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Physics Start‑class Mid‑importance

This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Misplaced Pages. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.PhysicsWikipedia:WikiProject PhysicsTemplate:WikiProject Physicsphysics Start This article has been rated as Start-class on Misplaced Pages's content assessment scale. Mid This article has been rated as Mid-importance on the project's importance scale.

Please enter all new discussion at the bottom. Nobody who actively follows this discussion is likely to notice it if it goes up here with the much earlier stuff. Thanks.

Modified diagram

I've modified the diagram according to criticism received on my talk page on Commons. The article mis-identifies "Glan-Thompson" as "Glen-Thompson," an easy mistake to make I guess. There was also an extra prism not pictured in the article's drawing (which is also missing several other features), so I had mis-labeled the prism that they did draw in. P0M (talk) 17:35, 7 November 2010 (UTC)

Removal of discussion section for alleged copyright violation

Somebody cut out this section, saying it was a copyright violation. I have restored it. The Discussion section has existed for nearly all the existence of this article. It has been brought into its present shape edit by edit. Anybody claiming a copyright violation must be able to show specifically what is wrong.P0M (talk) 03:33, 7 March 2011 (UTC)

Hi, that was me. No harm was intended. I removed the first two paragraphs of the "Discussion" section because they seem to have been substantially copied from the referenced paper with a little bit of rewording.

Here is the removed text, which was added in 2006 (emphasis mine):

In their paper, Kim, et al. explain that the concept of complementarity is one of the most basic principles of quantum mechanics. According to the Heisenberg Uncertainty Principle, it is not possible to measure both precise position and momentum of a quantum particle at the same time. In other words, position and momentum are complementary. In 1927, Niels Bohr maintained that quantum particles have both "wave-like" behavior and "particle-like" behavior, but can exhibit one kind of behavior only under conditions that prevent exhibiting the complementary characteristics. This complementarity has come to be known as the wave-particle duality of quantum mechanics. Richard Feynman believed that the presence of these two aspects under conditions that prevent their simultaneous manifestation is the basic mystery of quantum mechanics.

The actual mechanisms that enforce complementarity vary from one experimental situation to another. In the double-slit experiment, the common wisdom is that the Heisenberg Uncertainty Principle makes it impossible to determine which slit the photon passes through without at the same time disturbing it enough to destroy the interference pattern. However, in 1982, Scully and Drühl found a way around the position-momentum uncertainty obstacle and proposed a quantum eraser to obtain which-path or particle-like information without introducing large uncontrolled phase factors to disturb the interference.

And from the paper "A Delayed Choice Quantum Eraser published in 1999:

Complementarity, perhaps the most basic principle of quantum mechanics, distinguishes the world of quantum phenomena from the realm of classical physics. Quantum mechanically, one can never expect to measure both precise position and momentum of a quantum at the same time. It is prohibited. We say that the quantum observables “position” and “momentum” are “complementary” because the precise knowledge of the position (momentum) implies that all possible outcomes of measuring the momentum (position) are equally probable. In 1927, Niels Bohr illustrated complementarity with “wave-like” and “particle-like” attributes of a quantum mechanical object. Since then, complementarity is often superficially identified with “wave-particle duality of matter”. Over the years the two-slit interference experiment has been emphasized as a good example of the enforcement of complementarity. Feynman, discussing the two-slit experiment, noted that this wave-particle dual behavior contains the basic mystery of quantum mechanics . The actual mechanisms that enforce complementarity vary from one experimental situation to another. In the two-slit experiment, the common “wisdom” is that the position-momentum uncertainty relation δxδp ≥ ¯h2 makes it impossible to determine which slit the photon (or electron) passes through without at the same time disturbing the photon (or electron) enough to destroy the interference pattern. However, it has been proven that under certain circumstances this common interpretation may not be true. In 1982, Scully and Drühl found a way around this position-momentum uncertainty obstacle and proposed a quantum eraser to obtain which-path or particle-like information without scattering otherwise introducing large uncontrolled phase factors to disturb the interference.

Clearly this is the original source of those two paragraphs; several sentences or phrases are copied verbatim, and others are only slightly paraphrased. I couldn't find any Misplaced Pages policy on what exactly is considered a "derivative work" but my guess is that this qualifies. 72.177.91.150 (talk) 22:06, 7 March 2011 (UTC)

Good catch, and here I was feeling great admiration for the level of acumen of whatever Wikipedian had written the second of those two copied sentences.

Let's get some other opinion on how far a recapitulation of somebody else's work has to depart from the original to avoid breaking their copyright. Sometimes if I run something entirely through my own mind and write without looking at the original I get challenged regarding whether the original paper or book really said/meant what I wrote that it did, so I can see some justification for somebody's sticking pretty close to the original in this case. As people who only try to write encyclopedia articles, many of us are not deeply enough into a subject to boldly recast a technical explanation into a new form for fear of getting it messed up. (I just had to go back and fix the discussion pertinent to the second diagram above because in reading the article I somehow skipped over the Glan-Thompson prism. I got the diagram wrong, and that caused me to get the discussion of how the apparatus works wrong. Somebody with greater depth would not have made that mistake.) And, to be fair to whoever produced these two paragraphs, most of the first paragraph and one sentence in the second paragraph are such widely accepted ideas that Scully and Drühl can hardly be said to have been entirely original in writing them. It's not quite as cut and dried as asking how many ways one can say, "Paris is the capital of France." On the other hand, Scully and Drühl have an excellent technique for technical writing (just basing my judgment on this one paraphrased and copied sample).

What we really need is for somebody with real depth in the subject to abstract the content of these two paragraphs, reorganize the bare bones of it, and then rewrite without looking at how the original authors put it.

For the present I've put the two quoted sentences in quotation marks as surely ought to have been done in the first place. That way my guess is that Misplaced Pages is at least legally sort of covered -- but I'm not a lawyer, so who knows. P0M (talk) 04:13, 8 March 2011 (UTC)

Sounds fine to me. I'm certainly not knowledgeable enough about this subject to be confident about rewriting the section from scratch. (I was just using the article to find the arXiv paper out of my own curiosity, and happened to notice that the introduction looked awfully familiar. This details are mostly way over my head!) 72.177.91.150 (talk) 06:21, 8 March 2011 (UTC)

Questions on slightly changed setup

Based on the diagram for the experiment in the main article:

1) I understand the overall pattern at D0 is no interference.

If there were only that branch of the experimental apparatus, then there would be an interference pattern because a photon's wavefunction has gone through both slits, and the two wavefunctions overlap on the detection screen just as in a vanilla double-slit experiment. The question is, however, what will happen if the entangled twin of this photon has something done to it that will result in a physical change (absorption by one of the other detection screens), and in that process "which path" information is gained.

Let me clarify the paragraph above. Suppose that you took out the BBO, the crystal that is responsible for connverting single photons into entangled twin photons. You could leave the Glan-Thompson prism out too, since it is there basically to help separate pairs of entangled photons into more widely diverging paths, and now we don't have any entangled photons to worry about. So you would just have the laser (the blue part), the double slit (the black part), and then for D₀ you might as well just use a standard detection screen. There is no longer any need for measuring very faint signals or for seeing what signals coordinate with what other signals. All those complications have gone away because you've reduced the complicated apparatus to a very basic Young experiment apparatus.

Next, let's rebuild a part of the more complicated apparatus. We put the BBO back in, so we get entangled photons, and we put the Glan-Thompson prism back in and also the PS prism so that we can more easily make certain which stream of entangled photons we are looking at. If we left all the rest of the apparatus in the bottom limb of the experimental apparatus back in storage, there would still be some question about what happens to the entangled photons. What happens if you put something in front of both exits from the PS prism that blots up the photons before there is any chance of their interfering with themselves? Since the photons in the lower limb (now amputated to a stump) could not interfere with themselves, it would seem that their twins in the upper limb could also not interfere with themselves. So in this case, it seems that virtually anything that would sop up the photons in the lower limb without their having the opportunity to interfere with themselves would make interference in the upper limb disappear.

The other thing we could do would be to put in a couple of mirrors that would direct the photons coming out of the PS prism out into interstellar space. Our telescopes pick up photons that have never been absorbed by anything from some point near the beginning of the universe until they hit our telescopes, so it is also possible that their status as self-interfering or not-self-interfering would remain indeterminate for a very long time. I suspect that most of them will eventually be absorbed without having interfered with themselves. The delayed choice experiments are said to exhibit retrocausality, which means, I think, that a spark of light showing up somewhere a gazillion miles from here and far in our future will be matched by what shows up here in our present. I am not sure that it would make anybody happier about this alleged state of affairs to assert that while the appearance of the two entangled photons may be separated by a long time interval according to someone in our frame of reference, the appearance of the photons will be simultaneous for someone in an appropriate frame of reference. Anyway, because most of the photons that travel a gazillion miles or years before being absorbed will not interfere with themselves, we can expect that any interference pattern that may appear in D₀ will be very badly washed out by the photons that are matched by absorbed twins. On top of that, there will probably be a lot of photons reaching D₀ that are not entangled, not desired to be there, and were the reason that the original version of the experiment required the Coincidence Counter to be sure that we look only at entangled photons and discard the rest.P0M (talk) 02:09, 24 July 2012 (UTC)

Note that the experimenters have inserted a lens into the path of the wavefunctions in the upper branch. The reason they did that was to shorten the path. Shortening the path will make a photon show up at D0 before its twin shows up at D1, D2, D3, or D4. One would think that what happens first cannot be determined by what happens afterwards, and one would think that what is farther away from the laser must happen later than what happens at D0, and therefore what happens at D0 ought to determine what happens "later" in the bottom branch.

The problem with that picture is that what happens in the top branch and what happens in the bottom branch is all one event, and a single event happens when it happens.

Notice the Coincidence Counter. If something strikes D0 at t and something strikes, e.g., D2 at T = t + n, then the Coincidence Counter ought not to click -- because the events do not coincide in time.

To me, this part is extremely spooky. P0M (talk) 02:48, 15 April 2011 (UTC)

Hi, the part in your response about seeing an interference pattern in the the top half of the experiment if the bottom half does not exist has me confused. I would have thought that once the particles have passed the BBO and become entangled, there would always just be a blob detectable. Wouldn't the only way to get an interference pattern (or even just the slit pattern) out of the entangled particles be to measure the idler and correlate that information with the blob information to get the subset that forms a pattern? Do you have a reference to an experiment where the experimenter was able to directly observe either an interference pattern or a clear slit-pattern on entangled particles (i.e. without correlating the idler data with blob data to get a subset that makes one or the other pattern)? --89.253.76.71 (talk) 02:35, 21 July 2012 (UTC)

http://www.scribd.com/doc/58640003/Walborn may be what you want. However, I don't see any indication that they have actually looked at whether an interference pattern is delivered to D_p, their upper-limb detector. P0M (talk) 15:10, 24 July 2012 (UTC)

The problem may have been that I was not very precise about what not having the bottom half exist would entail. The only way to really get rid of the bottom half is to get rid of the BBO. Once you have entangled photons and their twins going in different direction, you have two halves, top and bottom. If you get rid of the BBO, you will have nothing to prevent an interference pattern from forming in the upper limb. If you trap all the photons in some carbon nano-tangle trap so that they must get "detected" without having interfered with themselves, then you will destroy the interference pattern topside. Fixing things in the bottom limb of the experiment so that the photon twins must interfere with themselves would mean that the bottom limb becomes a functional duplicate of the top limb, and so interference will show up in both limbs.

