Explaining DCQE - via coherence in layman terms

[San K]

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]

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 realize 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]

I've not time to dredge through all that but here's the basics.

Your original phase analysis for the Kim et al experiment:
https://www.physicsforums.com/showpost.php?p=2241460&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.

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.

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]

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.

 

[SpectraCat]

Ok Cthugha, it seems I was wrong in thinking that you were promoting a classical explanation of the DCQE

Thank you.

, so I should stop.

Don't stop questioning if you don't understand, we never objected to that. What we objected to were the blanket "refutations" of our detailed posts which ignored the specific points we were trying to explain why Cthugha's analysis was actually correct, and why you were mis-characterizing it in your replies.

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.

If you go back to my first posts in the other recent DCQE thread, you can see that I was referencing Cthugha's analysis because it shows clearly why there is no big mystery (other than entanglement itself) associated with the observed phenomena in those experiments. Personally, I think that is already clear from the fact that the "erasure" is only observable by using coincidence counting, so you never get any information about whether or not the polarizer had any effect until BOTH photons have been detected. Therefore, it doesn't matter if the p-photons are detected before or after the s-photons .. the results only depend on the full context of the experiment. Cthugha's analysis (well, to be correct, his rephrasing of Walborn's analysis) goes farther, and explains precisely why the observed results are obtained, using only the assumption that the entangled photons have a well-defined phase relationship.

 

[unusualname]

If you go back to my first posts in the other recent DCQE thread, you can see that I was referencing Cthugha's analysis because it shows clearly why there is no big mystery (other than entanglement itself) associated with the observed phenomena in those experiments. Personally, I think that is already clear from the fact that the "erasure" is only observable by using coincidence counting, so you never get any information about whether or not the polarizer had any effect until BOTH photons have been detected. Therefore, it doesn't matter if the p-photons are detected before or after the s-photons .. the results only depend on the full context of the experiment. Cthugha's analysis (well, to be correct, his rephrasing of Walborn's analysis) goes farther, and explains precisely why the observed results are obtained, using only the assumption that the entangled photons have a well-defined phase relationship.

Yes, by assuming non-locality and/or non-separability, the exact things which the experiment attempts to demonstrate. Nothing else is remotely interesting in these experiments.

 

[Cthugha]

Ok Cthugha, it seems I was wrong in thinking that you were promoting a classical explanation of the DCQE, so I should stop.

Thanks. Just for the records: I reread my old post you linked. The important things are the assumptions

a) In entangled photon experiments each photon on its own behaves like incoherent light. [...]

b) The two-photon state has a well defined phase. This means that the fields of both paths (signal and idler), which originate from the same point (A or B) have a fixed phase relationship.

because a well defined phase of a two-photon state without having also coherent one-photon state is to the best of my knowledge a trademark of a non-classical state. I did not broadcast this explicitly in that post, but thought that it was clear enough for the audience of that topic. It sure cannot hurt to spell out explicitly that this is a non-classical state, but it just seemed unnecessary.

Yes, by assuming non-locality and/or non-separability, the exact things which the experiment attempts to demonstrate. Nothing else is remotely interesting in these experiments.

One can argue about that. The first quantum eraser experiments were performed to find out whether uncertainty or complementarity is more essential. Other groups used similar setups to demonstrate how to beat the diffraction limit and how to get super-imaging using entangled particles and so on and so forth. There is lots of physics in these experiments. If you ask 10 people working in that field, you will most probably get 10 different opinions about what is interesting in these experiments.

 

[unusualname]

One can argue about that. The first quantum eraser experiments were performed to find out whether uncertainty or complementarity is more essential. Other groups used similar setups to demonstrate how to beat the diffraction limit and how to get super-imaging using entangled particles and so on and so forth. There is lots of physics in these experiments. If you ask 10 people working in that field, you will most probably get 10 different opinions about what is interesting in these experiments.

That's true. Cthugha, as we say in england, I would love to have a beer or two with you (and discuss things). I feel bad that I've been so aggressive, you seem like a really nice guy, and you are definitely not the "enemy" as I stupidly thought.

 

[Cthugha]

No, problem. I am glad to see that we are fine now.

Yes, a discussion with some beer wpuld always make things easier. However, I must admit that I am not too familiar with english beer. Last year I attended a conference in Nottingham and I think we ended up drinking Abbot Ale or something like that and it was not bad. Anyway, I am going offtopic. Good to see that the problem seems to be solved.

 

[San K]

I'm very much a layman but this is the way I see it (apologies if this is wrong, but I'm sure someone will correct me): -
You can't get which way info simply by looking at the p-photon. You can only get which path information by comparing the polarization of the p-photon with the polarization of the s-photon.
e.g.
if p-photon is vertical and s-photon is rotated left then s-photon went through slit 1
if p-photon is vertical and s-photon is rotated right then s-photon went through slit 2
if p-photon is horizontal and s-photon is rotated right then s-photon went through slit 1
if p-photon is horizontal and s-photon is rotated left then s-photon went through slit 2

So, without QWPs in place I would expect there to be an interference pattern, as we don't have which way information.

ok...we repeat the above experiment, but instead of polarizer we place double slit (with QWPs) in path of the -photon...

what happens to the pattern now (after pairing via co-incidence counter)?