explaining DCQE - via coherence in layman terms


by San K
Tags: coherence, dcqe, explaining, layman, terms
Drakkith
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#37
Jun11-11, 11:54 PM
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Let me ask this. If you don't assume any QM interpretations and simply look at the results, does this experiment have any use? Does it prove something?
Cthugha
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#38
Jun12-11, 07:23 AM
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Quote Quote by Drakkith View Post
Let me ask this. If you don't assume any QM interpretations and simply look at the results, does this experiment have any use? Does it prove something?
If you take the whole family of such experiments into account, then there are two main results/uses. These become clear in ghost imaging. Here you have the same setup, but use one large detector and one small detector which can be moved. If you now place some object in the beam pointing at the large detector and scan the small detector, you will find an image of the object in the coincidence counts although the detector behing the object is not position-sensitive at all.

The two main results of this experiment are as follows:
1) The image of the object provides superresolution. That means the resolution is not limited by the standard diffraction limit.
2) This superresolution is a result of the perfect anticorrelation of the entangled particles. Therefore the appearance of such superresolution can be interpreted as a criterion to identify entanglement which is somewhat easier to realize than Bell tests are.
unusualname
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#39
Jun12-11, 08:49 AM
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Well if everybody's suddenly happy with non-locality then of course the DCQE has no mystery. DUH!

Of course, you'll have a hard time convincing some people that the s-photons can correlate (via p-photon pairs) non-locally with a distant eraser, and there's nothing in the definition of QM which insists it has this kind of non-locality.

The WHOLE point I have been trying to explain is that you cannot simply say "this is how it works", there is no proof of "how it works". QM has a seemingly bizarre non-locality and/or non-separability (and/or non-realism) and there is no intuitive explanation of what is "happening" in a QM experiment.

Just saying that non-local sub-samples "explain" it, explains nothing in fact.

What you all mean, is that it is a mystery how it works. Like I have been explaininbg for the past year.
SpectraCat
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#40
Jun12-11, 09:18 AM
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The ONLY "mystery" is WHY (and perhaps the internal details of HOW) entangled photons behave the way they do, i.e. it is purely an issue of interpretation.

The non-locality is included in the subset based explanation by requiring the well-defined phase relationship between the entangled photons. There is NO classical explanation for that ... it's pure QM.
unusualname
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Jun12-11, 09:28 AM
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Quote Quote by SpectraCat View Post
The ONLY "mystery" is WHY (and perhaps the internal. Details of HOW) entangled photons behave the way they do, I.e. it is purely an issue of interpretation.
EXACTLY! There is (currently) no "explanation" of the DCQE by appealing to phases or whatever just like there is no "explanation" of the double slit experiment.

There are only interpretations.

If San K wants to interpret using the idea of nonlocal subsamples, then fine, that's a possibility. In fact I have often emphasised how simple the DCQE is to explain via the Bohmian non-local pilot wave. Now, I don't believe Bohmian mechanics is the correct interpretation, but I do believe wave functions are non-local. But what I believe is not an "explanation", it is one of many "interpretations".

So when people sometimes suggest the DCQE seems to show retrocausality, I can point out that it doesn't if you adopt an interpretation of QM (apart from Transactional Interpretation), but you certainly can't "explain" it by arguing about phase relationships at the detectors.
Cthugha
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Jun12-11, 09:58 AM
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Quote Quote by unusualname View Post
EXACTLY! There is (currently) no "explanation" of the DCQE by appealing to phases or whatever just like there is no "explanation" of the double slit experiment.

[...]

So when people sometimes suggest the DCQE seems to show retrocausality, I can point out that it doesn't if you adopt an interpretation of QM (apart from Transactional Interpretation), but you certainly can't "explain" it by arguing about phase relationships at the detectors.
This gets annoying as this is the point I tried to get across several times. The way how you get the information across from one photon to the other is subject to interpretation. You can have some non-local change, you can assume non-realism and say that both observables jump to some value upon the measurement of the first, you can assume superdeterminism, the flying spaghetti monster or whatever. From that point on, however, the remaining physics which is not subject to interpretation, is the same for any reasonable interpretation that survives Occam's razor. And from that point on there is indeed no way around taking the phase relationships and coherence into account for interpretations surviving Occam's razor.
As I said before, DCQE is as mysterious as entanglement is, but it does not add additional weirdness.