If you put back the entirety of the original lower limb, then you get a problem that the experimental design introduces by the way phase changes work out. On one detector you would get a pattern of amplitudes something like:

10203030403030202 vs.

20203030403030201

But you could flip the phase relationships by redesigning the experimental apparatus and get something like

10203030403030202 and

20203030403030201

and the interference pattern should be clear. (The interference patterns are not perfectly symmetrical left for right, but that is just the way things are.) The interference effects are there. There is no cheat or fudge factor involved. It's just a question of what strategy to use to get the results fixed so that they do not obscure each other.P0M (talk) 02:09, 24 July 2012 (UTC)

2) What if I extended the path to D0, say to 1 light minute, and let the photons hit D1 to D4 first. I believe pattern at D0 is unchanged.

That's what they do experiments for. My guess is that you are right. If D0 goes into operation after D1, D2, D3, or D4 go into operation, and if what happens to D0 is determined by the event at the other detector, at least the time sequence is "cause before effect." So maybe there is less cognitive dissonance in this experimental apparatus.P0M (talk)

3) What if I then change the setup so photons reaching at D3 and D4 to be like D1 and D2. That is no which path info. I believe pattern at D0 is unchanged. But I don't know why; in this experimental setup there doesn't seem to be any leak of which-path-info.

If there is which path information revealed in the bottom branch, then I believe the experimental results are that the photon that hits D0 will always have just the characteristic of a diffraction pattern, not the characteristic of an interference pattern. (They would need to collect and make an image for all the "hits" to D0 matched to hits at D1, an image for all the hits to D0 matched to hits at D2, and so on to sort these results out.) But if they do something to D3 and D4 as you suggest, and no which path information were available, then each of the four image "slices" should, over many runs, form an interference pattern.P0M (talk)

Thanks P0M, for taking the time to answer my questions.

In the scenario I give (ie: get D3/D4 to be like D1/D2), is there "which path information revealed in the bottom branch"? I think you are saying there is not, which means you are saying there will be an overall interference pattern at D0?

(I understand you are saying using info at D1 to D4, I can see 4 interference pattern within D0. But that is not I am interested in. I want to know the overall pattern at D0; don't care what additional info on D1 to D4 I can use.)110.175.53.141 (talk) 02:30, 16 April 2011 (UTC)

Because D3 and D4 were originally places where a photon could only reach by one route (imagine going from a two-lane interstate to a one-lane interstate) then for anything that showed up at the end of that route had to have come by a single path. If somebody makes it so now the same photon can reach it by two different paths (so it is getting whatever goes through each side of the double-slit apparatus), there is no longer any which-path information. So all photons and entangled twins that go through the apparatus will interfere with themselves. They will therefore do what would ordinarily form a single, very clear, interference pattern at D0. However, you asked about "overall pattern," and the article shows how by messing around with mirrors and beam splitters in the bottom branch the experimenters have created a situation in which one interference pattern looks something like:

.|.|.| vs.
|.|.|.

So its a little like throwing two movies from two projectors on the same screen at the same time. The bright spots from one movie will fill in the dark spots from the other movie. When you next make your contribution to the situation by letting D3 and D4 photons contribute to interference patterns, I think you can only make the "overall pattern" more like a continuous band. Well, actually, with just D1 and D2 contributing you already were seeing an "overall pattern" that was basically a band of light, brighter in the middle and dimmer at the ends. So there is even less variation than you would get by aiming four movie projectors at the same screen. The movies are still there. But each tends to wash out the other.

It's really spooky, to me, to see that what happens in the lower branch is mimiced, somehow, in the upper branch. It even goes to displacing the interference patterns associated with the entangled twins that end up at different detectors in the lower branch. Why something that happens in the lower branch should affect what happens in the upper branch is not clear at first. However, it looks like it boils down to the fact that in the lower branch the experiment forces the photon to "make decisions" as it were. In other words, in the original version of the experiment not enough was done to determine where and how D1 and D2 photons showed up there to make them unable to interfere with themselves. So their entangled twins also showed up interfering with themselves. Nevertheless, taking either the paths to D1 or the paths to D2 did do things to the phase relationships involved. D1 photons and D2 photons were originally in phase, but going through the lower branch of the apparatus put them out of phase. So their entangled twins were also put out of phase. So looking at what we get at D0 can tell us whether the entangled twin went through a D1 path or a D2 path. That's something interesting, but it still does not tell us which side of the double-slit apparatus the photon went through.P0M (talk) 10:00, 16 April 2011 (UTC)

4a) What If I then drop a polarizer in front of D0, does that bring back the interference? I think not, but worth a try...

If there is no which-path information available for the bottom branch, then an interference pattern (or, actually, four interference patterns that may be out of phase with each other. I'm just guessing) will form. Putting a polarizer in the upper path will only have an effect (not passing a photon with the wrong polarity) if a significant number of photons in the lower branch get polarized. If my memory is correct, what passes through a beam splitter has one polarity, and what reflects off the first surface of a beam splitter has the opposite polarity. So how a photon travels through the lower branch may affect its polarity. Then some of the wavefunctions that travel in the upper branch might be expected to have one polarity, and some might have the opposite polarity. So if that guess is correct, then some wavefunctions should be unable to pass through the polarizer. The result would be that the interference pattern observed at D0 would be dimmer. But then the question would be whether any photon that went through the polarizer to D0 could be associated with a path in the lower branch because that branch had the correct polarity to let the wavefunctions pass through the polarizer in the upper apparatus. P0M (talk)

4b) Is there anything I can drop just before D0 to bring back the interference pattern?

It's already been established that removing which-path information from the lower branch would mean that the upper branch would always show results consistent with an interference pattern. If there were any way that a single polarizer could have an effect in the upper branch, I think it would be that some incoming photons would be vertical in polarity while others would be horizontal, and that a polarizer would block one or the other. There might be some way that a circular polarizer could be used before the rectangular polarizer so as to undo in advance the blockage of wavefunctions by the rectangular polarizer. I'm too sleep at this point to try to see how that might be done.

Really, the best thing is to understand the experiment in the diagram correctly. What it all comes down to is that when a photon is emitted by a laser and goes through a double-slit device, then half of its wavefunction goes by one path and half of it goes by another. If anything is done to prevent those two halves from landing on the same detection screen in proper registration, then interference cannot occur. If, however, something is done after the two halves have been created by the double slits and one of those two halves is deliberately "lost,"then one may endeavor to do something else to get the two parts back together again. This experiment uses these relations to explore the idea of time and causation. The rest of what we can learn from this experiment is how correct are our ideas about what to do to a photon (or a wavefunction) to create or to block interference.

P0M (talk) 02:48, 15 April 2011 (UTC)

WhoperJ12 (talk) 05:45, 13 April 2011 (UTC) some sort of answer

I hope it is at least approximately correct. ;-) P0M (talk) 10:00, 16 April 2011 (UTC)

FTL possibility in no-communication theorem

http://en.wikipedia.org/No-communication_theorem#Opposing_viewpoint

For example B. Dopfer, a graduate student of Anton Zeilinger, has indicated via experiment that it is possible to cause or prohibit an ensemble of photons into making an interference pattern on a screen, by remotely manipulating their entangled twins. Physicist John Cramer is currently attempting to replicate Zeilinger's experiment for the purpose of communication. (The first experiment, attributed to A. Zeilinger, was actually done by Zeilinger's graduate student B. Dopfer).

Of course Zeilinger and Dopfer's experiment does not prove superluminal communication, but neither does the no-communication prohibit all forms of communication. If superluminal communication is prohibited, it is not because of the no-communication theorem. Thus, the question of superluminal communication remains open.

A discrepancy here. --21:34, 28 April 2011 (UTC) —Preceding unsigned comment added by 146.96.35.52 (talk)

Is your point that a theoretical statement ("theorem") cannot trump a consistent set of empirical observations?P0M (talk) 01:49, 29 April 2011 (UTC)

No, my point is that this:

"The total pattern of signal photons at the primary detector never shows interference, so it is not possible to deduce what will happen to the idler photons by observing the signal photons alone, which would open up the possibility of gaining information faster-than-light (since one might deduce this information before there had been time for a message moving at the speed of light to travel from the idler detector to the signal photon detector) or even gaining information about the future (since as noted above, the signal photons may be detected at an earlier time than the idlers), both of which would qualify as violations of causality in physics."

suggests that there is NO way to make this a way of communication, while the earlier paragraph states that they're experimenting on the possibility. --68.160.195.48 (talk) 13:19, 29 December 2011 (UTC)

By some coincidence I just added a little to the article that should make the time sequence problem clearer. The facts about this limitation are, I am sure, perfectly clear to people like Dr. Cramer who are interested in the possibility of faster than light communication. I believe that despite these obvious difficulties there are also probably other people who consider the possibility that these limitations may be overcome. For instance, one of the factors that confuses the picture in the experiment that is the topic of this article is that a phase change in the lower (four detector) part of the experiment puts the two interference patterns out of phase with each other. It should be possible to introduce a further phase change in the lower part to put the two interference patterns in the same phase relationship. The center of the Detector 0 interference pattern would be overlaid with the diffraction pattern, but the fringes more distant from the center would be visible. The lower part of the apparatus might periodically be manipulated to prevent interference, and that expedient would create an on-off situation that could be the basis of communication. If I remember correctly, this kind of modified set-up was the basis of the experiment proposed by Dr. Cramer. Even if it is "theoretically impossible", that does not preclude people from trying to do something. I'll have another look at the article to see whether this conflict can be made to disappear or to become more easily apparent.P0M (talk) 15:57, 29 December 2011 (UTC)

It has not been easy to reformulate the section in question. I have made a start at it, but I need to see whether I will need to remove some old stuff to avoid repeating the new first paragraph of this section. I don't think that we have an article on Cramer's work on an "ansible." Perhaps there are other attempts going on that I do not know about.

