# 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:** 17\ **Page:** 4
**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, 6:58pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/61 "2013-02-10T18:58:10Z")
> [@ZenBeam](#): > > :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: After eq 14, you can recover the interference pattern by the following method, which is what they did. I guess you didn’t follow the experiment too closely. > [@](#): > > According to the above expression, we can recover interference > projecting the state of photon p over u1&p or u2&p . > Experimentally, this can be done by placing a polarizer in the > path of beam p and orientating it at 145° to select u1&p or at > 245° to select u2&p . The interference pattern is recovered > through the coincidence detection of photons s and p. Notice > that the fringes obtained in the two cases are out of phase. > They are commonly called fringes and antifringes. ---
**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, 7:01pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/62 "2013-02-10T19:01:58Z")
> [@Bob\_X](#): > > The way it is described, it sounds as if you can FORCE the result to be wrong. Or is it the case that, for speed-of-light reasons, there can never be enough time to switch the polarizor OFF if the s-path photon indicated it was ON and vice versa? ---
**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, 7:06pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/63 "2013-02-10T19:06:10Z")
> [@Bob\_X](#): > > Or is it the case that, for speed-of-light reasons, there can never be enough time to switch the polarizor OFF if the s-path photon indicated it was ON and vice versa? If you make the p path long enough, I don’t see why not. A fiber optic loop should work. I know that’s been done before. ---
**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, 7:23pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/64 "2013-02-10T19:23:59Z")
> [@deltasigma](#): > > If you make the p path long enough, I don’t see why not. A fiber optic loop should work. I know that’s been done before. Interesting question, but since you know it’s been done before, what were the results? **Half Man Half Wit** and others knowledgeable about QM: If the fiber optic loop was so long it took 2 years for the particle to get to the polarized plate, and the polarization was switched 1.9 years after after S particle was detected, what would we observe prior to the polarization change? Is the problem with this approach that our observation of the S particle result prior to the polarization change un-entangles S and P? ---
**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, 7:47pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/65 "2013-02-10T19:47:05Z")
I thought it was obvious that what I meant was that fiber optic loops had been used as a means to extend the travel times of photons in experiments - not that this particular experiment had been done that way. Certainly if it had been, I would have shared that information, but I suppose I could have been more explicit. If the result really is time independent, then it should agree with whatever position the p polarizer ends up in. If it doesn’t, then maybe the result has to do with spatial or temporal coherence. But I don’t really understand those concepts well enough to speculate. ---
**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, 7:53pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/66 "2013-02-10T19:53:09Z")
> [@deltasigma](#): > > If the result really is time independent, then it should agree with whatever position the p polarizer ends up in. If it doesn’t, then maybe the result has to do with spatial or temporal coherence. But I don’t really understand those concepts well enough to speculate. I’m guessing, but I think the answer is as follows: 1 - If we wait 2 years to look at the S results then the experiment is identical to the OP 2 - If we look at S results prior to 1.9 years (when the polarized plate switches) then we have just altered the experiment by observing S causing the wave function collapse etc. ---
**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, 8:51pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/67 "2013-02-10T20:51:57Z")
> [@deltasigma](#): > > After eq 14, you can recover the interference pattern by the following method, which is what they did. First off, it’s at +45 and -45 degrees, not 145 and 245 degrees. > [@deltasigma](#): > > I guess you didn’t follow the experiment too closely. Nice. Yes I did follow it. See my post 52 (again): > [@ZenBeam](#): > > 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). Anyway, I’m done talking to you. > [@Bob X](#): > > Or is it the case that, for speed-of-light reasons, there can never be enough time to switch the polarizor OFF if the s-path photon indicated it was ON and vice versa? In principle, you can perform the experiment in a low noise environment, so that all of the _s_ photons detected are entangled with some _p_ photon, even though you don’t know its polarization. (That is you don’t need the coincidence detection to determine if the photon is part of your experiment, versus some extraneous photon.) If you wait two years to measure the _p_ photons, you have no way of separating the observed _s_ photons during that time. So you only have the sum of all _s_ photons detected, which shows no fringing. You can only separate them once you’ve brought the measurements of the _p_ and _s_ photons together. Then you can split the _s_ photons into x+y and x-y photons, which will then give the fringing and antifringing patterns. ---