Also, "there is no explanation of the double slit experiment" is quite a weird statement. The explanation is quite clear in a wave picture. The interpretation of what this wave means is of course not unambiguous and unclear, but the explanation is clear.
unusualname
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#43
Jun12-11, 11:05 AM
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Quote Quote by Cthugha View Post
Also, "there is no explanation of the double slit experiment" is quite a weird statement. The explanation is quite clear in a wave picture. The interpretation of what this wave means is of course not unambiguous and unclear, but the explanation is clear.
I said "(currently)". If the QM double slit experiment (ie for single particles) could be "explained", there would be no mystery or need for interpretations in QM.
SpectraCat
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Jun12-11, 12:55 PM
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[QUOTE=unusualname;3352678]EXACTLY! There is (currently) no "explanation" of the DCQE by appealing to phases or whatever just like there is no "explanation" of the double slit experiment.

There are only interpretations.
[/QUOTE ]

Wrong again ... Cthugha has offered an INTERPRETATION -INDEPENDENT EXPLANATION of the DCQE. All valid interpretations of QM agree on the experimental phenomenon that entangled photons have a well-defined phase relationship. Cthugha takes that as a given, and is able to reproduce the experimentally observed results. That is a scientific explanation ... your objections are dogmatic and vague, and are thus UNscientific.

If San K wants to interpret using the idea of nonlocal subsamples, then fine, that's a possibility. In fact I have often emphasised how simple the DCQE is to explain via the Bohmian non-local pilot wave.[/QUOTE ]

Really? Can you provide some details of that simple explanation? A link would be fine.

[QUOTE ] Now, I don't believe Bohmian mechanics is the correct interpretation, but I do believe wave functions are non-local. But what I believe is not an "explanation", it is one of many "interpretations".

So when people sometimes suggest the DCQE seems to show retrocausality, I can point out that it doesn't if you adopt an interpretation of QM (apart from Transactional Interpretation), but you certainly can't "explain" it by arguing about phase relationships at the detectors.
The DCQE does not show retrocausality in any interpretation ... the apparent retrocausal effect of the eraser results from a misunderstanding of two-photon coincidence measurements.
Cthugha
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Jun12-11, 01:20 PM
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Quote Quote by unusualname View Post
I said "(currently)". If the QM double slit experiment (ie for single particles) could be "explained", there would be no mystery or need for interpretations in QM.
I see your point, but I disagree. The "job" of physics is model building. If I manage to find a consistent mathematical picture which explains observations in experiments and predicts all observables and experimental outcomes, a topic is understood from the viewpoint of physics. I see the point that this stage of understanding might be dissatisfying for some. However, if you have two models leading to exactly the same predictions which are therefore not distinguishable by experiments, discussion about them is not within the realm of physics. It is philosophy or ontology. I do not say that these questions are uninteresting or unimportant, but they are not questions about physics. That is the reason I made a clear distinction between what can be explained in terms of physics and does not depend on the interpretation and what is indeed a question of interpretation.

The physics of the double slit is well understood. The ontology is not.
unusualname
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#46
Jun12-11, 01:23 PM
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[QUOTE=SpectraCat;3352949]
Quote Quote by unusualname View Post
EXACTLY! There is (currently) no "explanation" of the DCQE by appealing to phases or whatever just like there is no "explanation" of the double slit experiment.

There are only interpretations.
[/QUOTE ]

Wrong again ... Cthugha has offered an INTERPRETATION -INDEPENDENT EXPLANATION of the DCQE. All valid interpretations of QM agree on the experimental phenomenon that entangled photons have a well-defined phase relationship. Cthugha takes that as a given, and is able to reproduce the experimentally observed results. That is a scientific explanation ... your objections are dogmatic and vague, and are thus UNscientific.
You are clearly not aware of my debates with Cthugha. His initial stance was that the DCQE has a purely trivial explanation if you analyse the phases at the detectors, he even claimed that coincidence counters would ensure only photon pairs with the correct phases are matched or something similarly weird.