Does the new paragraph make it clearer that the theoretical impossibility of something has not made everybody give up? There are still people trying to make eternal motion machines.P0M (talk) 02:20, 30 December 2011 (UTC)

Move subsection here

I moved the following part here because it seemed very out of place where it was in the article. It was as if the image-description text stopped and the following was inserted, and then after the following text, the image-description text started up again. I believe it requires an expert to replace it in a more appropriate place in the article. Also, the usage of bold emphasis is not within guidelines. (MOS:BOLD)

Now it seems that, regardless of appearances, something may in all cases have traveled along both paths.

But what if the choice to "erase" the information is in fact delayed, until after the target phase?

Kim, et al.,<ref name="DCQE" /> have shown that it is possible to delay the choice to "erase" the quantum information until after the photon has actually hit its target.

Under those conditions an interference pattern can be recovered, even if the information is erased after the photons have hit the detector. The experimental apparatus is considerably more elaborate than that shown and described above.

– Paine Ellsworth ( CLIMAX )  04:06, 25 October 2011 (UTC)

Reading through the rest of the article, the info above that I removed is detailed later on in the article. So it did not need to be where it was, as it disrupted the explanation of the easier-to-follow diagram in that section. If any editor wants to put this back where it was in the article, then it needs to be discussed here before that happens. – Paine Ellsworth ( CLIMAX )  05:17, 25 October 2011 (UTC)

Critical missing piece of information in summary.

The summary doesn't mention the dates the experiment was first run. It's possible to guess it was the 1980's sometime from the discussion section, but it would be expedient and helpful to include that piece of information unambiguously in the summary. I don't have time right now to chase the references to find out the correct info. 162.111.235.36 (talk) 15:38, 28 December 2011 (UTC)

The article describing the experiment was submitted in January 1999 and published in January of 2000. That's probably as close as I can get without buying the article to see whether they tell when they made the first run of the experiment. However, once they had their apparatus set up it shouldn't have taken very much time to run the experiment enough times to be sure they were getting reliable results. So I would guess that most of the work occurred during 1998.P0M (talk) 16:09, 28 December 2011 (UTC)

That's grand. Thank you, but shows how much guessing from context in the article does. I think if I ever have the time, I'll fix that. 162.111.235.34 (talk) 16:49, 28 December 2011 (UTC)

I made mention of the date the article was submitted.P0M (talk) 23:07, 28 December 2011 (UTC)

Forward-propogating information too?

One statement "It is impossible to know which group a photon appearing at Detector 0 at time T1 may belong to until after its entangled partner is found at one of the other detectors and their coincidence is measured at some slightly later time T2." seems overly strong to me. The position of the photon at D0 landing on the camera at D0 is known, and because there is correlation of the D0 position with the D1/D2/D3/D4 detection, then by observing D0 we may already have some information about which of D1/D2/D3/D4 will be detected.

For instance, if I observe the photon at D0 landing in a node (zero) of the interference pattern associated with D1, then I know with certainty that D1 will not be detected, and there will be 50%/25%/25% chance of D2/D3/D4. Likewise for D2's interference pattern. If instead the photon lands halfway between node and antinode, then I can assign 25%/25%/25%/25% chances for D1/D2/D3/D4. --24.85.247.169 (talk) 19:06, 21 January 2012 (UTC)

The reason it is impossible to do what you propose is that the nodes are not in the same positions in all instances. One fringe pattern looks like the left-to-right mirror image of the other:

|_|_|_|_|_|

_|_|_|_|_|_

So the result is a more-or-less solid band. The swap is due to phase changes in the bottom part of the experimental apparatus.P0M (talk) 22:59, 21 January 2012 (UTC)

Yes, those offset stripes are part of the reason I would expect that partial information is obtained. To be specific, I'm going to refer to the figures as the end of this paper. I agree that if you add up the joint detection rates R01 R02 R03 R04, or even just R01+R02, a smooth bump is obtained, but that's beside the point.

Suppose that I am sitting at D0 and detect a photon at 1.2 mm position. The joint detection rate R02 is about three times larger than R01 for this position -- thus, I know that if a photon in the lower apparatus is observed, it is three times more likely to be in detector 1 than detector 2. In particular I calculate P(D1) = R01/(R01+R02+R03+R04) = 50/405 = 12%, P(D2) = ... = 31%, and P(D3) = P(D4) = 28%.

We could also examine at 1.33mm, halfway between node and antinode: 22%/20%/29%/29%. (I'm not sure why D4 and D3 are more probable than D1/D2. Losses in the path to D1/D2 I guess.) --24.85.247.169 (talk) 02:14, 22 January 2012 (UTC)

D3 and D4 receive photons by one path only, so they would be the ones that have the spread that is associated with diffraction. So almost all of the photons hit near the center. There is some spread, but far less than when an interference pattern is involved, as with D1 and D2. Or, to be more precise, the probability that a photon that has a known single path will appear anywhere very far from the center is vanishingly remote. (George Gamow wrote that it is possible that all the air molecules in a room might by random "choice" all head north at the same instant, but that one need not hold one's breath in preparation for this event.) It is not clear that the experiment is set up in such a way that it catches photons that diverge far from the center of the detection screens.

D1 and D2 receive photons by two paths each, and the difference between them lies in the phase relations among the photons they will receive, which in turn means that we get the two sort of mirror image interference patterns. If you will do what I just did and superimpose the image for D3 and for D4 (even though the article only gives one, they are both present and the twins of the photons which reach each of them will all reach D0, and also D1 and D2, then you will be in a position to add the amplitudes for all four beams. The peak will be about three times higher than the highest of the interference pattern peaks, and the sides will be fairly smooth.

It is more likely that a photon observed at any point represented on the graphs of the experimenters will be associated with D3 and D4. There are two of them, and they overlap in uninterrupted coverage of the central region, which is all the experimenters appear to have been measuring. If you look at a position on D0 that corresponds to the highest peak of D1, I think it is right to say that it is more likely that any photon that is found there corresponds to one that showed up at D3 or D4, or with less probability it will be associated with D1, and with even less probability that it will be associated with one that shows up at D2. But knowing the probability that something is the case is not the same as knowing what actually happened.

Let's say that you picked one of these photons and made a bet as to which detector its twin was associated with. You could not know whether you had won or lost until you checked with the coincidence indicator.

If you were bound and determined to bet on photons that were parts of an interference pattern, then you would do better if you could find an experiment that took into account more than the first three or four fringes. Interference patterns resulting from Young experiments are extremely broad. (I don't know whether anybody has tried to experimentally verify the greatest angular spread of a detectible pattern. Just with the naked eye, the pattern produced by interferences is dozens of times wider than the pattern produced by a single slit. So if you had a typical setup with the laser 3 meters from a detection screen, anything that was more than 25 cm. from the center would almost certainly be part of an interference pattern. In other words, if you set up a single slit and looked for photons at any distance from the central spot of light, you would probably need very sensitive apparatus to detect anything, and even then you would have to wonder whether there was a light leak in your darkroom.

I think you are right about there being varying probabilities, across the width of a detection screen, for which detection screen a photon may be associated with. If you bet on either D1 or D2 you already have a fifty percent probability of being wrong because the photon was associated with D3 or D4 plus some additional probability of being wrong because the photon was associated with the less likely of D1 or D2.

I think the reason that some people are interested in the "retrocausal" aspect of these experiments is because they hope that the phenomenon can be used for communications over such great distances that the speed of light is not good enough for us. (If you send a radio message to earth from a colony 100 light years away, you will not receive an answer in your lifetime.) If somebody could send a continuous electromagnetic signal to earth and keep the quantum twins running in circles at home, then after the signals reached earth the quantum twins at home could be modified so that they conveyed information to the people on earth. John Cramer had a plan he intended at one time to test, but he has not published anything further about it. Anyway, just knowing that a single photon was more likely to have been associated with D1 than with D2 would not be very useful in this context. P0M (talk) 08:25, 22 January 2012 (UTC)

Thanks for pointing out the additional complication of single-slit diffraction pattern. I had completely forgotten about that. I wonder why they didn't bother plotting the side-lobes in the arxiv paper?

Yes, I definitely agree that D3 and D4 tend to dominate and make things uncertain. The information from D0 is poor and indefinite at best. I know I wouldn't make any bets on it unless the house margins were very slim :). Now, here's a fun thing to think about: Let's say the photon is detected at D0, and the photon on the lower apparatus is stuck in a super long delay line (okay, two matched delay lines, one for each path). After D0 detection, I randomly decide to remove the beam splitter in front of the D1/D2 detectors, so that I am guaranteed to measure which-path information (D1 acts like D4, and D2 acts like D3). Or, instead I could remove the beam-splitters in front of D3/D4, so that I can never measure which-path information. Would this change my D0 result? I think that the overall pattern of D0 intensity should be the same, no matter what devious manipulations I perform with the lower photon. If not, I would say it's a very good case for retrocausality (and ability to send messages back in time / faster than light).

By the way, Hyperphysics has a nice graphic showing the difference between single and double slit. The double-slit intensity there can rise up to at most 4 times higher than the single-slit intensity curve, due to constructive interference, but on average it will only be 2 times brighter. I know what you mean about interference patterns looking a lot wider than single-slit patterns (I've noticed the same things in a few texts), but as far as I know that's mathematically not supposed to happen... it might just be an artifact of the increased brightness. --24.85.247.169 (talk) 20:50, 22 January 2012 (UTC)

Even in the classical account of this phenomenon (see Sears, Optics, p. 214ff) the math is expressed as a continuing series of values, i.e., the equations involve a series of integer multiples and there is no reason to stop calculating except that generally speaking the intensities of the peaks get smaller as you calculate fringe positions farther from the center. Eventually you will get to the point where your eyes are too weak to see anything. Some charge-coupled devices can be fired by only one photon hitting them, so the width of the detectable fringe pattern ought to be much more than the easily visible width of more than 4 feet observable at 20 feet or so from the double slit apparatus.

I made the experiment and the photograph I made is still on Commons. In the article on the Double-slit experiment it has been replaced by a better image made with nicer equipment. The number of fringes in both images are approximately the same. The image I photographed was only the central 8 inches or so. The remaining 40 inches or so was weaker, and I saw no point in trying to photograph it. (I had to get close enough to the image to make my digital camera fire its shutter. Any farther away and the camera would refuse to fire because the image was "too dark" for its settings. With my present camera I could have put it on open shutter so I would have gotten a much broader image that would have been impossible to see without printing it horizontally on 8 x 10 paper. The photo by Sears shows about 36 fringes (opposite p. 222). I think I have seen more than a couple of books that assert directly that Psi functions are not bounded. The central portion, i.e., the portion where experience tells us to look for a photon to show up, is the central portion because that is where the probability of a photon showing up is highest. But there are lesser probabilities away from the center, and while the probabilities may drop down so low that experimenters might have to wait for a lifetime to actually see a photon show up there, one eventually will. (Somebody wins the Sweepstakes.) Richard Feynman says at one point something to the effect that if you fire off a laser, any given photon has some probability of showing up anywhere in the universe. Anyway, it should be clear just from the photo above that the visible fringes are not restricted to the width of the visible diffraction pattern.

