many details that it discusses. His books usually reference many experiments, both from others and his own. Perhaps it his writing style that sometimes isn't very clear on details. Still, his two books are the most informative writing on the subject of entanglement that I have found so far, without comparison.

Most of his work is based on conducting experiments, and in the field of entanglement, they are often cutting edge work. After all he is an experimental physicist, not a theoretical physicist.

As far as Cramer 's experiment is concerned: it hasn't been performed yet, so in this regard he still has to earn the title you are already assigning to him.


The experiment itself:

I find Zeilinger's argument quite convincing: he argues that the source of light needs a minimum size to guarantee the the impulses (momentum) are precise enough opposite of each other, so that path information about one photon also applies to the other. The reason is that Heisenberg Uncertainty requires an uncertain position for the source point of the photon, in order to guarantee the precision of opposite impulse.

Once the source of light has a certain minimum size, it is quite obvious that patterns from different points will overlap and eventually, if the size is large enough, produce only a blur.

If the source of light is smaller, according to this argument, then the entanglement on the impulse isn't strong enough to cause an effect of one photon on the other, in this regard, and the interference will always be there, not usable to send a signal.

Cramer Update

I was able to speak to Cramer for a few minutes yesterday and got an update on his experiment.
The experiment does continue but must be fit around other commitments – like his real job, relocation it to another lab in the building, travel comments etc.

The prep testing last Summer convinced him that the “Camera” he had obtained was not good enough and is changing to an array of detectors.

He expects receiving as few as ten photons in the “Camera” area with none in the “dark areas” will be enough for a statistical confirmation of an interference pattern, but only if “noise” can be first tested as at a low enough level.

I don't know... 10 photons sounds like a very small number, given the interference images I've seen so far.

He is confident the size of the down conversion (type 2) crystal and distance used well avoid any “Walkoff” issues. But producing an adequate volume of test photons is yet to be confirmed using the special cut SPDC. It is to produce the H V photon pairs in a single beam together. I didn’t even know there was an option other than the double cone (figure 8) distribution you find in most Type II Down Conversion experiments. That explains a lot as to how and why the prism will remove the remaining pump beam and the polar prism separates the H & V beams into separate testable areas. I had always assumed both type of PDC produced “cones” and had not been able to figure out how those prisms would work.

So, the experiment is still there, has a lot of work to do in confirming the set up is valid, but is not on a fast track as he works through the details mostly on his own with some other help as needed.
No projected time frame for when to expect results.

Well, I'm all in favor of exploring the possibility of non-random FTL effects, however it seems that requires a theoretical advancement first. Entanglement as described by current theory seems to always have the randomness built in, since entanglement seems to require Heisenberg Uncertainty where the wave-functions are dependent.

My impression (which, as now has been established :wink:, can be false) is that the success of this experiment depends on the possibility to build a source of light (or in this case, it seems, a crystal) which will strongly entangle the impulse (momentum), without causing a blur of always overlapping patterns, when there are patterns. (As described above).

[RandallB]

I agree with your sentiments re: Zelinger. He seems like a brilliant and well-respected scientist but he says things in his articles that just don't have experimental support. Kudos to Cramer for actually putting his money where his mouth is and testing his ideas!

My money is on there always being an interference pattern.

I agree with you, especially on the ALWAYS part.
Provided the PDC is positioned far enough away for a “far field” to avoid the “walk off” problem.
Meaning I disagree with Prof. Cramer about being able to lose that interference pattern in the signal branch (the “camera” test area) to a dispersion pattern, by doing something in the other leg (idler branch or switching area) of the experiment.
IMO only when correlations are done between the two areas to select a smaller set of photons in the "Camera area" can the interference pattern seem to be erased into the needed standard dispersion pattern "signal". And of course the whole point is to get the signal before idler information can be transmitted to the signal area to do such correlations to confirm his reverse causality.

I actually think there are some other experiments that would be more worthy of his time, and have suggested them to him. But certainly my conviction that his experiment at best can only falsify his TI ideas, is no reason for him to change his mind about doing the work. As long as he feels there is some doubt, although slim, and his idea might be provable he should continue his work even if only to get a firm falsification.


As far as Cramer 's experiment is concerned: it hasn't been performed yet, so in this regard he still has to earn the title you are already assigning to him.

I don't know... 10 photons sounds like a very small number, given the interference images I've seen so far.

What title? I did say he was right – the only title I assigned him is a scientist that does not overstate his case as fact, but proposes real experiments to test his ideas and then peruses have the tests preformed. His experiment does not need to succeed or fail to merit that well deserved “title”. While have seen many others that IMO do not merit that.

As to “10 photons sounds like a very small number …” isn’t that a rather seat of the pants, common sense evaluation on your part?
I’m sure the Prof. used the science of a statistical probability. You know, in line with a “Shut up and calculate” approach more in touch with QM HUP. If you have doubts why not try it yourself, for a interference pattern with 10 to 12 areas each of dark, gray and bright areas, then overlay those thirty some areas with a standard dispersion pattern. What do you figure the odds are of not one of ten randomly chosen photons in the dispersion pattern would overlap with one of the dark areas in the interferance pattern?

As to further comments on Zelinger I don’t have time now, but I’ll provide a link here back to the other thread you referred and comment there. That way we do not to clutter up this discussion on the Cramer Experiment (which if I get any other updates on I’ll add).

[colorSpace]

What title? I did say he was right – the only title I assigned him is a scientist that does not overstate his case as fact, but proposes real experiments to test his ideas and then peruses have the tests preformed. His experiment does not need to succeed or fail to merit that well deserved “title”. While have seen many others that IMO do not merit that.

This would make sense as a general judgement only if Zeilinger was someone who never makes experiments. The reality is he made tons of experiments regarding entanglement. So I don't see a big problem with him making a thought experiment once in a while. That Cramer merely 'promises' an experiment in this specific case doesn't outweigh the substantial record that Zeilinger has in actually performing experiments.

