# Please explain the 'delayed erasure' form of the quantum erasure experiment **URL:** \ **Category:** Factual Questions\ **Created:** [February 7, 2013, 7:32am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570 "2013-02-07T07:32:31Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [February 10, 2013, 12:46am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/41 "2013-02-10T00:46:01Z")
> [@deltasigma](#): > > The OP is more complicated since operations performed on one photon are used to manipulate the other. Also in the OP there are at least 3 different types of modifications to the polarization. But in what substantial way do you think the **results and what can be understood about QM** are different? Can’t they be equivalent but not identical? ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 10, 2013, 12:49am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/42 "2013-02-10T00:49:29Z")
Those are very good questions. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 10, 2013, 1:09am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/43 "2013-02-10T01:09:51Z")
[The delayed quantum erasure experiment](http://grad.physics.sunysb.edu/~amarch/Walborn.pdf) and the [delayed choice quantum eraser](http://arxiv.org/pdf/quant-ph/9903047v1.pdf), both illustrate the same physical principles. The difference between the two experiments is how the ‘which path’ information is marked and that the pattern is observed depends on initially choosing one of a variety of different experimental set-ups in the first whereas the choice in the second is how the data is combined after the experiment has been performed. ---
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [February 10, 2013, 4:04am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/44 "2013-02-10T04:04:23Z")
> [@Senegoid](#): > > Or, to put it as my big brother, the math major, did: It’s all in the mathematics. The math involved is beyond mind-bending for anyone other than a very advanced mathematician and physicist, and provides the “mathematical” explanation of quantum phenomena. My brother’s point was, it’s all just so alien (in light of our normal experience) that there’s just no real macro-world metaphor or analogy that can compare to quantum phenomena. You just gotta do the mind-bending math, and then you just gotta understand it from that, and believe that it works like the math says it does, just because the math says it does. > > Then you go out and do some real-life experiments, like these quantum erasure games, and lo and behold, you find that you really get the incomprehensible results that the mind-bending math says you get. And that may just be about as comprehensible as it gets. This is one of the best summaries of quantum mechanics I have ever heard. And it’s really quite true. The world we perceive just does not match up with the quantum world. You have to accept that the math does; it expresses what normal human language and thought cannot. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 10, 2013, 4:54am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/45 "2013-02-10T04:54:19Z")
> [@RickJay](#): > > This is one of the best summaries of quantum mechanics I have ever heard. And it’s really quite true. The world we perceive just does not match up with the quantum world. You have to accept that the math does; it expresses what normal human language and thought cannot. I sort of agree up to a point, It is very correct to think of the maths of being central importance. If you look at the postulates of QM they’re nearly all of them formulated mathematically. For example one postulate describes what happens when you combine two quantum mechanical systems together and references something called the “tensor product”. There’s no real interpretation for that except mathematical, though quantum entanglement comes directly and straightforwardly from this postulate. Maths itself could be expressed purely in English without the need for symbols, but it would be extremely tedious and unwieldy and, beyond the very basics, understanding one mathematical definition requires you to understand several other more basic definitions. However I would say the maths for basic quantum mechanics is not anywhere near as hard as many assume it is. ---
**Author:** ![Bob\_X](https://avatars.discourse-cdn.com/v4/letter/b/4491bb/32.png) [@Bob\_X](https://boards.straightdope.com/u/Bob_X)\ **Post date:** [February 10, 2013, 5:21am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/46 "2013-02-10T05:21:36Z")
> [@deltasigma](#): > > BobX: I’m not sure that would be any different. In both cases, the photon in the p path doesn’t “know” what kind of polarizer it’s going to hit until it actually does. > > Even so, it might be an interesting question to investigate any time dependent aspect of the experiment. I suppose if there were a way that you could change the polarizer on a nanosecond or femtosecond basis and you got a different result, that would be pretty interesting. The way it is described, it sounds as if you can FORCE the result to be wrong. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [February 10, 2013, 8:16am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/47 "2013-02-10T08:16:58Z")
> [@deltasigma](#): > > Clearly you haven’t even looked at the link in the OP. They aren’t even remotely the same so until I have some indication that you actually know what you’re talking about I won’t be responding to you. They’re exactly equivalent. The detector on the p-path in the Walborn et al. version is equivalent to detectors 1 and 2 in the wiki version if the polarizer is placed in front of it, and thus, it doesn’t record which-path info, and equivalent to detectors 3 and 4 if there’s no polarizer and which-path info is present. The reason an interference pattern appears at the detector in the s-path is that the polarizer only permits about half of all photons to pass through, such that only half the photons arriving at the s-path detector are counted (because only they are in coincidence with counts in the p-path). If there were no coincidence counting, i.e. you were to just observer the photons incident on the detector in the s-path, you would never observe an interference pattern. You can see this by considering the ‘antifringe’ pattern that is generated when the polarizer is rotated by 90°: compared to the fringe pattern, it is slightly shifted, and effectively, it is the pattern generated when you reject the ‘other half’ of the photons. And adding fringe and antifringe patterns—counting all photons at s—gives again the interference-free distribution. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [February 10, 2013, 8:23am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/48 "2013-02-10T08:23:02Z")