You would enjoy the article on retrocausal communications published by John Cramer in Analog a few years ago. I think you are describing a version of his experiment. You can google it up. P0M (talk) 08:44, 23 January 2012 (UTC)

Heh, alright, you got me. I'm not entirely convinced but I myself don't have any experimental pictures to back up what I want to say, nor have I gone through the math myself. I would make one suggestion though, related to this single vs. double-slit pattern: In the third and fourth figures of this wikipedia article, the coincidence results for D3 and D4 have diffraction bands on them (good), but the spacing of the diffraction bands is the same as the interference bands on D1 and D2 (confusing). You might want to make them look more like the Kim data (plus side-lobes if you prefer). Anyhow, it's been a pleasure talking to you. Cheers. --24.85.247.169 (talk) 17:40, 24 January 2012 (UTC)

why not one photon

Why not try this experiment with just one photon instead of stream of them? Then you wouldn't need the coincidence counter would you? That should then enable looking at the D0 detector before the other ones and see if the information is there. — Preceding unsigned comment added by 64.22.160.1 (talk) 17:03, 3 February 2012 (UTC) One photon would not provide the information needed to detect a pattern. However shooting a series of photons at intervals that allowed recording of each before the next is sent would allow us to view the patterns as they are assembled. The results would not change but it might provide some insight and would be an interesting way to share the data with the community. — Preceding unsigned comment added by 76.178.252.56 (talk) 01:30, 30 June 2012 (UTC)

I'm just working from memory here, but I think the reason is that in equipment currently being used is "noisy" in the sense that while it is designed to produce entangled photons it also produces lots of photons that do not get entangled. It would be difficult, probably impossible, to filter out the unentangled photons somehow, and easier to just deal with everything that is produced and look for the photons that show up together.

It should be possible, however, to do what the very early experimenters did to pick out individual photons going through a double-slit apparatus, just cut down the delivery rate so that, generally speaking, only one photo would be delivered within a certain time period. As long as the separation was clear, then a CCD or whatever detection device being used could detect the positions and times of all photons received, wash out the photon records that were not matched, and then you would have a record of individual photon pairs produced under virtually identical conditions (same apparatus, same power supply, etc.).

To me it would seem one way to deal with the claim of some that quantum mechanics only deals with ensembles of measurements, as though the interference effects would disappear if they did not occur amidst hundreds or more other measurements that would show interference.P0M (talk) 15:59, 1 July 2012 (UTC)

No citations in section: Problems with using retrocausality

The discussion in the subsection "Problems with using retrocausality" is very clear, however, it has no citations whatsoever. I would like to dig deeper into understanding that part, but couldn't find the source of the main explanation. 89.253.76.71 (talk) 12:46, 13 August 2012 (UTC)

I'll see whether Dr. John Cramer still has his speculations (published in Analog Science Fiction and Fact a number of years ago, and on his website) in an available form. That may lead to citations of people who have published the critiques of this idea. P0M (talk) 18:28, 14 August 2012 (UTC)

There is a little information here: http://nextbigfuture.com/2009/09/john-cramers-retrocausal-experiments.html

P0M (talk) 18:32, 14 August 2012 (UTC)

File:Beam_Split_and_fuse.svg

This diagram incorrectly labels which beams receive a k phase shift and which ones receive a lambda/2 shift.

The beam that passes through the glass should receive the k phase shift and the beam reflected from the mirror should receive the lambda/2 shift.

In addition, when the top beam reflects from the back of the mirror of the second beam splitter (instead of passing through it) it should pick up a phase shift of 2k.

The final result is labeled correctly.

Regards. KC. — Preceding unsigned comment added by Kcerb (talk • contribs) 01:30, 30 December 2012 (UTC)

I'll try to fix the diagram. Thanks. P0M (talk) 21:29, 13 January 2014 (UTC)

Horrible infix subclauses

The article contains a lot of really obfuscated hard-to-parse sentences. For example:

• In the basic double slit experiment, a very narrow beam of coherent light from a source that is far enough away to have almost perfectly parallel wave fronts, is directed perpendicularly towards a wall pierced by two parallel slit apertures.

The largest problem here is the large infix subclause providing a long range dependency. The distance from the subject "a very narrow beam of coherent light from a source" to its verb "is directed" really taxes the short-term memory, and long range dependencies are established as troublesome to learn and understand. Subclauses are quite OK and makes the language flow nicely, for example the postfixed "pierced by two parallel slit apertures", but infix subclauses should preferrably be short. Just as a general advice: the alternative to large infix subclauses, is to express their information in a separate sentence before or after the sentence. I'm not sure what's the best solution in this case. Rursus dixit. (bork!) 07:56, 1 October 2013 (UTC)

Explanation by physical optics

I understand that this chapter puts questions to the standard explanation. I added some. But I don't understand the "By changing the detector D0 position, different phase shifts at detector D1 and D2 lead to a different statistic in observed correlated events". In the first place, as I added, there will not be a red and blue photon the same time. But even if it would, I don't understand why a phase difference between detector Do and D1 will result in a detection interference pattern. This depends on the (not explained) working of the coincidence counter. DParlevliet (talk) 17:14, 1 December 2013 (UTC)

Again, I have trouble understanding what you have written. What do you mean by "I added some"? Do you mean you changed the article at some point(s)?

As to there not being "a red and blue photon at the same time," nobody is making any such assertion. The laser (blue box at the left) emits a photon, which encounters a diaphragm with two slits (the heavy black vertical line. From there, two pathways for the photon emerge, a blue path and a red path. The BBO crystal sends two entangled photons forward on two pairs of red and blue paths. Paying attention now to what choices in paths open up on down the line, the red path and blue path hit prism PS which causes these paths to diverge. Each path hits its own beam splitter, or, I should say, path splitter to be more precise. The blue path hits BS_a and from there one path goes to D₃, and that is the only path that terminates at that detector. So if a photon hits there it cannot display any interference phenomena because there is nothing in the physical circumstances that support any interference. On the other hand, instead of being reflected at that beam splitter there is a path that passes through it, encounters mirror_a whereupon the path is directed toward BS_c whereupon the path again splits, one branch leading to D₂, and another branch reflects and that path terminates at D₁. So, to encapsulate all of this path following, when you check the endpoints of the splitting and resplitting blue path, you find that the blue path terminates in D_1,2,3 but not in D₄.

As you can confirm for yourself, the red path goes through PS and then terminates in D_1,2,4 but not in D₃.

What that means is that D₃ is a terminal point for only a "stream in a river delta" of the blue path, and D₄ is a terminal point for only an "stream in the river delta" belonging to the red path. D₁ is an end point for both a red and a blue path. D₂ is also an end point for both a red path and a blue path.

            ____ red   blue ____                 (interference possible)
        ___/                    \___
       /   \____ red   blue ____/   \            (interference possible)
RED __/                              \__BLUE
       \___            blue\____/            (no interference possible)
           \____ red                            (no interference possible)  

When you get right down to it, all you can do is to report the results in the physics lab. Those results have very consistently shown that when a photon has two paths to the same detection screen it is possible to observe interference effects, and when a photon has only one path to the same detection screen it is impossible to observe interference effects. Therefore, one may predict (and the Kim et al. experiment showed this), you get no interference at D₃, no interference at D₄, interference at D₁ and at D₂.

I don't know who wrote the "Explanation by physical optics." Offhand, it doesn't look right to me. Let's get straight on your fundamental problem and go on from there. P0M (talk) 03:30, 7 January 2014 (UTC)

I did ask about the "Explanation by physical optics" chapter, not about the normal explanation as you gave (not yet). So you don't understand (or don't agree) the arguments in that chapter either. DParlevliet (talk) 11:53, 11 January 2014 (UTC)

I don't think we will get anywhere unless you will respond by indicating whether you understand things like the diagram that constructed above. I said, immediately above your stated assumptions about what I understand or don't understand, that while the "Explanation by physical optics" section seems wrong I wanted to get straight on the basic physics.

Do you accept that there will be interference only in those situations so marked in the diagram above? We can go on from there once I know whether you understand what I am talking about.P0M (talk) 19:54, 12 January 2014 (UTC)

Citation needed of photon detection

I have never seen a published measurement where the photons are detector at the slit. DParlevliet (talk) 22:15, 6 January 2014 (UTC)

Haven't you read the Scientific American article on the quantum eraser yet? They have such a set-up in that article. Misplaced Pages is blocking the link, but use Google to search for "Rachel Hillmer" "Paul Kwiat" and look down the list for a PDF file on the quantum eraser experiment.

Photons are "detected" or labeled as to which path they came through, by polarizing whatever came through the left slit with one polarizer in vertical orientation and the other polarizer in horizontal orientation.P0M (talk) 22:48, 7 January 2014 (UTC)

They are marked, not detected. Those articles claim that because of the marking/labeling you can in principle later on detect through which slit the photon went, so not at the slit. DParlevliet (talk) 11:42, 11 January 2014 (UTC)

As long as photons are what you call "marked" they will not show up on the screen as components of an interference pattern. They will show up as part of a diffraction pattern, basically a somewhat blurry main dot and a couple of comparatively much dimmer side dots.

The language used in these experiments is often confusing. What physicists are calling a "detector" can be part of an apparatus that can in principle be used to determine what path a a given photon has followed. For instance, with the polarizers in place in the Scientific American apparatus it is possible to put a second polarizer in front of the detection screen. Any photon that shows up must correspond to a path that, at the slit, had the same polarization as the polarization of the polarizer nearer the detection screen.

The whole point is that anything that allows determination of a path, anything that would allow determination of a path if you bothered to do something about it, is enough to prevent interference. What that boils down to, if you follow through the experiments and see what the physicists have really done, is that if a physical change is made anywhere close enough to a slit to localize its effect to a photon that could find a path through that slit, then it will prevent interference at the detection screen. Polarization is one of the more subtle ways that one can interfere with the wavefunction that is associated with one slit because all it does, in effect, is to rotate the wavefunction so that it has one orientation and not the other. The wavefunction itself does not get changes as to its plot. It is just as though you took the wavefunction as plotted on graph paper and rotated the graph paper. So at the screen if you plotted the two wave functions corresponding to the two slits, one would be going along an x-axis and the other would be going along a y-axis. "Erasing" the marking of the photon amounts to re-rotating it (and its "twin") so that wavefunctions are aligned again in such a way that they can interfere. You must familiarize yourself with the Kim et al. article. It is not possible to go by ordinary macro-world intuition in this case. That's the whole reason that the double-slit experiment is so important and so revealing.P0M (talk) 18:19, 12 January 2014 (UTC)

"The Experiment"

About POM's explanation: "The laser (blue box at the left) emits a photon, which encounters a diaphragm with two slits (the heavy black vertical line). From there, two pathways for the photon emerge, a blue path and a red path. The BBO crystal sends two entangled photons forward on two pairs of red and blue paths.....". If the laser photon goes through the red slit, in the BBO two red entangled photons are emitted, on going up and one down (in the diagram). There is no blue photon(s) at the time (it went though the red slit). So if the up-going red photon reaches Ds, where does its wave interfere with? DParlevliet (talk) 13:45, 11 January 2014 (UTC)

What possible evidence do you have for saying that there are two red entangled photons and no blue entangled photons? There is no possible evidence for your claim since the situation with photons is different from the situation with bullets or the like. We can photograph bullets in flight. Some people can see baseballs in flight. Nobody can see photons in flight.