The promise of an experiment is not an experiment.

As to “10 photons sounds like a very small number …” isn’t that a rather seat of the pants, common sense evaluation on your part?
I’m sure the Prof. used the science of a statistical probability. You know, in line with a “Shut up and calculate” approach more in touch with QM HUP. If you have doubts why not try it yourself, for a interference pattern with 10 to 12 areas each of dark, gray and bright areas, then overlay those thirty some areas with a standard dispersion pattern. What do you figure the odds are of not one of ten randomly chosen photons in the dispersion pattern would overlap with one of the dark areas in the interferance pattern?

Take a look at the image labelled "Electron buildup over time" at http://en.wikipedia.org/wiki/Double-slit_experiment

It shows five images "a"-"e", with progressively more particles. Image "b" already has a substantial number of particles in the dark areas. And even in image "a", where I count about 10 particles, it looks like the third and especially the fourth from the left are in dark areas.

One thing is for sure: I wouldn't believe any experiment using only 10 photons to make a statement.

[AndreasP]

Hello,

Zeilinger's use of a small versus a big light source in his book "Einstein's Schleier" always seemed a bit confusing to me. Here is why.

(1) CASE ONE: SMALL LIGHT SOURCE:

By a small light source, Zeilinger simply means a light source that is smaller than the distance between two interference fringes on the detection screen (assuming for simplicity that the distance between the light source and the double slit on the right side is roughly the same as the distance between the double slit and the detection screen).

In his book, Zeilinger in effect says that there are two reasons why a small source is necessary for interference to occur in the double-double-slit experiment:

(1a) A small source means that different points on the source are very close together. So patterns originating from different points on the source will NOT overlap on the detection screen and therefore will NOT produce a blur. So no washing out here, but interference. This is definitely true. As a side comment, this condition has nothing to do with entanglement per se. It must always be met, regardless of whether there is a second entangled photon around somewhere or not, that may or may not carry information about the original photon. For already in the simple double-slit experiment, as described by Feynman (where there is no entangled photon at all, but just a single light source emitting single photons towards a single double slit on the right side and then on to a detector) this condition must be met for interference to occur.

(1b) A small source means high uncertainty in the momentum of the emitted photons in either direction, making it impossible to determine the precise direction of photon 2 on the left hand side, and consequently the direction of photon 1 on the right hand side.

So far so good. This was the easy part.

(2) CASE TWO: LARGE LIGHT SOURCE

Here is where - at least in my opinion - the book is less clear and, perhaps, a bit misleading.

First, let's see what Zeilinger says about large light sources. Two things:

(2a) A large source means that patterns originating from different points on the source will overlap and produce a blur on the detection screen. That is, the interference patterns originating from different points on the source will wash each other out, producing a uniform grey pattern on the detection screen. So a large source can produce interference patterns from different points on the source, but they wash each other out (one can view all this differents points on the source as a set of many "mini-sources" but let's not get into that).

(2b) A large source means small uncertainty in the momentum of the two entangled photons in either direction, making it POSSIBLE to determine the precise direction of photon 2 on the left hand side (by registering it in a detector on the left hand side), and thus the direction of photon 1 on the right hand side (because they are entangled). In other words, the entangled photon 2 on the left hand side carries information about the path information of photon 1 on the right hand side, and this is why there cannot be any interference pattern on the right side: In Zeilinger's words, the which-way-information is still "present somewhere on the universe" - here in the form of the momentum carried by photon 2 on the left side.

He then describes a way to make the interference pattern re-appear, even in the case of a large light source. He says all one needs to do is to "destroy" the which-way-information of photon 2 on the left side (possibly at a later time), simply by letting it also go through a double-slit itself on the left hand side. A simple trick, he says, will allow the experimenter to "recover" the interference pattern of photon 1 on the right side. The trick consists in making precise measurement of time, allowing the experimenter to detect the twin pairs, i.e. which photon 1 on the right side belongs to which photon 2 on the left side.

MY QUESTION IS:

But what happend to reason (2a) ?? Even if the which-way-information is destroyed using the method described in Zeilinger's book, there is still a large source emitting the entangled photon pairs. So, won't the different patterns originating from the different points on the large light source STILL overlap and wash each other out on the detection screen, producing a blur?

If, what Zeilinger says, can be done in an experiment, then recovering the various interference patterns ex-post would in effect mean to recover the individual points on the (large) source, ie the interference patterns from the many mini-sources.

COMMENTS?

[RandallB]

A follow up on Prof. John Cramer's Backward Causality Experiment

First:
for anyone reading through this thread concerned about an expensive experiment basing conclusions on only 10 detected photons thinking --
“I wouldn't believe any experiment using only 10 photons to make a statement.”

It should be obvious he needs to minimize the number photons used in a “snapshot” to determine if he has a pattern YES or NO.
Between 10 or 100 per snapshot he still needs to something like 90 out of 100 snapshots that come up Yes in the ‘Alice’ Test Area with 10 uncertain (I would think any “sure NO’s” in the setup would call for adjustment) just to confirm the sampling is within reason.

Then once the results are established he needs to repeat the same with some change external to Alice and the source path in the Time and Distance separated ‘Bob’ test area that will have the “Backward Causality” affect of giving a NO result for the pattern. And deciding how many “sure NO’s” in a 100 snapshots is enough to indicate “Backward Causality” and if any “sure YES’s” should be allowed.
Remember The Cramer set up has no “coincidence correlation requirement” only a setting at B that should turn the pattern at A on or off which A should be able to see without any info from B about the setting. It is not a DCQE set up.

Even with a small number of photons per snapshot the complete experiment will run into 10’s if not 100’s of thousands of photons tested.