The point of DCQE is not to produce interference patterns, it is to demonstrate delayed choice and eraser. The interference patterns are only roughly required to be observered to indicate that delayed choice and eraser are working exactly as QM predicts.

Cthuga's analysis misses this point completely, and explains nothing relevant, and worse, misleads people that the DCQE is a trivial experiment.

In a classical optics experiments with coherent beams, like you might analyse at school, the phase analysis would be relevant, since the position and distribution of the pattern is something actually relevant and interesting in the experiment, and can be explained by a phase analysis.

In this experiment it is the delayed choice and erasure effects that are the relevant points to understand and explain, not the exact shape of the inteference patterns. Which is why the detectors are shuffled back and forth quite roughly.


The DCQE does not show retrocausality in any interpretation ... the apparent retrocausal effect of the eraser results from a misunderstanding of two-photon coincidence measurements.
No, it results from an attempt to understand or explain the experiment classically and not adopt a QM interpretation, and btw in the Transactional Interpretation, retrocausality is built in.
Cthugha
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#47
Jun12-11, 02:06 PM
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Quote Quote by unusualname View Post
You are clearly not aware of my debates with Cthugha. His initial stance was that the DCQE has a purely trivial explanation if you analyse the phases at the detectors, he even claimed that coincidence counters would ensure only photon pairs with the correct phases are matched or something similarly weird.
Stop misrepresenting my posts. It seems like you clearly did not bother to read or even understand them. That phase matching idea was a straw man you constructed. I never posted anything like that. If you want to talk about my posts, please quote them exactly in the future, like it is good scientific practice. I am really annoyed by seeing you claim again and again that I made some statements which I never made.

The fact that you repeatedly bring up this claim although I already clarified several times that I did not post such a statement makes it look like you are misrepresenting my posts on purpose.

So could you please just cite the passage you mean here and I will explain what I really said. But do not put words in my mouth.
unusualname
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#48
Jun12-11, 02:34 PM
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Quote Quote by Cthugha View Post
Stop misrepresenting my posts. It seems like you clearly did not bother to read or even understand them. That phase matching idea was a straw man you constructed. I never posted anything like that. If you want to talk about my posts, please quote them exactly in the future, like it is good scientific practice. I am really annoyed by seeing you claim again and again that I made some statements which I never made.
I've not time to dredge through all that but here's the basics.

Your original phase analysis for the Kim et al experiment:
http://www.physicsforums.com/showpos...60&postcount=8


See, no invocation of QM to explain it, the novel features of delayed choice and erasure are obscured.

But Walborn has a much simpler setup, now can this also be explained by a simple phase analysis, recalling that the detectors are shifted around roughly?

http://www.physicsforums.com/showpos...2&postcount=30

...The main point of my argument is that the spatial coherence of the subset chosen by coincidence counting is significantly higher than the spatial coherence of the whole set of sdetected photons in one arm without doing coincidence counting and that it is this increase which causes the interference pattern to appear. This is, however, much easier to see in the Kim et al. quantum eraser paper....
er, really? The point is that the coincidences match s-photons with p-photons that had which way info erased.
San K
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#49
Jun12-11, 02:40 PM
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[QUOTE=SpectraCat;3352949]
Quote Quote by unusualname View Post
EXACTLY! There is (currently) no "explanation" of the DCQE by appealing to phases or whatever just like there is no "explanation" of the double slit experiment.

There are only interpretations.
[/QUOTE ]

Wrong again ... Cthugha has offered an INTERPRETATION -INDEPENDENT EXPLANATION of the DCQE. All valid interpretations of QM agree on the experimental phenomenon that entangled photons have a well-defined phase relationship. Cthugha takes that as a given, and is able to reproduce the experimentally observed results. That is a scientific explanation ... your objections are dogmatic and vague, and are thus UNscientific.