The double-slit experiment works when photons are emitted one at a time. If, as you claim, a photon goes through one slit and therefore nothing goes through the other slit at the same time, then how do you explain interference? The clear experimental evidence show that if a photon has the possibility of going by two paths it will show interference. If it can only go by one path then it will not show interference. Are you trying to claim that this is not true?P0M

I don't know what to do in this situation. You should read the original experiment http://arxiv.org/abs/quant-ph/9903047 and depend on it and other legitimate sources, not make up rationales on your own.P0M (talk) 08:32, 12 January 2014 (UTC)

I just asked if somebody knew the explanation. Of course I did read the original and that is not clear while the "Explanation by physical optics" mentioned that is was wrong, but that explanation I understand neither. DParlevliet (talk) 12:01, 12 January 2014 (UTC)

Again, I cannot understand your syntax. If you read the Kim article you appear not to have understood it. You appear not to have understood the basic physics of the double-slit experiment. You should be able to follow Greenstein's Quantum Challenge and you will probably find it the best source against which to check your understanding.P0M (talk) 19:10, 12 January 2014 (UTC)

One other thought, is it possible that you do not understand that the BBO does pass the red and blue path wavefunctions on through. It is difficult, using ordinary logic, to see why this would happen since, as you point out, the photons presumably originate in different regions of the BBO. Nevertheless, the experiment shows interference fringes and that couldn't happen unless the red and blue paths are linked in such a way that whatever got set up at the original double-slit part of the experiment is continued all the way through on both the top part of the diagram and the bottom part of the diagram.P0M (talk) 20:00, 12 January 2014 (UTC)

You are right, that is what I mean. If the red and blue area would generate a double photon at the same time, that would be a very remarkable discovery. I have not read that anywhere. Take also into account the remark in the article that the lens is focussed on infinity and not to the red/blue area, as it should be. So the red/blue waves will not be focused on the detector. Interference can be caused by other effects, even errors. But I have no other explanation or reference, so have nothing to change in the article. DParlevliet (talk) 20:51, 12 January 2014 (UTC)

Citation to the "focused at infinity" remark, please.

Have you studied entanglement yet? Nobody I know of has small enough eyes to watch what goes on in the BBO, but the experimental results clearly show that there will be interference between red path and blue path whenever they meet in a properly arranged detector, D₁ and D₂. You can see what you get when you pick out the hits on D₀ with D₁ (figure 3) and D₂ (figure 4) in the Kim et al. article, 4th page.

The BBO does produce two photons, two entangled protons that is, for each photon that enters from the double-slit diaphragm. Moreover, each entangled photon that emerges from the BBO has a red path origin and a blue path origin (or, to use more technical vocabulary, there is no path information available that could say that whether a photon came out of the red path or the blue path area). If this were not true, then there could not be a photon that ends up at the ends of both red and blue paths on D₁ or D₂.

There are experiments that show when two different lasers are placed close together side by side and are controlled so that they jointly can only emit one photon at a time, then an interference pattern will emerge on the detection screen. The argument is that it is impossible to determine which laser actually emitted the photon, so that there is no path information, and therefore there must be interference. If you don't believe me, look it up in Greenstein's Quantum Challenge.

Also, it is not clear that a BBO absorbs a photon by having an electron boosted in orbit and having it emit two photons in a two-hop trip back to its equilibrium state. All that can really be said is that one kind of thing goes in and two things of another kind go out. As far as I know, the assertion that different physical parts of the BBO are responsible for the two photons that emerge and go in different directions is just a matter of interpretation. The things that go out behave just as if they were emerging from a double-slit apparatus. In other words they seem to "inherit" the history of the earlier passage through the double-slit apparatus. This is a black box situation, pretty much. We know what goes into the box. We know what comes out of the box. We don't know what the "machinery" in the black box is. (Actually, I guess people know something about the crystalline structure of the BBO, but I don't know that they can explain what really happens to a photon in transit through the BBO.)P0M (talk) 21:37, 12 January 2014 (UTC)

Point by point

I am trying to understand and make your section more understandable.

You say:

There seems to be a misconception, regarding where this pattern originates. One might think that the signal photons shape the pattern on the way to detector D0 by interfering with each other. But at detector D0 only a blurred image of the double slit is projected by the converging lens, effectively smearing any direct interference patterns from the slits.

Whose misconception is this supposed to be? Are you challenging Kim et al.?

How can you assert that there is only a blurred image of the double slit at Detector 0? Give me a quotation from the Kim article in support of that.

You seem to attribute the blurring to the converging lens, but that assertion is not correct. All the lens does is to shorten the time it takes for photons to reach Detector 0 so that Detector 0 will do whatever it is going to do before any of the other detectors go into action.

Please answer these points with citations.P0M (talk) 19:35, 12 January 2014 (UTC)

Not seeing any reply, I'll just have to challenge the next part that I find dubious:

To understand the source of the derived interference pattern, one has to focus on the third beam splitter BSC, where the photon paths from both slits merge. At this point a phase difference exists between the merging paths, which is dependent on the different path lengths from slit A or B respectively to the splitter. Furthermore path length and phase difference depend in part on the deviation angle of the idler photon leaving the Glan-Thompson prism.

If I am reading this correctly, I think this part may be o.k. I guess you are saying that if you observe the paths going vertically (on the diagram) from BS_c then you will understand that both a blue path and a red path approach D₂ therefrom, and that if you examine the paths going from the same beam splitter diagonally down and toward the left, you will also find both a blue and a red path. "At this point" is confusing to the reader since according to the rules of syntax "this point" should refer to some point in or on BS_c, but you surely must mean instead the surfaces of D₁ and D₂. Any phase difference that exists will not ordinarily be present only at some point along any of the paths. The wavefunctions will be out of phase or in phase just depending on how far along their paths and what the exact geometry of the situation may be. (If you've ever tried to do this general kind of experiment you will realize that the slightest misalignment will throw everything out of whack and it is a painstaking business to get all the mirrors beam splitters and other components to line up properly.) The only point where phase differences make a difference to what is actually observed is along the surfaces of the detectors. With suitable tampering the two wavefunctions could be made to slide closer or farther apart horizontally just as two sheets of paper on a desk can be made so that one exactly covers the other or pushed sidewise far enough that no part of one is covered by or covering a part of the other. But all of this stuff is just details about the touchy adjustments that have to be made in the lab.

While a fixed position on detector D0 is correlated to events at detector D1 or D2, only events at D1 or D2 are inspected, which share the same phase differences.

I don't know what you are trying to say. It is not true that there is one "fixed position" on D₀ to which all signal protons are brought. Is this part of the unsubstantiated business about the lens focusing everything in the signal path at infinity? You have thus-far ignored my correction on that point. I have re-read the article and there is such assertion.

Furthermore, according to the precise syntax of your next sentence you are saying that researchers examined events at D₁ and D₂. In that case they would have ignored events at the other two idler detectors. That is not true. You then further assert that "events at D1 or D" happen to "share the same phase differences." I doubt that what you have said is what you meant, but you can't burden readers to try to figure out what you intended to say.

Are you trying to say that events at D₁ and at D₂ share the same phase dfferences, and so those are the events that are preferentially observed? Or are you trying to say something else?

After leaving the beam splitter BSC, each phase difference may lead to constructive or destructive interference on the paths from the splitter to the detectors D1 and D2, while also allowing any arbitrary intermediate values. But one has to note, that the combined probabilities for both paths behind BSC always add up to 1, as the difference of the phase shifts between that paths amounts to π or 180°.

Interference is not manifested "on the paths." Interference is manifested only when wavefunctions are superimposed on some surface (object) and also the wavefunctions "collapse."

The business about the "combined probabilities" is only significant in the context of explaining what is seen at D₀ (or if for some reason someone decided to sum the results for D₁ and D₂.

Then, out of the blue, you bring up the business about the phase shifts pertaining to the two detectors just mentioned. The difference occurs because of the different sequences of reflections and transmissions leading up to those two detectors. For the first it is: blue: Tr Rf Rf red: Tr Rf Tr For the secnd it is: blue: Tr Rf Tr red: Tr Rf Rf What happens to blue in one situation happens to red in the other situation, and vice-versa. There are real consequences to this fact.

That is only significant because it explains why the curves in diagrams 3 and 4 are flipped, and their being flipped explains why, if you projected one pattern on top of the other the maxima of one would fill in the minima of the other and vice-versa.

These are all experimental details. What the experiment does, basically, is to run four interlaced experiments at random.

Every so often an idler photon lands on the first detector and will start to fill out one kind of interference pattern. It is interesting to see what the behavior its entangled twin manifests on D₀. It turns out that it must contribute to an interference pattern of type one on that detector also.

Every so often an idler photon lands on the second detector and interference patterns of type 2 start to fill out on that second detector but also on the detector for signal photons.

Every so often an idler photon lands on the third detector and no interference can occur (only one path goes to that detector), and on the upper detector (O) a photon contributes to a diffraction pattern but not to an interference pattern.

Every so often an idler photon lands on the fourth detector and no interference can occur there either. As before, the upper detector sees only a photon being added to a diffraction pattern.

This experiment would be useless if we could not keep the four interlaced experiments separate, which is what the coincidence counter is used for. When there is a coincidence between D₀ and one of the four detectors below, the pattern being added to (and needing to be sorted out) on D₀ should be graphed on the sheet that corresponds to inputs that correlate to whichever of the four detectors lit up at the same time.

So what about erasure? The experiments are saying, as it were, if a photon shows up on the third detector of the fourth detector we are in the position of being able to stand with our eyeball where that detector is and see the bottom slit "light up." Or if a photon shows up on the fourth detector it is like being able to stand there and look directly into light blinking at us from the top slit. So we would say that we have found "which path" information. However, we can take that information away by standing at either D₁ or D₂, in which case we will not be able to tell whether the blip came from the top slit or the bottom slit. We cannot have "which path" information from those positions because both slits are superimposed in our visual field.