Amazes me how much work this guy is putting himself through; and at the end of the day he fully recognizes that his results may well confirm what I and others have predicted in this thread that changes in the ‘Bob’ (“Image Slits” path) test area have absolutely no affect on the observed results in the ‘Alice’ (double-slit path) test area; I.E. No Backward Causality!

the update:

I spoke Prof. Cramer today:

After discarding the “CCD Camera” in ’07 as not good enough for this experiment (Jan. 2008 update) progress this year has been stalled on on getting an array of photon detectors to replace it to perform to the standard he needs.

It is basically a technical issue that, short of super cooling the detectors which could require putting the entire experiment in a vacuum to avoid frost fogging, needs a practical economical solution.

Thus no new real progress on the experiment itself has been made this year. But has far as John is concerned he is pressing on to resolve the tech stuff to enable and get back to the real experiment, so you can’t call it a dead effort yet.

I’ll check with him again after some time and update again here, – maybe the Fall semester will bring in some fresh solutions to the tech issues.

[peter0302]

That doesn't make any sense. The experiment should generate a visible interference pattern on a screen just as easily as it can generate one on a CCD - if he's right.

[RandallB]

That doesn't make any sense. The experiment should generate a visible interference pattern on a screen just as easily as it can generate one on a CCD - if he's right.
But you missing the point and purpose of Cramer’s work and the paper you referenced early on in this thread.
Of course the pattern will be produced to give his “0” signal on a screen instead of a ‘Camera’ in the test area. And even if he were to change that “0” to an “1” signal continually displayed on the screen by changing something in the B area – what would that prove?
Nothing to him because he is trying demonstrate BACKWARDS CAUSALITY.

And if he were to produce such a change on a SCREEN I would immediately assume that some phase interference communication was traveling back up the B path to where it split from the A path to effect the display on the screen with a undetected time delay. I.E. a normal forward time causality.

If it were as simple as changing what is seen in a long time interval screen dispaly he wouldn’t be dealing with taking snapshots of patterns.
It is also why he needs to be concerned with the number of photons required to decide on a “0” or “1” pattern result in a snapshot. The longer it takes to collect enough photons the greater the optical distance between A & B needs to be to eliminate a hidden slower than light forward time solution. This is not as simple as you make it sound. And this experiment has a long way to go.

Sure you and I (probable for differert reasons) "know" he will never get a pattern to go blank without a real time connecting link as used in a DCQE. But Cramer is intent on testing what we think we “know” as “opinions” that might be wrong.

[peter0302]

What really needs to be proven first before any more money is wasted on this is whether a change at A can have any effect on the pattern at B. That's never been shown and would defy QM. Now, sure you'd assume there was some phase interference with an undetected time delay, but if can't do it my way, which is much cheaper, at all (which he can't!) then there's no reason to waste more money. If he can do it my way, then move onto eliminating loopholes as you suggest.

In other words, what inspired all this was Dopfer's experiment (Zelinger's student), which Cramer couldn't understand why required coincidence counting, so he decided to remove the coincidence counter. If he can make Dopfer's experiment work without a coincidence counter - even slower than light - I will be highly impressed. But he can't even do that, so why waste our time and a lot of other peoples' money trying to do more?

[mickeyp]

Cramer makes a big mistake when he states that the coincidence circuit only filters a little noise. Actually the coincidence circuit is necessary to see any fringing at all. As with all the previous quantum erasure experiments, the Dopfer experiment is no different. In a quantum erasure experiment there usually is a fringe pattern and an equal antifringe pattern. The coincidence circuit removes the antifringe pattern.

Nothing here that could be used as a non-local communicator.

Check some of the early work by Mandel and his associates at Rochester to understand this more thoroughly.

[RandallB]

Nothing here that could be used as a non-local communicator.

Check some of the early work by Mandel and his associates at Rochester to understand this more thoroughly. Of course there is something there to serve as a non-local communicator.
You need to recognize “non-local” as meaning; Not maintaining Locality and/ or Not Realistic.
The unrealistic appearance of Backwards Time Causality would obviously be the non-local communicator.

Most of us think that this nor any other experiment is capable of demonstrating Backwards Time Causality as better than say OQM as a Non-Local solution. The hallmark of a free Science is we get to criticize, but we don’t get to control how Cramer and his supporters spend their experimental resources.

Also, do the many readers of Physics Forums a favor;
If you have a useful resource from Mandel give a detailed reference.

[peter0302]

Cramer makes a big mistake when he states that the coincidence circuit only filters a little noise. Actually the coincidence circuit is necessary to see any fringing at all. As with all the previous quantum erasure experiments, the Dopfer experiment is no different. In a quantum erasure experiment there usually is a fringe pattern and an equal antifringe pattern. The coincidence circuit removes the antifringe pattern.

In DCQE yes. In Dopfer, no. That is why Dopfer is so tantalizingly close to what Cramer is trying to do. No one had shown, prior to Dopfer, that you could make a fringe pattern by sending an entangled photon through a double slit (DCQE sends the original photons through double slits and THEN creates the entangled pairs).

But what is even more tantalizing about Dopfer is that as you move the detector away from the focal point slowly, you get something inbetween an interference pattern and a gaussian pattern - at the other detector - showing that the quality of the pattern is directly proportional to the number of photons for which you destroy which-path info. In other words it doesn't necessarily have to be perfect.

In dopfer, there is no doubt that all the coincidence circuitry does is eliminate noise. No doubt. It is a filter that makes sure that we're only looking at photons which we _know_ correspond to ones we've done something to (either destroyed which-path or not destroyed). But if we could ensure that we do the same thing to MOST of the photons -we should not need the counter. But how do we ensure that we get them all without violating the HUP? That's the problem.

