explaining DCQE - via coherence in layman terms

Cthugha

Sure it can. It is of course necessarily a non-local filter, as you would need to get the information about which state photon B has to go to once A is detected across instantly, but definitely sufficient to explain all results. For the first experiments on DCQE (before 2004) the results were, however, not tested for violations of Bell or GHZ inequalities and could therefore indeed have been explained completely classically in terms of a common history in the SPDC process. Or in other words: hidden variables have not been ruled out back in the early days of DCQE. These tests started in 2004. By the way I still do not get your resistance against the subsampling picture. You do realize that also the subsampling explanation requires a non-classical description and non-locality, do you?

I do not care much about interpretations, but a lot of people caring about them on these forums will tend to disagree with that statement. It might be that nature is non-realistic and there are no properties before measurement. However, at current it is not experimentally possible to distinguish between non-realistic and non-local descriptions of qm as both can lead to the same results. It is a well established experimental fact that Bell and other inequalities are violated for entangled photons. Non-realism is one possible and appealing conclusion, but not a fact that can be tested experimentally at current.

San K

Sub-sampling can explain the results of DCQE, sub-sampling does include non-local assumptions/interpretation.

There is no need to assume future can change the past or causality is violated by QM/DCQE or other hypothesis.

Scientists/experimenters, such as Yoon, do discuss sub-sampling (and the associated patterns) in their papers to explain appearance and disappearance of interference patterns.

Drakkith

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

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

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

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

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

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

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

[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.

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

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

[QUOTE=SpectraCat;3352949]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.


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

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

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

er, really? The point is that the coincidences match s-photons with p-photons that had which way info erased.

San K

[QUOTE=SpectraCat;3352949]
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

[QUOTE=San K;3353105]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

[QUOTE=unusualname;3353117]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.