What this all boils down to is saying that we can direct light from a double-slit apparatus along diverging paths and can reliably expect one photon to show up for one photon emitted by the laser, and we can never see "half a photon" show up at the end of the two divergent paths, or else we can direct light from a double-slit apparatus along converging paths and exactly line them up, in which case there would be no interference because maxima and minima in the two copies of the wave function would be in exactly the same place, or else we can arrange for light from a double-slit apparatus to be slightly out of phase in which case we get superimposed wavefunctions, maxima filling in for minima at some points, maxima doubling the amplitude at some points (and therefore quadrupling the intensity at those points), and so forth.

It would be much better if you would discuss changes and gain a consensus in discussion before making changes. Making changes that have to be substantially changed creates turbulence that is bad for those who come to this article for information.P0M (talk) 05:40, 13 January 2014 (UTC)

Please read my starting post. I did not write that section. I don't understand it so asked if someone knows. That seems not to be so. DParlevliet (talk) 08:51, 13 January 2014 (UTC)

Sorry. I got confused by what you wrote in your starting point. I can't well understand what it's supposed to communicate. I'm just going to delete it. Thanks. P0M (talk) 14:19, 13 January 2014 (UTC)

I work in quantum optics and I am kind of shocked when looking at the state of this article. Much of what is claimed here is plain wrong like the claim that "The experiment can also be explained with classical waves, but with another conclusion." for the DCQE using entangled photons. There obviously are no classical entangled photons. It is also generally agreed upon that all the consequences of entanglement cannot be explained classically. That part of the article also does not cite any references and is not acceptable the way it is. Also many of the recent changes introduced odd wordings ("This Delayed quantum eraser claimes to be a real double-slit measurement.", "If the photon is recorded at detector D1 or D2, it has a 50% chance to be a "blue" photon and 50% chance to be a "red" one.") or sloppy wordings ("A Quantum eraser which often used in education is based on placing two orthogonal polarizers at the slits of a Double slit experiment, causing the interference to disappear. ") and the article is very incoherent. Right now, it more or less reads like pushing crackpot agenda. In my opinion the last acceptable version was the one from 07:41, 1 October 2013‎ by Rursus. As I am not actively editing Misplaced Pages, I will leave the decision to others, but in my opinion the article should be reversed to that state. 129.217.159.124 (talk) 12:56, 30 January 2014 (UTC)

I have to agree with you. I have tried to work with DParlevliet without being able to convince him that there are problems with what he has written. I'm involved in other things that are taking up lots of time right now.

Anyone who makes substantial changes to established articles should be willing to discuss the changes and convince those who have already spent a great deal of time and trouble to make the clearest explanation possible that the change proposed actually improves the article.

I, too, am in favor of reverting to the 1 October 2013 version.P0M (talk) 01:19, 31 January 2014 (UTC)

I have been adding parts bit by bit and got no remarks until now. But perhaps I went too fast. With POM I had a misunderstanding but no remarks or discussion about the latest parts. According Wiki rules edits should not be deleted but discussed and improved. I know the classical way is controversial, and if it is wrong I will change it. But then with arguments. Not just "classical is wrong" by definition. If words are sloppy, change it, that is Wiki-style. There are several parts changed, most just to compress multiple or not relevant explanation without change of description. There are several experiments added and combined with another article and with most also is mentioned if it can be classical explained or not. This is with simple High-School goniometry which does not reference I suppose. So if someone has remarks about specific parts, please mention where and discuss. DParlevliet (talk) 10:56, 1 February 2014 (UTC)

It is somewhat hard to take that comment seriously as you deleted major parts of a reasonably well written article yourself without discussing beforehand. Do you have any peer-reviewed references backing your point of view? If so, there is something to discuss. Otherwise in my opinion the old version of the article should be restored and kept.129.217.159.124 (talk) 14:39, 3 February 2014 (UTC)

To add to what I just wrote. Sections like " So 2 is not an absence of interference, but the sum of two interference patterns which are shifted 180°." are a good example of what is wrong with the article. any non-interference pattern seen when using both slits can be decomposed into several interference patterns. A double slit measures spatial coherence. Or simply speaking a light field with narrow angular spread will show an interference pattern. A light source with large angular spread will not. One can easily check that by taking a light source and placing it closer to the double slit. At some distance the pattern will vanish. If one wants to, one can now filter that light source and allow only the emission from one tiny spot on the surface of the emitter to pass through the double slit. The pattern will reappear. If one repeats that for every tiny spot on the surface of the emitter, one gets different patterns for every spot on the sample. They will in total sum up to no pattern at all. This IS the absence of interference. 129.217.159.124 (talk) 14:49, 3 February 2014 (UTC)

Delayed quantum eraser of Walborn e.a in classical way

From 129.217.159.124 I understand he objects to this part. What is wrong here? It is just an explanation following strict and simple classical rules. Because entanglement is not classical, it is not used here. Only that the polarization of POL determines the polarization before Q1/Q2, and that was from the article of Walborn. I will make a reference of that. DParlevliet (talk) 11:16, 1 February 2014 (UTC)

I will let 129.217.159.124 respond since he has his own set of questions. Actually, I think he already told you. Have you read and tried to understand his criticisms?

Since you have brought up this one section for examination, however, I would like to ask you why you cite a document that it costs money to acquire instead of arXiv:quant-ph/0106078v1 13 Jun 2001? (http://www.arxiv.org/pdf/quant-ph/0106078%E2%80%8E).

Second, I would like to understand why you begin by casting doubt on the qualifications of a number of physicists with responsible positions in universities by saying, "This Delayed quantum eraser claimes to be a real double-slit measurement." The sentence does not exhibit proper syntax, but the "claims to be," to which I have added emphasis, displays a clear rhetorical intent. We owe individuals who have spent decades of study to achieve their high level of competence some modicum of respect. As for the syntax, erasers can't make any claims. You appear to be talking about the experiment and about what several people claim it to be.

In the beginning period of the development of quantum mechanics the Copenhagen group had a clearly articulated understanding that anything that was well explained in classical physics could not acceptably have an explanation of inferior predictive power in quantum physics. Or, to put it another way, all of classical physics that was correct would be found to be subsumed in quantum physics. The unsuccessful parts of it, such as attempts to explain heat and radiation that led to the Ultraviolet catastrophe, would be replaced by entirely new explanations, and then there were some items of theory that were not even mentioned in classical physics. Unless somebody comes up with a mistake in quantum mechanics that blows it up and leaves classical physics standing or revived, there cannot be one explanation according to quantum mechanics and an equal contender in line with classical physics that is equally acceptable or superior.P0M (talk) 02:58, 2 February 2014 (UTC)

I have answered 129.217.159.124 above. The document reference I have take over from quantum eraser, with I merged in this article. The "claim" had no rhetorical intent, but the article itself claimed that this was the first real double slit. Because it can be misunderstood (and is not so relevant) I will remove it. Copenhagen mentioned that photond has wave properties, until a limit. There is no harm giving also a classical explanation and mention where is differs from QM, so where is its limit (or the description of the article). Look to double slit experiment, which has a separate classical part, and I just did the same in this article. Take into account that most readers will understand classical waves much better then QM formula, but it must be right. So if something in the description is wrong, please mention. Anyway I will change it to be less defiant. DParlevliet (talk) 13:11, 2 February 2014 (UTC)

You say:

In 2 the incoming polarization can be resolve in two polarizations on the F and S axis, which each give the same result as 3 en 4. So 2 is not an absence of interference, but the sum of two interference patterns which are shifted 180°.

This statement is clearly wrong.

There is nothing significantly different, for this experiment, between a classical understanding of polarization and a quantum mechanical understanding of it. In other words, for the purpose of understanding this experiment we need only know what the various polarizations are, and how they determine the observed results. We do not need to get into explanations of how polarization occurs.

The important quantum mechanical features of this experiment are: (1) the fact that entangled photons can be created, and that when photons are entangled they must retain certain correlations, and (2) the way that photons passing through a double-slit diaphragm behave, something that is already in fairly substantial common knowledge.

In this experiment, the light emitted by the argon laser is polarized to begin with. When it encounters the BBO it can produce a pair of photons that divide the energy (and frequency) of an original photon and also inherit the wavefunction of the original photon.

The behavior of the photon in the lower branch of this experiment when encountering the double-slit diaphragm is no different from that of a photon in any regular double-slit apparatus. When the wavefunction identified with the photon passes through the double slits there will come to be identical wavefunctions issuing forth from each slit. These wavefunctions will find their centers at different places along the x axis of the detection screen, they will be superimposed, and the squares of the sums of their values across the screen will determine the probability of the photon showing up at all points.

When circular polarizers rotated 90° to each other are placed in slits 1 and 2, they are said to be marked. Being marked means that by observing the polarization of a photon arriving at the detection screen it would be possible to determine whether it picked its polarization up by passage through the circular polarizer at slit one or slit two. It is possible to say, dogmatically, that when the which-path information can be known interference cannot occur. It is also possible to say that when the two wavefunctions of a single photon are differently polarized it is impossible that they fall in superposition and produce interference.

In order to get interference, two copies of the same wavefunction must come into superposition. The members of each wavefunction pair are effectively separated by being in different states of polarization. Unless these polarizations are changed in some way there can be no interference. Schematically, the polarizations in the following way: Initially the members of the wavefunction pairs that came through slit A all have one polarization. Let's call it C. Their twins that came through slit B all have the opposite polarization. Let's call it D. C and D can never get together, and the problem isn't solved by somehow turning the polarization of group A photons into D polarization, nor is it solve by turning the polarization of group B photons into C polarization. However, it is possible to change all photons polarized C into two sets of polarization E and F, and change all photons polarized D into two sets of polarization, also E and F. So now we have four groups of photons, all falling on the same detection screen. We have some photons that came through slit A that are in polarization E. We have some photons that came through slit B that are also in polarization E. They can interfere now that the last polarity change has been engineered. We also have some photons that came through slit A that are in polarization F, and some photons that came through slit B that are also in polarization F, and they can interfere once that final polarity change has been provided. The only problem is that the the sets of interfering photons have phase difference so we can't see their interferences patterns unless the two groups are separated. That task is performed by using the coincidence counter.