[RandallB]

then move onto eliminating loopholes as you suggest

In dopfer, there is no doubt that all the coincidence circuitry does is eliminate noise. No doubt. As I don't believe there are any real Loopholes in EPR testing worth considering I don't recall suggest any need to eliminate them - I consider those efforts just as pointless as Cramer's.

I do not see 'that all the coincidence circuitry does is eliminate noise' at all.
Coincidence tracking is much more significant than that and is subject to noise itself.
The important demonstration in the Dopfer example is showing how a DCQE configuration is affected by the near field walk off issue as that setup is in the very near field.
An issue Cramer is accounting for very carefully in his set up to ensure he is just far enough into the far field to eliminate walk off.

[mickeyp]

In Dopfer the double slit is close to the pdc. Furthermore, the pdc has a finite aperature and a thickness -- which is important. The wavefunction at the double slit is phase scrambled. It is equally likely that the wavefront at bolth slits is in-phase or completely out-of-phase. Thus there is no significant fringing behind the double slit.

The coincidence circuit, with the lens at f from the detector, filters out an antifringe, thus the coincidence only records photons with a similar phase condition in front of both slits.

One could also produce fringes after the double slit with a pinhole at the pdc output, or one could move the double slit further away from the pdc. Both of these approaches limits the phase variations in the wavefunction but in neither case would one be capabile of controlling the fringes from the other path.

[mickeyp]

The Dopfer / Cramer thing probably is a lost cause.

However, an experiment that met my eye as might show some reverse causality is:

B. Hessmo, P. Usachev, H. Heydari,and G. Bjork, PhysRevLett.92.180401, 7 MAY 2004

If you can get a hold of this please note that the rotation of the birefringent plate changes the photon rate at detectors D1 and D2.

As with Dopfer, Hessmo et. al. uses coincidence, but in this case the coincidence isn't to remove an antifringe but is to remove background noise and increase the fringe visiblity to exceed the Bell inequality.

[RandallB]

In Dopfer the double slit is close to the pdc. Furthermore, the pdc has a finite aperature and a thickness -- which is important. That is where “walk off” comes from “close” means near field relative to the size and thickness of the down converter.

Also wrt B. Hessmo and if they “might show some reverse causality”:
If as you say they are using coincidence counting or correlations then they Cannot.

[mickeyp]

That is where “walk off” comes from “close” means near field relative to the size and thickness of the down converter.

Also wrt B. Hessmo and if they “might show some reverse causality”:
If as you say they are using coincidence counting or correlations then they Cannot.

In Hessmo, the control in one path, after a beamsplitter, changes whether a "hit" on a detector is one photon or two photons. Due to the constant photon rate this is half as many two photon "hits" as one photon "hits". This could be thought of as the "signal" - the rate of photon bunch "hits" where a bunch is either one or two photons.

Since the "hits" from noise is much higher than the controlled one/two photon signal, Hessmo et. al. utilized coincidence to filter the noise -- achieving a near 100% signal.

Unlike Dopfer / Cramer where the antifirnge term is exactly ( and I mean exactly) the negative of the desired fringe term, in Hessmo there is no such exactly equal negative term -- just a "signal" which is covered by a random noise term.

[peter0302]

The Dopfer coincidence circuitry does not filter out an antifringe. It is quite clear that when the one detector is placed at the focal point of the Heisenberg lens, the other detector (for the entangled twin), as it scans along the x-axis, shows a visible interference pattern. There is no filtering of an anti-fringe.

This is in stark contrast to DCQE, where there are two interference patterns which must be differentiated from one another or else they show up as gaussian at the signal photon detector. That is *not* the case in Dopfer. There is *one*, perfect interference pattern at the signal photon end - not one half of a fringe/anti-fringe pattern.

The coincidence circuitry is necessary to ensure that the photons we're looking at at the signal end COINCIDE with the ones that were sent to the focal point of the Heisenberg lens. Its function is nothing more than that. *If* all of the idler photons could be sent to the focal point of the Heisenberg lens somehow, there would be no need for coincidence circuitry at all. But, I believe it is impossible to force all the photons to a single point, which is why we filter out only those that happen to reach that point.

[mickeyp]

That doesn't make any sense. The experiment should generate a visible interference pattern on a screen just as easily as it can generate one on a CCD - if he's right.

I think you misread the actual Dopfer experiment. At no time was there any measurements without the coincidence circuit involvement. Both detectors were on some sort of movable platform, the detector on the lens leg was kept stationary and then the detector on the double slit side was moved to create the interference pattern in coincidence.

the process is similar in the other direction, that is, the detector in the double slit leg is kept stationary while the detector in the lens leg is moved - again providing some sort of pattern.

Without coincidence the actual photon distribution in front of both detectors is just a fog. It takes the coincidence circuit to bring out any pattern.

[peter0302]

I read Dopfer correctly. I never said she didn't use coincidence circuitry. She did. But no one seems to understand _why_ that circuitry is necessary.

Again, it has nothing to do with fringe/anti-fringe patterns. It has to do with finding the idler photons that COINCIDE with the signal photon whose which-path information was destroyed. When we detect signal-photons with ambiguous which-path information, we see interference in the idler-photons directly proportional to our certainty as to the which-path. This is reflected in the fact that as the signal photon detector is moved away from the focal point, we have a slightly better idea of where it came from and so get less and less of an interference pattern.

Bottom line: when we destroy which-path information for the signal photon by detecting it at a point where it is impossible to tell where it came from (the focal point) we see a perfectly symmetric (not fringe/antifringe) pattern from the idler photon. If we could detect EVERY signal photon in this fashion, we'd see a pattern in EVERY idler photon, and hence a visible interference pattern without coincidence counting.

That's what Cramer's TRYING to do. Unfortunately he has not shown us a viable way of doing it.

