explaining DCQE - via coherence in layman terms


by San K
Tags: coherence, dcqe, explaining, layman, terms
SpectraCat
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#55
Jun12-11, 04:15 PM
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Quote Quote by unusualname View Post
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
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#56
Jun12-11, 04:21 PM
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Quote Quote by SpectraCat View Post
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
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#57
Jun12-11, 05:51 PM
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Quote Quote by unusualname View Post
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

Quote Quote by old post
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.

Quote Quote by unusualname
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
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#58
Jun12-11, 06:00 PM
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Quote Quote by Cthugha View Post

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
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#59
Jun13-11, 06:57 AM
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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
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#60
Jun17-11, 11:47 AM
P: 915
Quote Quote by Joncon View Post
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)?


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