Explaining DCQE - via coherence in layman terms

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Discussion Overview

The discussion revolves around the concept of Delayed Choice Quantum Eraser (DCQE) and the role of coherence and interference patterns in quantum mechanics. Participants explore the implications of measurements on photon behavior, particularly in relation to interference patterns and the effects of polarizers.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant suggests that interference is caused by coherent waves and that attempting to determine which-way information breaks this coherence, leading to a partial interference pattern.
  • Another participant proposes that interference patterns arise from individual photons interfering with themselves, and that polarizers can alter these patterns by filtering certain polarization states.
  • A participant questions how a sub-sample of an interference pattern can appear as a non-interference pattern, expressing confusion about the filling-in of blank spaces between fringes.
  • One reply clarifies that when a polarizer is added to the p-photon path, only photons passing through are counted, which affects the observed interference pattern in the coincidence measurements.
  • Another participant discusses the Walborn version of the DCQE, explaining that distinguishable photon paths due to polarization lead to no interference in single-photon measurements, and that interference patterns are only observed under specific conditions with polarizers.
  • Further clarification is provided regarding the superposition of interference patterns and how they can cancel each other out, leading to the observed results in the absence of a polarizer.
  • A participant expresses ongoing confusion about the relationship between the s-photons and p-photons in terms of pairing and the resulting patterns, indicating a need for further understanding.

Areas of Agreement / Disagreement

Participants express a range of views on the mechanisms behind interference patterns and the effects of measurements. There is no consensus on the explanations provided, and several points remain contested or unclear.

Contextual Notes

Participants reference specific experimental setups and theoretical models, such as the Walborn experiment, but there are limitations in the clarity of how these relate to the observed phenomena. Some assumptions about photon behavior and measurement outcomes are not fully resolved.

  • #31
Joncon said:
Fair enough, I accept entanglement. But that suggests to me that when the p-photon meets the final BS, the chances of it passing through or reflecting are not 50/50, but are influenced somehow by where the s-photon landed.

yes, when s-photon landed, the entanglement was broken since s-photon position has been fixed/locked...and the p-photon also becomes probabilistically determinable...that’s how I think...some on the forum agree, some disagree and some have another explanation, etc
 
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  • #32
San K said:
yes, when s-photon landed, the entanglement was broken since s-photon position has been fixed/locked...and the p-photon also becomes probabilistically determinable...that’s how I think...some on the forum agree, some disagree and some have another explanation, etc

Well what happens if I just (by mechanical means) put the eraser in place (by microseconds) after EACH s-photon is detected, or if I remove the eraser (by microseconds) before the entangled p-photon can reach it EACH time.

Or I put the eraser so remotely that all s-photons are measured before a single p-photon can reach the eraser, and then I put the eraser in place just before they reach it?

Will the (timing offset if necessary) coincidence counts show interference?

You see, not so simple is it?
 
  • #33
unusualname said:
Well what happens if I just (by mechanical means) put the eraser in place (by microseconds) after EACH s-photon is detected, or if I remove the eraser (by microseconds) before the entangled p-photon can reach it EACH time.

Or I put the eraser so remotely that all s-photons are measured before a single p-photon can reach the eraser, and then I put the eraser in place just before they reach it?

Will the (timing offset if necessary) coincidence counts show interference?

You see, not so simple is it?

the eraser is simply a filter/sieve, so is the co-incidence counter and all of the above can be explained by this/that
 
  • #34
San K said:
the eraser is simply a filter/sieve, so is the co-incidence counter and all of the above can be explained by this/that

NO IT CAN'T!


Until photons are MEASURED/DETECTECTED they have UNKNOWN quantum properties (like polarisation). This is a well establsished experimental fact, it is highly nonintuitive and unsettling but it is the way the world is.

Sorry San K, but this experiment is kinda old school compared to what's been shown with multi-entangled states recently. Reality just ain't really real the way you think. You can allow a non-local interpretation to retain some idea of reality, which is what I thought you were suggesting.
 
  • #35
unusualname said:
NO IT CAN'T!

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?

unusualname said:
Until photons are MEASURED/DETECTECTED they have UNKNOWN quantum properties (like polarisation). This is a well establsished experimental fact, it is highly nonintuitive and unsettling but it is the way the world is.

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.
 
  • #36
unusualname said:
NO IT CAN'T!


Until photons are MEASURED/DETECTECTED they have UNKNOWN quantum properties (like polarisation). This is a well establsished experimental fact, it is highly nonintuitive and unsettling but it is the way the world is.

Sorry San K, but this experiment is kinda old school compared to what's been shown with multi-entangled states recently. Reality just ain't really real the way you think. You can allow a non-local interpretation to retain some idea of reality, which is what I thought you were suggesting.

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.
 
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  • #37
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?
 
  • #38
Drakkith said:
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.
 
  • #39
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.
 
  • #40
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.
 
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  • #41
SpectraCat said:
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.
 
  • #42
unusualname said:
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.
 
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  • #43
Cthugha said:
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.
 
  • #44
unusualname said:
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.
 
  • #45
unusualname said:
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.
 
  • #46
SpectraCat said:
unusualname said:
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.
 
  • #47
unusualname said:
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.
 
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  • #48
Cthugha said:
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:
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.

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?

https://www.physicsforums.com/showpost.php?p=2794532&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.
 
  • #49
SpectraCat said:
unusualname said:
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.
 
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  • #50
San K said:
SpectraCat said:
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.
 
  • #51
unusualname said:
San K said:
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.
 
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  • #52
San K said:
unusualname said:
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.
 
  • #53
unusualname said:
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.

unusualname said:
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.

unusualname said:
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.
 
  • #54
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.
 
  • #55
unusualname said:
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.
 
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  • #56
SpectraCat said:
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.
 
  • #57
unusualname said:
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

old post said:
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.

unusualname said:
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.
 
  • #58
Cthugha said:
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.
 
  • #59
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.
 
  • #60
Joncon said:
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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