The DCQE does not show retrocausality in any interpretation ... the apparent retrocausal effect of the eraser results from a misunderstanding of two-photon coincidence measurements.

The explanation (involving phases and sub-samples) is more than a decade old and offered by Kim, Kulik, Shih, Walborn etc and physicists/scientists before them.

Cthuga/SpectraCat are re/para-phrasing/supporting (the already understood/established understanding among QM physicists) the explanation on this forum.

This has a value-add too because it saves time/efforts of forum members.
unusualname
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#50
Jun12-11, 02:48 PM
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[QUOTE=San K;3353105]
Quote Quote by SpectraCat View Post


The explanation (involving phases and sub-samples) is more than a decade old and offered by Kim, Kulik, Shih and physicists/scientists before them.

Cthuga/SpectraCat are re/para-phrasing/supporting (the already understood/established understanding among QM physicists) the explanation on this forum.

This has a value-add too because it saves time/efforts of forum members.
Yes, if you assume non-locality is trivial, then the explanation is trivial. Well done for not getting the point for the umpteenth time.
San K
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#51
Jun12-11, 02:58 PM
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[QUOTE=unusualname;3353117]
Quote Quote by San K View Post

Yes, if you assume non-locality is trivial, then the explanation is trivial. Well done for not getting the point for the umpteenth time.
we are all assuming its non-local from the beginning itself, no one ever said its local. Unless you get into bohemian, and no one mentioned that.

Well done/bravo/hurrah/hooray/three-cheers for not getting the point for the (umpteenth + million)^million time.

entanglement, by definition/understanding, is non-local. DCQE starts with entanglement.
unusualname
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#52
Jun12-11, 03:09 PM
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[QUOTE=San K;3353133]
Quote Quote by unusualname View Post

dude, we are all assuming its non-local from the beginning itself, no one ever said its local.

Well done for not getting the point for the umpteenth + million time.

entanglement, by definition/understanding, is non-local. DCQE starts with entanglement.
haha. You realise that there are quite a few people who don't think non-locality is required to explain nature.

To be honest, maybe this is the point I've missed. Cthugha, San K, SpectraCat all assume non-locality so naturally that it doesn't occur that it might be a point to mention when distinguishing between classical explanations and QM explanations.
Cthugha
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#53
Jun12-11, 03:22 PM
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Quote Quote by unusualname View Post
I've not time to dredge through all that but here's the basics.

Your original phase analysis for the Kim et al experiment:
http://www.physicsforums.com/showpos...60&postcount=8


See, no invocation of QM to explain it, the novel features of delayed choice and erasure are obscured.
Obscured? I explained the origin of the patterns in a simplified way just as I said in that post. I see no obscuring stuff in that post. The OP also already knew that entanglement exists as can be seen from his first post. As I said later in that thread I tried to clarify why the choice can be made afterwards, but does not change the detections made before. Not more, not less.

Quote Quote by unusualname View Post
But Walborn has a much simpler setup, now can this also be explained by a simple phase analysis, recalling that the detectors are shifted around roughly?
Yes. The Walborn experiment is even simpler to explain. See section 6 in Walborn's review paper "Spatial correlations in parametric down-conversion" where conditional interference patterns are explained in more detail than can be given on these forums. Equation 96 gives all the relevant phase differences. I linked it earlier here, so I do not give the full citation again. Let me know if you cannot find it.

Quote Quote by unusualname View Post
er, really? The point is that the coincidences match s-photons with p-photons that had which way info erased.
I do not get your point. What do you think spatial coherence is about? High spatial coherence means that momentum is well defined which corresponds to having which-way info erased and also means high visibility in double slit experiments.
unusualname
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#54
Jun12-11, 03:33 PM
P: 661
Ok Cthugha, it seems I was wrong in thinking that you were promoting a classical explanation of the DCQE, so I should stop.

I still don't get the relevance of the detailed phase analysis to what is a remarkable experimental illustration of the non-classicality of QM, but maybe I jumped to the wrong conclusions.


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