When you say, "The experiment can also be explained with classical waves, but with another conclusion," you are wrong.P0M (talk) 16:12, 2 February 2014 (UTC)

In the last version your last remark was already taken into account. In your explanation you follow QM-rules. That is good of course, but the part I added follows strictly classical rules. I suppose you are familiar with that too. A classical wave vector can be resolved in two vectors on the X and Y axis, calculate what happens with both, and combine the output vectors. In this case I have chosen X and Y along F and S. If this is wrong, then explain, but with only classical rules. DParlevliet (talk) 17:17, 2 February 2014 (UTC)

I just explained why your explanation is wrong. There is nothing in what I said that needs any special "quantum" "rules" to understand. P0M (talk) 17:22, 2 February 2014 (UTC)

Alright, I see. "The members of each wave function pair are effectively separated by being in different states of polarization": It is not orthogonal polarised, but rotating. Opposite rotating polarizations does interfere. During rotation they are 4x orthogonal, 2x in line horizontal, 2x in line vertical and all in between. When polarization is resolved, then beams from both slits has vectors in X and Y which are in superposition. Calculate with trigonometry and you will see. Probability is P =(1-sinφcos2α) with α is the angle of incoming polarization to polarisers axis. DParlevliet (talk) 20:43, 2 February 2014 (UTC)

You are telling me that the polarizations need to be "resolved," and then they will be in superposition, right?P0M (talk) 03:42, 3 February 2014 (UTC)

Yes, you are allowed to, without changing the outcome. I don't know if the word is right, I mean as in the pixture

. Then after the polarisers Y = sinαcos(ωt-φ) + sinαsin(ωt+φ-π/2) and X = cosαsin(ωt-φ-π/2) + cosαsin(ωt+φ). If you calculate with those the probability, you get above formula. You would be right if it were linear polarisers, those does not interfere. DParlevliet (talk) 08:19, 3 February 2014 (UTC)

Anyway, for QM it still fits. It is an error of thought that marking the wave gives information about the path of the particle. According Copenhagen it is not allowed to combine wave and particle in this way. In Walborn, if you add a right and left rotating polarisation, it resulting polarisation does not rotate, so information is lost again. But more general, if the particle goes through one or the other slit, the waves through both slits are the same in both cases. So the wave, marked or not, does not hold information about the particle (path), so there is still interference. And that is the outcome of Walborn (and classical wave).DParlevliet (talk) 08:32, 3 February 2014 (UTC)

In Walborn's experiment we are discussing single photons. If we add countercircular polarizations classically, we will get a beam of linear polarization. For single photons, linear polarization can only be a property of the ensemble. When detected, every single photon must transfer some spin to the absorbing system as in vacuum only two states of helicity are allowed. A spinless photon does not exist. That means that there is a 50% chance to have either spin up or down. The detector receives that amount of angular momentum and thus will always to allow you to get which-way information. The artificial distinction between particle and wave made here is arbitrary and even wrong. Wave and particle are not seperate entities as estimated in the 1920s. A proper treatment in terms of quantum field theory shows that a treatment using quantization of the em field solves the issue nicely. Terms like "the wave goes through both slits in both cases" are meaningless. QM is a statistical theory and whether the wave function actually corresponds to anything physical is questionable. It may or it may not. 129.217.159.124 (talk) 14:34, 3 February 2014 (UTC)

@DParlevliet: I am unable to deal with illogical communications. I asked you if you need to resolve the polarizations for superposition to occur, and you say, "yes, you are allowed to." I can read what you have written to mean that there is superposition without anything being done to resolve the polarizations. P0M (talk) 14:47, 3 February 2014 (UTC)

129.217.159.124: I am using classical waves according Copenhagen interpretation, which also predicts the results of the experiment. Just an addition to the QM explanation which is mentioned in the first place. That is all. If the calculation is wrong, you can comment.

P0M: I am not sure what you mean. Probably there are other ways to calculate, with the same result, but this one is easy. It is a standard way of working with vectors in linear systems. When you use orthogonal polarization in the same way it will result in a constant, so independent of φ, so no interference pattern. DParlevliet (talk) 18:20, 3 February 2014 (UTC)

Why do you evade my question? It is a clear question and should have a "yes" or "no" answer. P0M (talk) 05:04, 4 February 2014 (UTC)

Because, as I mentioned, I am not sure what you mean. If your question was do I need to resolve the polarizations, then my above answer was: no.DParlevliet (talk) 07:56, 4 February 2014 (UTC)

Then you are clearly wrong. See the new section below. Why wouldn't these researchers both anticipate and notice these interference phenomena that you claim to exist? They are professionals.P0M (talk) 16:36, 4 February 2014 (UTC)

Sorry, but your statements do not make much sense. "using classical waves according Copenhagen interpretation" is just a random bunch of words. Copenhagen is an interpretation of quantum mechanics and as such of course incompatible with classical waves. In classical physics, measurements are non-invasive. You can measure the system again and again and again. In quantum physics, once you detect a photon, it is gone. This is of course of highest importance when discussing single photons as in this experiment. Single photons never have a classical counterpart for this reason. Also, I must second that your responses to P0M are not satisfying. You seem to be evading a clear answer.129.217.159.124 (talk) 10:45, 4 February 2014 (UTC)

According Copenhagen when you measure with classical wave equipment, you will see wave behaviour. I only show that when you calculate which classical wave formula, you get the outcome of the experiment. About POM, he is an QM expert and I am not. So I sometimes misunderstand what he is asking. Perhaps he is not so familiar with the wave formula I use. Just rephrase (classical) and perhaps I see his point.DParlevliet (talk) 14:45, 4 February 2014 (UTC)

So could you please show us, how you get single photons classically? Your "math" does not give you single photons. Single photon sources like the ones constructed from photon pairs and heralding show antibunching: The probability to detect a second photon directly after you detected the first one is significantly reduced. Unless you can show that effect, your formula does not describe the experiment in question because you are not discussing single photons. So please show that your formula gives you single photons or provide a good reference for that. 129.217.159.124 (talk) 14:52, 4 February 2014 (UTC)

Graphical analysis of the experiment for concreteness

http://www.china-learn.info/Nature_Photos/z_JPG_images/CircularPolarizationsInDouble-SlitExperiment.jpg

Let's start over. If you will open a second window and access the above URL you will see graphic representations of what happens to a single photon that goes both ways through a double-slit apparatus and is also given clockwise circular polarization and counter-clockwise circular polarization, a different polarization for each wavefunction copy. After hitting the double-slit diaphragm, one copy of the wavefunction of the photon is on what I've labelled as the red path, and one copy of the wavefunction of the same photon is on what I've labelled as the blue path. When the two copies of the wave function arrive at the detection screen they will be slightly displaced from each other due to the difference in time to reach any point on the screen from either slit. (There are innumerable diagrams of this "slight displacement" phenomenon on the WWW including in the Misplaced Pages article on the double-slit experiment, so I will not reproduce that stuff here.) On the diagram that should now be on your other open web page, the top right diagram shows what would happen if the two copies of the wavefunction both went through the same kind of polarizer and both received the same circular polarizations. The maxima and minima of each wave function would be overlapping with some difference in phase due to the way things work out if you project two images from two different (separated slightly) remote points. Unless the experimenter makes a mistake when grabbing quarter-wave plates or whatever polarizer is being used, this result would never be seen in the lab when doing the delayed choice quantum eraser experiment.

What you get in the lab for any single photon is shown in the bottom right diagram. If you want a physical example, think of putting a bolt with an unusual left-threaded design next to an ordinary bolt of the same size with a right-threaded design. Expecting these two wavefunction copies to superimpose would be like expecting to thread a left-threaded bolt through a right-threaded nut. If you have good powers of visualization, imagine that the two bolts can be moved through each other so that on the outside surface you would see a sort of criss-cross pattern for the threads.

The several authors of the article describing this experiment are the real experts in this field. They are the ones who have been using polarizers and the math involved and experiment after experiment and calculation after calulation for the last several decades. It is utterly incomprehensible if they would set up an experiment in which by conscious design they use clockwise and counter-clockwise polarizations to keep superposition from happening, but they fail to notice that they are actually getting superpositions and interference on the detector screen. Please give these researchers the respect that they deserve. Also note that their article is a peer-reviewed publication, so to believe that the whole experiment is based on some kind of Physics 101 lab experiment mistake insults the intelligence of the whole community of physicists.

The whole business about having another polarizer that can be inserted or removed so as to "switch on" or "switch off" an additional kind of polarization is that by performing this additional step there can be superpositions and interferences manifested on the detection screen. At that point everything gets mixed up, so the experimenters have to use the coincidence counter to pull out one group or the other of the mixed-together results.(See above: "We have some photons that came through slit A that are in polarization E. We have some photons that came through slit B that are also in polarization E. They can interfere now that the last polarity change has been engineered. We also have some photons that came through slit A that are in polarization F, and some photons that came through slit B that are also in polarization F, and they can interfere once that final polarity change has been provided. The only problem is that the the sets of interfering photons have phase difference so we can't see their interferences patterns unless the two groups are separated. That task is performed by using the coincidence counter.") You can't get to the stage where you can sort out groups of self-interfering photons and see a fringe pattern unless you do the part of the experiment that changes all these wave functions from their circular polarizations.P0M (talk) 16:26, 4 February 2014 (UTC)

I see your point. Now look to wiki how a quarter wave plate works. It has two orthogonal axis from which one (slow) delays the wave with π/2 compared to then other (fast). This is causing the rotating effect. The incoming polarization vector is first decomposed to the fast and slow axis, then after the polarisers composed again to the output vector. Because the two waves on both axis are now π/2 out of phase, the resulting vector will rotate in time. Your picture is when the input polarizing vector is π/4 to polariser axis. When decomposed axis F and S are equal and a circular rotation results. If the input differs from π/4 you will get ellipse rotation. But now consider the input polarization parallel to the fast axis, as Waldborn is doing with POL. Then decomposing gives a vector only one the F axis, not on the S axis. Then the output does not rotate (also explained in wikipedia). What you get is from the first slit a wave from the F axis while from the other slit the wave is π/2 delayed by S axis (both polarizers are orthogonal to each other) but in the same polarization direction. Those interfere like shown in Walborns result. Walborn is not wrong. He gives a QM explanation, I give the classical explanation. Both have the same outcome. DParlevliet (talk) 16:59, 4 February 2014 (UTC)

If you "see my point," why are you still arguing with me? Why do you claim that physics professors are getting first-year physics stuff wrong? Why have you ignored what other people have told you? All you do is repeat the same mish-mash of assertions about classical physics "rules." And what do you mean about the two copies of the photon's wave function being "in the same polarization direction"? They are contrary rotations to each other, not the same.

If Walborn is not wrong and you are not wrong how come you get incompatible results? You see the clear graphical results that experimenters get, a right-rotating helix and a left-rotating helix. They are what you get in (2) as you termed it above. Experimenters get the bottom right result on the linked-to illustration. You may mistakenly think they get the upper-right result. They would only get that result if they used the same polarizers at both slits. They would not make that mistake. There is no way for these contrarily rotating wavefunctions to interfere, but you say that you get interference. That is false. Oppositely revolving circular polarizations cannot superimpose. Only by involving the additional polarizer inserted into the path of the entangled photon do you get the circular polarizations to be replaced by other polarizations that can superimpose (your 3 and 4). That is why the experimenters set their apparatus up the way that they did.