[mickeyp]

If the experimenter reduces the size of the area of radiation of signal and idler photons from the pdc, let's say by use of pinhole(s), then one would see an interference pattern behind the double slit without the use of the coincidence circuit.

However, in this case, and with the Heisenberg detector at 2f, the two detected triangle pattern intensities behind the Heisenberg lens are broadened wherein both are on top of each other, thus the experimenter cannot deduce path.

The pinhole between the pdc and the double slit acts to reduce the uncertainty in the momentum which increased the uncertainty in position. For there to be signaling from the lens leg to the double slit leg the Heisenberg's uncertainty principle must be revoked -- a violation of quantum mechanics.



I read Dopfer correctly. I never said she didn't use coincidence circuitry. She did. But no one seems to understand _why_ that circuitry is necessary.

Again, it has nothing to do with fringe/anti-fringe patterns. It has to do with finding the idler photons that COINCIDE with the signal photon whose which-path information was destroyed. When we detect signal-photons with ambiguous which-path information, we see interference in the idler-photons directly proportional to our certainty as to the which-path. This is reflected in the fact that as the signal photon detector is moved away from the focal point, we have a slightly better idea of where it came from and so get less and less of an interference pattern.

Bottom line: when we destroy which-path information for the signal photon by detecting it at a point where it is impossible to tell where it came from (the focal point) we see a perfectly symmetric (not fringe/antifringe) pattern from the idler photon. If we could detect EVERY signal photon in this fashion, we'd see a pattern in EVERY idler photon, and hence a visible interference pattern without coincidence counting.

That's what Cramer's TRYING to do. Unfortunately he has not shown us a viable way of doing it.

[mickeyp]

Being new to this foum I hadn't realized, until now, that I could upload an attachment.

Here is the Hessmo, et. al. experiment that I mentioned earlier.

Although this experiment uses coincidence to extract a high fringe visibility, one can logically say this:

observation 1: With the phase shift at 180 degrees (FIG. 3) there is no coincidence between detectors D1 and D2 (FIG. 2)

observation 2: With the phase shift at 0 degrees there is maximum coincidence between detectors D1 and D2

assumption: The photon rate going to beamsplitter BS is independent of the rotation of the birefringent crystal (FIG. 2)

conclusion 1: No coincidence implies that two (or more) photons are striking D1 or D2 together.

conclusion 2: Maximum coincidence implies “single photons” are striking D1 and D2 at the same time

conclusion 3: with the assumption of a constant photon rate illuminating the beamsplitter BS then there must be ½ as many two photon hits on D1 and D2 as there are single photon hits – a signal controlled by the rotation of the birefringent crystal.

Conclusion 4: the control of the number of hits at detector D1 by the rotation of the birefringent crystal, which is after the beamsplitter in the D2 path, constitutes a non-local communicator.

[RandallB]

Being new to this foum I hadn't realized, until now, that I could upload an attachment.

Here is the Hessmo, et. al. experiment that I mentioned earlier.
You can also just provide the appropriate link for us to access whatever is available.

This is still a corralation example, it doesn't address J C's experiment.

[peter0302]

If the experimenter reduces the size of the area of radiation of signal and idler photons from the pdc, let's say by use of pinhole(s), then one would see an interference pattern behind the double slit without the use of the coincidence circuit.

However, in this case, and with the Heisenberg detector at 2f, the two detected triangle pattern intensities behind the Heisenberg lens are broadened wherein both are on top of each other, thus the experimenter cannot deduce path.

The pinhole between the pdc and the double slit acts to reduce the uncertainty in the momentum which increased the uncertainty in position. For there to be signaling from the lens leg to the double slit leg the Heisenberg's uncertainty principle must be revoked -- a violation of quantum mechanics.

Why? In neither case do we ever actually learn which-path info. It just so happens that in one, we see an interference pattern, and in another, we see a single blob. But technically in neither case did we deduce which slit the photon came from.

I have seen complimentarity (I think incorrectly) described as we cannot simultaneously view particle-like and wave-like behavior. In fact, what it means is that we cannot know which-slit a photon came through and still be certain enough as to momentum to generate a coherent interference pattern. But that is not happening here. In the interference case, we don't know which-slit because it was intentionally destroyed. In the blob case, we still don't know which slit because both blobs are superimposed onto one. I don't see how the HUP is violated.

[RandallB]

If the experimenter reduces the size of the area of radiation of signal and idler photons from the pdc, let's say by use of pinhole(s), ... . Mickey your missing the point in Dopher – which has nothing to do with backwards causality.
Contiune the Dopher discussion in:
http://physicsforums.com/showthread.php?t=251158

I’m sure Peter will catch up with you there.
I know your new to the forums so I thought I’d helping out –
There are always tangents in a discussion but it helps to avoid hijacking a thread into a different topic.

Just opening a new thread helps keep each one a little better focused each different subject rather than becoming a string of random topics like you find in a blog.
Plus by including Dopher in the title of the thread makes it easier for others to find the discussion even years from now. Just as you can finds a lot of past info with a title search on “Dopher”

[peter0302]

Randall, all due respect, but I think Dopfer's experiment is *directly* relevant to Cramer's experiment. Cramer got hsi entire inspiration from the Dopfer experiment. He believes it proves that his concept will work, so the question of whether/how the coincidence counting could be removed from Dopfer is really the central question to Cramer's concept.

[RandallB]

Randall, all due respect, but I think Dopfer's experiment is *directly* relevant to Cramer's experiment. Cramer got hsi entire inspiration from the Dopfer experiment. He believes it proves that his concept will work, so the question of whether/how the coincidence counting could be removed from Dopfer is really the central question to Cramer's concept. No his real inspiration comes from the peer reviewed and published work of the Shih group. The Dopher notes help illustrate his concern and attention to the near fields vs. far field effects in setting up his modified Shih group experiment to test Backwards Causality.