> [@RaftPeople](#): > > A couple points: > 1 - **Half Man Half Wit** , in my experience, is not only very knowledgeable, he is one of the most polite and unbiased posters on this website. I think you might want to give him a little more credit than you seem to be. Oh, and thanks! (But really, I wouldn’t want anybody believing what I wrote just because of my perceived ‘authority’… It’s always better to ask if anything’s not clear.) ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [February 10, 2013, 8:54am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/49 "2013-02-10T08:54:16Z")
I’ve a question. In many of these experiments, theorists _already know_ what to expect, and run the experiment in part just to demonstrate that reality really is as “spooky” as predicted. Is that the case in the experiment of OP? ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [February 10, 2013, 9:57am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/50 "2013-02-10T09:57:34Z")
> [@septimus](#): > > I’ve a question. In many of these experiments, theorists _already know_ what to expect, and run the experiment in part just to demonstrate that reality really is as “spooky” as predicted. > > Is that the case in the experiment of OP? Yes. The behavior was exactly as predicted; anything else would have been surprising (the possibility of such a surprise being the reason one runs these experiments). ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [February 10, 2013, 11:08am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/51 "2013-02-10T11:08:40Z")
> [@Half\_Man\_Half\_Wit](#): > > Yes. The behavior was exactly as predicted; anything else would have been surprising (the possibility of such a surprise being the reason one runs these experiments). Or to quote Isaac Asimov yet again: The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka” but “That’s funny…” ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 10, 2013, 2:50pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/52 "2013-02-10T14:50:18Z")
In the OP’s link, there are two paragraphs that are at best misleading, that I think it’s important to understand: > [@](#): > > In case you might be suspicious of the quarter wave plates, it is worth noting that given a beam of light incident on a double slit, changing the polarization of the light has no effect whatsoever on the interference pattern. The pattern will remain the same for an x polarized beam, a y polarized beam, a left or a right circularly polarized beam. This paragraph is true if the beam passing through _both_ of the slits is x-polarized OR y-polarized OR left-circularly polarized OR right-circularly polarized. But in their setup, they have left-circularly polarized passing through one slit and right-circular through the other, destroying the interference pattern. They would have similarly not have had an interference pattern if they had x-polarized light passing through one slit and y-polarized light passing through the other. > [@](#): > > It is peculiar then, that the presence of the quarter wave plates causes the s photons to so drastically change their behavior. One can’t help but ask, how do these photons know that we could know which slit they went through? The simple answer is that left-circular polarized photons won’t interfere with right-circular photons in that way. Nor will x-polarized photons interfere with y-polarized photons (x and y perpendicular). Note that the authors Walborn et al in the paper **Asympotically fat** linked to understand this: > [@Walborn et al](#): > > Since Psi1 and Psi2 have orthogonal polarizations, there is no possibility of interference. The polarization for the beam incident on the quarter wave plates could be at any angle, but’s helpful to consider four possibilities: x, y, x+y, and x-y (so every 45 degrees). Without the quarter wave plates, for any polarization, a fringe pattern would be observed, with the peaks and valleys in the same location. It doesn’t matter, then, if you know the polarization. For x or y beams, after the quarter wave plate the two slits have orthogonal polarizations. For an x+y polarized beam, you get the fringing pattern, and for an x-y beam you get antifringing (see figures 4 and 5 in Walborn et all). The antifringing looks just like fringing, except that the locations of the peaks and valleys are swapped. Without the polarizer in beam _p_, you’d have all four cases equally likely, and since the fringing and anti-fringing have their peaks on the others valleys, you don’t see any fringing in their sum. When you put in the polarizer, you only have x+y (or x-y if you rotate it 90 degrees), so you do get the fringing. You could also imagine _measuring_ the polarization of beam _p_, and keeping track the s-beam pattern for all four cases separately. This ties in well with **Half Man Half Wit** ’s conclusion above: > [@Half Man Half Wit](#): > > But this is the crucial thing: the detections of the different detectors must be brought into coincidence in order to observe the interference pattern. If only the counts at the detector D0 were analyzed, no interference pattern would be observed, ever. This is the reason you can’t send a signal into the past. That is, the physical pattern of photons incident on D0 does not change due to the choice made later on; the knowledge of the which-path info merely changes the probability distribution used to describe the detections. The “If only the counts at the detector D0 were analyzed” case corresponds to analyzing the pattern of the s-beam without including the polarization information. “brought into coincidence in order to observe the interference pattern” corresponds to looking at only one of the four polarization possibilities (or, in the actual experiment, adding the polarizer). ---
**Author:** ![whitetho](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whitetho/32/1059_2.png) [@whitetho](https://boards.straightdope.com/u/whitetho)\ **Post date:** [February 10, 2013, 3:33pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/53 "2013-02-10T15:33:58Z")
> [@FuzzyOgre](#): > > The resolution of what is actually going on will be reality shaking. That year there will be no doubt as to who gets the Nobel prize. You have to be alive to be awarded the prize, so Schrödinger’s living cat is eligible, while the dead one is not. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 10, 2013, 4:10pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/54 "2013-02-10T16:10:54Z")