Specifically, where you say: "So 2 is not an absence of interference, but the sum of two interference patterns which are shifted 180°," Walborn et al. indicate that there is no interference whatsoever，There are no interference patterns to be shifted. That is all there is to it at "2" as you list the various experimental configurations in the article.P0M (talk) 18:12, 4 February 2014 (UTC)

I said the results are the same, but the explanation is different. Walborn's results are described by 1-4 and with classical calculation those results are the same. Only the explanation is different, because QM and classical use different rules. But from your answer it is clear to me that you are not familiar with calculating with classical optical waves. I advise to consult someone in that field and let him check the calculation.DParlevliet (talk) 19:55, 4 February 2014 (UTC)

I just quoted your own words directly above. Do you deny them? P0M (talk) 20:08, 4 February 2014 (UTC)

The problem is that you are not following the classical calculations. I can try to talk you through, but you must be willing to do. For instance start with the polarization image in the article. It shows the situation of Walborn fig 5, explained on page 5. Green is in incoming polarization (forged by POL). Can you give the classical formula output of both slits, so in sin/cos? DParlevliet (talk) 08:28, 5 February 2014 (UTC)

The problem is that in the delayed choice article you say:

2. With Q1/Q2 there was no interference.

On this talk page you say,

So 2 is not an absence of interference.

You contradict yourself and you are not even aware of it.

You can't have it both ways. Which way is what you really think is correct?

P0M (talk) 08:35, 5 February 2014 (UTC)

The first is the Walborn QM conclusion, the second the classical wave conclusion. Those differ. But the measured result is the same. If you add two waves which are shifted 180 degree (fig 4 and 5) you get the same graphic as no interference. Walborn mentioned that too in his article, but did not draw that conclusion.DParlevliet (talk) 09:03, 5 February 2014 (UTC)

Finally you give a responsive answer to my question, and in the same entry you also reveal that you are in fact talking about the wavefunctions when they are no longer in circular form. You refer to "fig 4 and 5," and the article says:

FIG 4. Coincidence counts when QWP1, QWP2, and POL1 are in place. POL 1 was set at θ.

FIG 5. Coincidence counts when QWP1, QWP2, and POL1 are in place. POL 1 was set at θ+π/2.

Experimenters depended on this manipulation to "erase" the which-path marking that was originally provided by putting one member of each wavefunction pair in clockwise polarization and the other member of each wavefunction pair in counterclockwise polarization. Because POL1 has to be in two orientations to get at all of the wavefunction pairs, it is necessary to use the coincidence counter to filter for those photon pairs that have been "straightened out" using one orientation and with that set one can create one interference pattern. Then the other setting of POL1 must be used to get at the other set of wavefunction pairs. With that set one can create a second interference pattern. P0M (talk) 13:22, 5 February 2014 (UTC)

I am glad it is clear now (but not "finally", because in the wiki article you can read about the polarization which gives no output rotation, and the wave plate article reference also explained this. DParlevliet (talk) 14:50, 5 February 2014 (UTC)

This is still not true. As I pointed out above already (and you ignored) you can decompose pretty much any non-interference pattern into many interference patterns. This is simple primitive basic optics. Spatial coherence in a double slit is related to the angular width of the source as seen at the position of the double slit. If you make the angular width narrow, you will get an interference pattern. If you make it broad, you will not get an interference pattern. So you can simply create a double slit interference pattern from a situation without interference pattern by placing a pinhole at the appropriate position. If you move it around, you will notice different interference patterns which add up in total to no pattern at all - what you see without placing a pinhole. This is absolutely trivial. As a consequence also the definition is clear and strict: If you do not see an interference pattern, this is called "no interference". It does not matter that you could get one from filtering because that is always and in any situation possible. So, yes: you contradict yourself. Also your comments above about "But from your answer it is clear to me that you are not familiar with calculating with classical optical waves" is nothing but insulting. You have yourself stated clearly that you are far from an expert in qm and your posts clearly reflect that you do not understand the basics of the Walborn experiment. Also you refuse to reply to any comment pointing that out. The fact that single photons are used in this experiment is fundamental and is what makes this experiment interesting. When you start explaining some setting with classical waves you are in fact describing a completely different experiment. So unless you can show that you can describe single photons classically, any other claim that you are actually describing Walborn's experiment is pointless. 129.217.159.124 (talk) 10:04, 5 February 2014 (UTC)

You are right, and I redraw the "not familiar" part. But as I said before, is it a matter of expression. Is a bag of sugar one kilo or two half kilo's? Both is right, but are expressed different. In QM one looks to the output: depending on a visible pattern the path is known or not. That was Walborn's right conclusion and that I wrote in the article without changing or comment. But in classical waves it is common to see it as added interference patterns, indeed also in the wide slit example you mention. Most optics formula are calculated that way. So as I said before, the conclusions between QM and classical are different in this case. Because nobody yet knows what a photon really is, everyone can choose the conclusion he agrees with. About the wave I use the Copenhagen interpretation that a single photon has wave properties, which includes polarisation. DParlevliet (talk) 11:35, 5 February 2014 (UTC)

You are again avoiding the single important point here: Can you reproduce single photons (antibunching) classically? If you cannot, you are describing a completely different experiment. This is a very simple question. So yes or no? 129.217.159.124 (talk) 12:04, 5 February 2014 (UTC)

What do you mean with single photons antibunchin? I am not explaining SPDC classical. DParlevliet (talk) 14:50, 5 February 2014 (UTC)

Of course you are. You are applying classical fields to the experiment in question. A measurement on a classical field is non-invasive. You can repeat the measurement again and again without changing the classical system. This is the essence of classical physics. In quantum physics, you can detect every photon only once. It is gone afterwards. This backaction is the essence of non-classicality. This effect cannot be reproduced classically and is the one effect that makes experiments using single photons interesting. The Walborn experiment relies heavily on the fact that you do not use classical coherent laser beams, but works on the single photon level. So if you claim that there is a classical way to model the experiment, you must model the exact experiment, including single photons. What you are claiming right now is: "I get similar results when replacing single photons by a classical coherent light beam". That is of course true, but trivial and completely misses the surprising element of the experiment. The point of the experiment is that it indeed works for single photons.129.217.159.124 (talk) 15:26, 5 February 2014 (UTC)

Also what do you mean by saying "I use the Copenhagen interpretation that a single photon has wave properties"? Copenhagen is intrinsically non-classical. It seems to me you do not understand the basics about this experiment and you should definitely not edit this article.129.217.159.124 (talk) 12:11, 5 February 2014 (UTC)

I use: "An experiment can show the particle-like properties of matter, or the wave-like properties", see Wikiarticle.DParlevliet (talk) 14:50, 5 February 2014 (UTC)

How does that support your point? Having wave-like properties does not mean that you can take a classical wave-approach. The wave-part comes from quantum field theory. I already pointed you towards that: Wave–particle_duality#Treatment_in_modern_quantum_mechanics. 129.217.159.124 (talk) 15:19, 5 February 2014 (UTC)

Classical wave-like properties can be calculated with classical formula, until a certain limit. Look to the wiki Double slit article, which also has a classical wave part with classical formula. DParlevliet (talk) 16:41, 5 February 2014 (UTC)

The same is the conclusion in QM that the polarisers mark the beam in a way that in principle the path of the photon can be found. But according classical wave(duality) it is not possible with the marked beams to find the photon path. These conclusion differ, but fit with the formula used in Quantum- or classical physics. So in QM articles you will find the first conclusion, in classical books the second. And as long as the formula are right, both can be mentioned in Wiki articles. DParlevliet (talk) 11:35, 5 February 2014 (UTC)

Duality does not mean that you have a classical wave. See for example Wave–particle_duality#Treatment_in_modern_quantum_mechanics for details on how the "wavey" part makes sense only in terms of quantum field theory. To me the changes you made are simply vandalism of the article and pushing some "classical physics is enough to explain seemingly quantum experiments" agenda. We should come to some conclusion and you just try to evade answering questions and do not seem to be willing to admit that you are wrong when you are shown so which makes a discussion with you incredibly cumbersome: do you agree that it would be best to revert the article to the version from 1. October 2013? 129.217.159.124 (talk) 12:27, 5 February 2014 (UTC)

See above. I calculate the "wave-like properties". The article gives conclusion of both QM and classical and the reader may choose or think that both are right in their own area. The article can only be edited when you show that the classical calculation (which give the same result as the QM!) is wrong. DParlevliet (talk) 14:50, 5 February 2014 (UTC)

"wave-like properties" in the Copenhagen interpretation of qm and classical waves are two completely different things. The experiment deals with single photons from an entangled photon source. It is basic knowledge that "The correlation of quantum entanglement can not be explained simply using the concepts of classical physics." (see: Quantum_entanglement) and that "Antibunching, whether of bosons or of fermions, has no classical wave analog." (see Hanbury_Brown_and_Twiss_effect). Your approach does NOT give the same results as qm because your approach does not yield single photons. I have asked you several times now to show explicitly how you get single photons or entangled photons out of your treatment. Up to now you evaded any answer. So please provide evidence that your approach yields single photons. If you cannot, your calculation necessarily is wrong as it does not apply to the experiment in question. 129.217.159.124 (talk) 15:05, 5 February 2014 (UTC)

I told above that I am not explaining SPDC classical, so I don't get single photons or entangled photons. I start after that. I calculated with the wave property of the photon which goes through the double slit.DParlevliet (talk) 15:24, 5 February 2014 (UTC)

So you agree that your calculation is not valid for this experiment? You need to use single photons.129.217.159.124 (talk) 15:29, 5 February 2014 (UTC)

My explantion is about the single photon entering the double slit. The entangled properties I took over from Walborns explanation. DParlevliet (talk) 16:41, 5 February 2014 (UTC)

You cannot simply do some math and take some properties here and there and afterwards claim you did some classical treatment. Once you admit that you are discussing single photons, you cannot go back to a classical treatment because single photons are intrinsically non-classical. If you "take entangled propertiesfrom Walborns explanation" or if you discuss single photons your treatment is automatically not classical anymore. Absurd discussions like this one are the reason why I typically do not bother to work on wikipedia articles. Someone without basic optics knowledge is trying to tell a PhD in quantum optics how to do explain optics experiments correctly. Feel free to continue your vandalism of this article. I will leave this discussion as I do not have the time or patience for this. For the books: I strongly recommend undoing DParlevliet's changes as they are contrary to the scientific consensus as expressed even within the wikipedia articles on the HBT effect and wave-particle duality I already linked to. 129.217.159.124 (talk) 16:57, 5 February 2014 (UTC)

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