In any case a detailed discussion of Dopher warrants its own thread IMO.

[peter0302]

In any case a detailed discussion of Dopher warrants its own thread IMO.

I do agree with you there. :)

Do you have any arix links to the Shih group papers?

[RandallB]

Do you have any arix links to the Shih group papers? There’s a Phys Rev Lett. Reference on page 2 of the Cramer doc you’re familiar with.
http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

Trim off the file name on the link and you’re on the Prof’s Home page with more info.
I seem to recall one of his lectures listed there going into a little more detail about the Shih group – but that was when I first read about this;
The original old discussion on Backwards C in ’06 ’07 is at:
http://physicsforums.com/showthread.php?t=144298

[AndreasP]

Randall, all due respect, but I think Dopfer's experiment is *directly* relevant to Cramer's experiment. Cramer got hsi entire inspiration from the Dopfer experiment. He believes it proves that his concept will work, so the question of whether/how the coincidence counting could be removed from Dopfer is really the central question to Cramer's concept.

Peter,

having read this thread one more time, I think I am totally with you. Dopfer's experiment is in essence what Cramer is trying to do, or at least is very close to it. The conincidence circuit in Dopfer's setup is not used to filter out the antifringe but merely to find out which photons belong together.

Do you conclude therefore that Cramer's experiment will work?

It seems to me that by using a precise enough clock one should in principle be able to match photon pairs without the coincidence ciruit. Alternatively, one may leave enough time between the emission of the individual photons to know for sure which photons belong together. This may seem to "slow down" the tranmission rate, but the main point is here is of course that the two detectors can be very far away from each other.

Another thing I am wondering:

If Cramer cannot find funding for his experiment, why does he not just ask Dopfer or Zeilinger to allow him to come to Innsbruck and modify Dopfer's original experiment that she performed in 1998 as part of her PhD thesis in Zeilinger's group (assuming the setup is still available)?

That seems like a more pragmatic approach. Why reinvent the wheel? In the process he could update the experiment with the latest available technology which I am fairly sure is available in the Innsbruck/Vienna labs of the group - which are among the most advanced in the world.


Andreas

[Dmitry67]

Interesting, what if we do that experiment with 1 'open' leg?

So we observe an interference pattern pointing 2 other rays to cosmos, to the infinity.
If, in 1000000 years these rays hit an alien astronomer looking at us at telescope (so he can determine which path) our interference pattern dissapear right now.
If we accidently point it to the black hole, then our pattern is restored :)

I suspect that this wont work for some reason, and Cramers experiment must fail, however, I did not find any explanations WHY it wont work so far.

[DrChinese]

Do you conclude therefore that Cramer's experiment will work?

It seems to me that by using a precise enough clock one should in principle be able to match photon pairs without the coincidence ciruit. Alternatively, one may leave enough time between the emission of the individual photons to know for sure which photons belong together. This may seem to "slow down" the tranmission rate, but the main point is here is of said course that the two detectors can be very far away from each other.

Another thing I am wondering:

If Cramer cannot find funding for his experiment, why does he not just ask Dopfer or Zeilinger to allow him to come to Innsbruck and modify Dopfer's original experiment that she performed in 1998 as part of her PhD thesis in Zeilinger's group (assuming the setup is still available)?


Zeilinger said (click to see reference) (http://www.hep.yorku.ca/menary/courses/phys2040/misc/foundations.pdf) this will not work and explains why, see Fig. 2 on page 290. Basically, the which-slit information is available in principle - and thus no meaningful interference pattern arises (i.e. so nothing changes at S1 based on what you do at S2. He also indicated that his paper is based on experiments that have been performed, so I guess he thinks there is no need to re-run the experiment. Probably would explain why everyone is not rushing to do it.

BTW, in Cramer's version of the experiment, coincidence counting is not necessary. If it were, then this would not be a non-local quantum communication device.

Dopfer's setup is discussed specifically as well, see Figures 3 and 4 on pages 290 and 291. Now, here is where I think there is something more to consider. In Dopfer's version, there is an interference pattern formed for a subset of the photons at D2. If the person at D1 changes the location of the detector, that interference pattern at D2 disappears. Clearly, to get the subset you must coincidence count. But consider the entire set at D2, including the data points for which there is not a coincidence (i.e. where the which-path could not be erased. Presumably it does not change based on the actions of the person at D1 (otherwise Dopfer would have noticed this and commented). So that means that as the interference pattern disappears, the data points for the remainder of D2 change in just such a way that the total pattern (a single peak/crest/bar) remains essentially static. Now clearly, the number of clicks at D1 changes as D1 is moved to and from the focal point; that will also affect the coincidences (between D1 and D2).

So the subset of D2 in which could not have the which-path info erased - that subset will see a shift in its D2 pattern based on the actions at D1 as well. And yet the total pattern at D2 doesn't change. So more weird stuff to think about!

[RandallB]

Peter,
having read this thread one more time, I think I am totally with you. Dopfer's experiment is in essence what Cramer is trying to do, …..

Do you conclude therefore that Cramer's experiment will work?
Andreas
Welcome to PF
Peter will not be replying – when you see a line though a name, as on his, it means for whatever reason they are no longer a member here.

No the Cramer set up is not the same as Dopfer, Cramer requires a Far Field set and Dopfer specifically explained the need for a near field set up in her work (hard to find as here paper in only available in Germen).

And as I’d reported Cramer considers the “Shih group” as the foundation to his plan, but does reference Dopher especially as to the need to use a Far Field rather than a Near Field.

I know this from personal conversations with him.
I did promise to update his progress, but I would not expect a contact from him on lack of progress. It is been almost a year so I’ll see if I can get in touch with him to post a current update.