> [@ZenBeam](#): > > In the OP’s link, there are two paragraphs that are at best misleading, that I think it’s important to understand: > This paragraph is true if the beam passing through _both_ of the slits is x-polarized OR y-polarized OR left-circularly polarized OR right-circularly polarized. But in their setup, they have left-circularly polarized passing through one slit and right-circular through the other, destroying the interference pattern. They would have similarly not have had an interference pattern if they had x-polarized light passing through one slit and y-polarized light passing through the other. > > The simple answer is that left-circular polarized photons won’t interfere with right-circular photons in that way. Nor will x-polarized photons interfere with y-polarized photons (x and y perpendicular). > > Note that the authors Walborn et al in the paper **Asympotically fat** linked to understand this: > > The polarization for the beam incident on the quarter wave plates could be at any angle, but’s helpful to consider four possibilities: x, y, x+y, and x-y (so every 45 degrees). Without the quarter wave plates, for any polarization, a fringe pattern would be observed, with the peaks and valleys in the same location. It doesn’t matter, then, if you know the polarization. > > For x or y beams, after the quarter wave plate the two slits have orthogonal polarizations. For an x+y polarized beam, you get the fringing pattern, and for an x-y beam you get antifringing (see figures 4 and 5 in Walborn et all). The antifringing looks just like fringing, except that the locations of the peaks and valleys are swapped. Without the polarizer in beam _p_, you’d have all four cases equally likely, and since the fringing and anti-fringing have their peaks on the others valleys, you don’t see any fringing in their sum. When you put in the polarizer, you only have x+y (or x-y if you rotate it 90 degrees), so you do get the fringing. I think you might be wrong about that. The point is that each photon interferes with ITSELF, not with other photons of different polarizations. But even if we assume that were the case, I don’t think it would matter. I’m not sure I completely understand the optics but here are a couple of things maybe you would like to look at. The first is open access, the second you would probably have to get through a university library. [http://arxiv.org/pdf/0904.0204.pdf](http://arxiv.org/pdf/0904.0204.pdf) > **[AJP - AJP Website landing](https://www.aapt.org/publications/ajp/index.cfm)** > > AJP Website landing ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 10, 2013, 4:28pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/55 "2013-02-10T16:28:50Z")
> [@deltasigma](#): > > The point is that each photon interferes with ITSELF, not with other photons of different polarizations. This is correct. With the polarizer in place in the _p_ beam to give _x_ polarized (or _y_ polarized) photons incident on the quarter wave plates, _each photon_ is a superposition of left-circular through one slit and right circular through the other slit. Those two “components” of the photon are orthogonal to each other, so you don’t get the self-interference pattern. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 10, 2013, 4:40pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/56 "2013-02-10T16:40:28Z")
> [@ZenBeam](#): > > This is correct. With the polarizer in place in the _p_ beam to give _x_ polarized (or _y_ polarized) photons incident on the quarter wave plates, _each photon_ is a superposition of left-circular through one slit and right circular through the other slit. Those two “components” of the photon are orthogonal to each other, so you don’t get the self-interference pattern. x just means horizontally polarized and y just means vertically. Orthogonal just mean ‘at right angles’ and I don’t think that applies when comparing circular polarization to linear polarization - unless you can show me something that demonstrates otherwise. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 10, 2013, 4:43pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/57 "2013-02-10T16:43:56Z")
> [@deltasigma](#): > > I think you might be wrong about that. Actually, what do you mean by “that”? One particular sentence? Every sentence in the entire post? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 10, 2013, 4:47pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/58 "2013-02-10T16:47:54Z")
> [@deltasigma](#): > > Orthogonal just mean ‘at right angles’ and I don’t think that applies when comparing circular polarization to linear polarization - unless you can show me something that demonstrates otherwise. [Orthogonal](http://en.wikipedia.org/wiki/Orthogonal) means much more than just “at right angles”. See especially [Orthogonal Functions](http://en.wikipedia.org/wiki/Orthogonal_functions). ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 10, 2013, 4:53pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/59 "2013-02-10T16:53:38Z")
> [@ZenBeam](#): > > [Orthogonal](http://en.wikipedia.org/wiki/Orthogonal) means much more than just “at right angles”. See especially [Orthogonal Functions](http://en.wikipedia.org/wiki/Orthogonal_functions). Even so, can you show me a cite for your position? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 10, 2013, 6:45pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/60 "2013-02-10T18:45:02Z")
> [@deltasigma](#): > > Even so, can you show me a cite for your position? :dubious: Cite for what, exactly? Cite that orthogonal means more than at right angles? Right there in my post that you quoted. Cite that the wavefunctions going through each of the two slits with the quarter-wave plates in place are orthogonal? Right there in my post 52: > [@ZenBeam](#): > > Note that the authors Walborn et al in the paper **Asympotically fat** linked to understand this: > > > [@Walborn et al](#): > > > > Since Psi1 and Psi2 have orthogonal polarizations, there is no possibility of interference. [Previous page](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570.md?page=2) [Next page](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570.md?page=4)