I expect no real progress,
If you go through Peters posts another time I think you can see that he agreed with me that the experiment would never succeed in even the early set up before adding in sufficient time delays to finally address the Causality issue. At the time at least I thought both Peter and I had given enough detail to explain why it could not work even at the level where the sampling intervals were still longer than time separation.
In fact if I remember right Peter doubted the proposal so strongly the he questioned the value of anyone providing the first dime of funding for it.

[DrChinese]

No the Cramer set up is not the same as Dopfer, Cramer requires a Far Field set and Dopfer specifically explained the need for a near field set up in her work (hard to find as here paper in only available in Germen).

And as I’d reported Cramer considers the “Shih group” as the foundation to his plan, but does reference Dopher especially as to the need to use a Far Field rather than a Near Field.

I know this from personal conversations with him.


RandallB,

As you mention, Cramer's setup is different from Dopfer's. I noticed that too, and was trying to understand the reasoning there. Cramer has the focal lens before the beamsplitter, that sticks out most to me. Is that what you refer to as the "far field" setup? Can you explain the reasoning for that?

I am guessing that by placing the focal lens there, both photons of *every* entangled pair has its which-path info erased. That way, interference is expected on the "uncorrelated" image at both S1 and S2, and that is why coincidence counting would not be required. Thus is born the idea for the FTL signaling mechanism (although I assume that no one actually expects that to result). Am I close?

So where might this ingenious scheme goes wrong? My guess: the camera at S1 is not actually at the focal point of those photons, and therefore their which-slit info is not truly erased. Ergo there is never an interference pattern at S1 anyway, and consequently nothing done at S2 makes any difference at S1. All you ever see at S1 is the "1" pattern.

[RandallB]

RandallB,

As you mention, Cramer's setup is different from Dopfer's. I noticed that too, and was trying to understand the reasoning there. Cramer has the focal lens before the beamsplitter, that sticks out most to me. Is that what you refer to as the "far field" setup? Can you explain the reasoning for that?

I am guessing that by placing the focal lens there, both photons of *every* entangled pair has its which-path info erased. That way, interference is expected on the "uncorrelated" image at both S1 and S2, and that is why coincidence counting would not be required. Thus is born the idea for the FTL signaling mechanism (although I assume that no one actually expects that to result). Am I close?

So where might this ingenious scheme goes wrong? My guess: the camera at S1 is not actually at the focal point of those photons, and therefore their which-slit info is not truly erased. Ergo there is never an interference pattern at S1 anyway, and consequently nothing done at S2 makes any difference at S1. All you ever see at S1 is the "1" pattern. No - not close
The Lenses in Dopfer are after the Slit locationS (both real and image slits).

Cramer is using a Type II PDC; the only purpose of the lens here is to turn the diverging H & V beams onto the same vector (parallel) so that they can both go through the same beam splitting polarizer before moving on to the double slits (one path to real slits the path other to 'image' slits).

Where does his scheme go wrong,
Just my opinion, (and I have given it to him).
As you know I am convinced that an individual beam from a pair of beams produced by any “entanglement” process when measured in isolation will produce the same results as a “normal” beam of light that is produce to be identical with the exception of not having an “entangled twin” beam to have ever been produced of any type.

Meaning:
A given in classical optics is:
1) A single ‘normal’ beam can never produce a two slit interference pattern when in a near field set up.
2) the same ‘normal’ beam will always produce a two slit interference pattern when in a near field set up.
I am convinced:
any single 'entangled' beam will always produce the same 2 classical results – period.

I know you and I disagree on this point – but unless Cramer can prove me wrong on this point and show that Far field interferance is sometime failed to be seen; there is no reason to moving onto the delayed changes on the idler beam step protion of his testing. I.E. a failure.

He will only be able to prove what I’ve already discussed with you about the experimental results you found elsewhere. (we just disagree on how to read those results).
Unfortunately, when Cramer’s testing confirms my position on this, it will result in his approach FAILING and no report will be produced. And that means no formal report or formal confirmation of my positon.

I.E If I'm wrong it will be reported - If I'm right it will not!
A bit of a catch 22 for me.

[AndreasP]

Zeilinger said (click to see reference) (http://www.hep.yorku.ca/menary/courses/phys2040/misc/foundations.pdf) this will not work and explains why, see Fig. 2 on page 290. Basically, the which-slit information is available in principle - and thus no meaningful interference pattern arises (i.e. so nothing changes at S1 based on what you do at S2. He also indicated that his paper is based on experiments that have been performed, so I guess he thinks there is no need to re-run the experiment. Probably would explain why everyone is not rushing to do it.


Thanks for pointing to this reference. My comments are included below.

Comment 1 (Dopfer's experiment):

In Fig 2. on the top of page 290 of Zeilinger's 1999 article that you reference above, it is definitely true that there cannot be an interference pattern because which-slit information is still available, by virtue of particle b/b' in the same figure.

However, Cramer's setup (see Comment 2 below) looks closer to Fig. 3 on the bottom of page 290, which is Dopfer's experiment.

In Dopfer's setup, the question of whether there is an interference pattern of photon 2 behind the double slit or not, depends on where the Heisenberg detector is placed to register photon 1. There are two cases (quoting from Zeilinger's article):

Case A: The Heisenberg detector is placed in the focal plane of the lens, i.e. at distance f from the Heisenberg lens. In that case registration of photon 1 (in the Heisenberg detector) projects the state of photon 2 (in the double slit) into a momentum eigenstate which cannot reveal any position information about slit passage. In other words, which-slit information is not available. Therefore, in coincidence with a registration of photon 1 in the focal plane, photon 2 exhibits an interference pattern.

Case B: The Heisenberg detector is placed in the imaging plane of the lens, i.e. at distance 2f from the Heisenberg lens. In that case registration of photon 1 (in the Heisenberg detector) projects the state of photon 2 (in the double slit) into a position eigenstate which can reveal position information about the path photon 2 takes through the slit assembly. In other words, which-slit information is available. Therefore, in coincidence with a registration of photon 1 in the focal plane, photon 2 cannot exhibit an interference pattern.

So, which-slit information does not have to be available, at least not in Dopfer's experiment, as described in Fig. 3 on page 290 of Zeilinger's article. It is of course present in Fig. 2 on the same page. But that is not Cramer's experiment.

Comment 2 (Cramer's experiment):

It seems to me that what Cramer is trying to do is to essentially take Dopfer's experiment and (a) increase the distance between the crystal and the double slit, in order to assure that photon 2 behind the double slit is always detected after photon 1, and (b) eliminate the coincidence logic. So it would still very much look like Fig. 3 on the bottom of page 290 in Zeilinger's article, just with a longer distance between the crystal and the double slit, and without the coincidence logic.

The main difference to the situation in Fig. 2 on page 290 in Zeilinger's article is of course that in Fig. 2 one of the entangled photons first goes through the double slit before something else happens to the twin photon, whereas in Cramer's experiment it is the opposite. There, one of the photons is detected in a Heisenberg detector first, then the other one goes through a double slit, possibly at a much later time.

Cramer's question now seems to be this: Would one still see an interference pattern in case A, and no interference pattern in case B?

For example, let's say we are in case A, i.e. the Heisenberg detector is placed in the focal plane of the lens. For simplicity, let's assume that photon 1 is registered in the Heisenberg detector at T1=1s and photon 2 in detector D2 behind the double slit at time T2=2s. Let's also assume that the experimenter always leaves the Heisenberg detector in the focal plane of the lens, i.e. he makes the same type of measurement for all photons that are sent through the apparatus.

Then the registration of photon 1 in the Heisenberg detector (located in the focal plane of the lens) at time T1=1s projects the state of photon 2 (which is still underway to the distant double slit) into a momentum eigenstate. From that moment on photon 2 cannot reveal any position information about slit passage anymore because the which-slit information was erased at that very moment T1=1s, once and forever.

Later, photon 2 is registered in detector D2 behind the double slit at time T2=2s.

According to Cramer, there should be an interference pattern on detector D2 if a large number of photons is sent through the apparatus, (presumably) because the state of photon 2 at time T2=2s is still in the momentum eigenstate that it was projected into back at time T1=1s (PS: I wonder whether this statement is actually true. Could photon 2 actually change its state between T=1s and T=2s and become more dispersed again? In any case, I think it will never be able to "re-acquire" the which-slit information that it lost at T1=1s. How would it?)

Finally, Cramer seems to believe that he doesn't need the coincidence logic, if one just uses a sufficiently large number of photons. However, in practice only a small fraction of all pairs emitted by the source is actually registered, as detectors just are not perfect. This is the detection loophole. So how can Cramer filter out the pairs that actually belong together? Perhaps he thinks this is just a practical issue. Of course, if both the source and the detectors were of very high quality (meaning that a large fraction of all pairs emitted by the source is actually registered) I can see why he thinks he doesn't need the coincidence logic. What for? There is no fringe/antifringe pair to be filtered through.

What is going on here?

Perhaps the following can shed some more light into this debate. Cramer's transmission protocal in essence seems to be this: For each bit of information to be transmitted:

(a) The sender (the one in possesson of the crystal and Heisenberg detector D1) decides what he wants to send. He does so by selecting the location of the Heisenberg detector, either in the focal plane or the imaging plane of the lens (representing "0" or "1" by convention).

(b) The sender then sends a large number of entangled photon pairs, say N=1000, through his side of the apparatus. For each photon pair emitted, "his" twin will immediately be detected by the Heisenberg detector, while the other one is still underway.

(c) The receiver (the one in possession of the double slit and detector D2) checks whether or not an interference pattern emerges at detector D2.

I think one key issue here simply is that the photon twin travelling to the receiver still needs a finite amount of time to reach the received, even though the registration of photon 1 at the sender's side instantaneoulsy projects the state of photon 2 into a momentum or position eigenstate.

Where is the catch?

Comment 3 (comment independent of Comment 2):

Another point on Dopfer's experiment I have always wondered about after heaving read her thesis:

In both cases of comment 1 (case A or case B), the distance between the Heisenberg lens and the double slit (i.e. the distance from the Heisenberg lens back to the crystal plus the distance from the crystal to the double slit) seems to be the same -- namely 2f (I am not sure whether I read Fig. 3 correctly, but that is what it looks like in the picture anyway).

In any case, it seems to me that in order for the Dopfer experiment to work, photon 1 must be registered (in the Heisenberg detector D1) before or at least at the same time as photon 2 (in detector D2 behind the double slit).

Otherwise, one would in essence be in the situation of Fig. 2 on the top of page 290 in Zeilinger's article: photon 2 first goes through the double slit, while photon 1 is still underway. In other words, at the time when photon 2 is detected one does not know yet what type of measurement of photon 1 (still underway to the Heisenberg detector) will be made in the future. This scenario therefore represent a kind of delayed choice situation (registration of photon 1 is delayed). In this case I agree with your oroginal comment that one would of course never see an interference pattern on detector D2.

One can only try to filter out the fringe/antifringe pair ex-post, but that would of course require the conincidence circuit (otherwise one could not find out which photons 2 belong to which photons 1).

One curious question here is: What if all photons 1 are detected with the Heisenberg detector in the focal plane of the lens (so always in case A, meaning no which-slit information is available). So there really isn't a pair of fringe/antifringe patterns, but just a single fringe pattern.

On the one hand one clearly cannot see an interference while photon 1 is still underway, so no interference pattern on detector D2. But on the other hand there cannot be an antifringe pattern because all photons 1 are measured with the Heisemberg detector in the focal plane of the lens.

Where is the catch? (it can't be just the detection loophole of course).