**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, 9:22pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/68 "2013-02-10T21:22:52Z")
> [@ZenBeam](#): > > First off, it’s at +45 and -45 degrees, not 145 and 245 degrees. > > Nice. Yes I did follow it. See my post 52 (again): > Anyway, I’m done talking to you. > In principle, you can perform the experiment in a low noise environment, so that all of the _s_ photons detected are entangled with some _p_ photon, even though you don’t know its polarization. (That is you don’t need the coincidence detection to determine if the photon is part of your experiment, versus some extraneous photon.) If you wait two years to measure the _p_ photons, you have no way of separating the observed _s_ photons during that time. So you only have the sum of all _s_ photons detected, which shows no fringing. You can only separate them once you’ve brought the measurements of the _p_ and _s_ photons together. Then you can split the _s_ photons into x+y and x-y photons, which will then give the fringing and antifringing patterns. OMG, you never even bother to look at the original paper did you? I cut and pasted that from Adobe Acrobat and didn’t bother to check it. If you look at the original it says +45 and -45. When I did the cut and paste the plus and minus were corrupted. LOL!!! If you had actually looked at the original paper you would have known that. I’m grateful for the fact that you won’t be speaking to me. ---
**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, 9:41pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/69 "2013-02-10T21:41:25Z")
> [@deltasigma](#): > > I thought it was obvious that what I meant was that fiber optic loops had been used as a means to extend the travel times of photons in experiments - not that this particular experiment had been done that way. Certainly if it had been, I would have shared that information, but I suppose I could have been more explicit. > > If the result really is time independent, then it should agree with whatever position the p polarizer ends up in. If it doesn’t, then maybe the result has to do with spatial or temporal coherence. But I don’t really understand those concepts well enough to speculate. OK, so it sounds as if every time the s-path photon ends up in a position which would have higher probability if the p-path had polarizer ON, you can turn the p-path OFF, and vice versa, and thereby force this experiment to come out wrong. Am I misunderstanding something? ---
**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, 10:18pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/70 "2013-02-10T22:18:46Z")
> [@Bob\_X](#): > > OK, so it sounds as if every time the s-path photon ends up in a position which would have higher probability if the p-path had polarizer ON, you can turn the p-path OFF, and vice versa, and thereby force this experiment to come out wrong. Am I misunderstanding something? I’m not sure what you mean. If the quarter wave filters are in place and the p polarizer is in place, you should get the interference pattern. It’s not a matter of probabilities. If the p polarizer is not in place, the no pattern. ---
**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, 10:42pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/71 "2013-02-10T22:42:13Z")
> [@RaftPeople](#): > > **Half Man Half Wit** and others knowledgeable about QM: > If the fiber optic loop was so long it took 2 years for the particle to get to the polarized plate, and the polarization was switched 1.9 years after after S particle was detected, what would we observe prior to the polarization change? I’m not sure I understand you correctly, but if you look at the s detections at any time without taking into account the detections from the p-path detector, then you will not observe an interference pattern. Only if you bring both into coincidence can you observe the 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, 10:46pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/72 "2013-02-10T22:46:09Z")
> [@Half\_Man\_Half\_Wit](#): > > I’m not sure I understand you correctly, but if you look at the s detections at any time without taking into account the detections from the p-path detector, then you will not observe an interference pattern. Only if you bring both into coincidence can you observe the pattern. The fact that each detector records each hit I’m pretty sure qualifies. ---
**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 11, 2013, 7:18am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/73 "2013-02-11T07:18:37Z")
> [@deltasigma](#): > > I’m not sure what you mean. If the quarter wave filters are in place and the p polarizer is in place, you should get the interference pattern. It’s not a matter of probabilities. If the p polarizer is not in place, the no pattern. I’m not sure why you can’t understand my question. Whether the p-polarizer is or isn’t in place is something you can set either way-- and you can set it either way AFTER the s-path photon has hit the screen. So, can you make the decision deliberately go the opposite of what the s-path photon told you? ---
**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 11, 2013, 7:42am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/74 "2013-02-11T07:42:18Z")
Just to be clear, you don’t observe interference patterns in single individual photons. This [video](http://www.youtube.com/watch?v=MbLzh1Y9POQ) should give you an idea of how interference patterns are observed when you pass one photon at a time through a double slit. But as **HMHW** has already pointed out it’s the coincident count that actually appears on the graphs that demonstrate interference. If you don’t have knowledge of that then you can’t the interference pattern from detector D[sub]s[/sub]. ---
**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 11, 2013, 8:13am UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/75 "2013-02-11T08:13:32Z")
> [@deltasigma](#): > > The fact that each detector records each hit I’m pretty sure qualifies. With the polarizer in front of it, the p-path detector records only about half the photons the s-path detector does; this information tells you which s-detections to keep, so to speak, in order to build the interference pattern. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [February 11, 2013, 12:11pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/76 "2013-02-11T12:11:59Z")
Some questions about the double slit experiment: 1. When we are sending electrons one by one, it appears as if the same electron passed through both slits at the same time. But how many hits are registered on the screen? Is it just one or two? Or we do not have the technology to detect single electrons so the need to make several iterations of the experiment to get the interference pattern? 2. In all those double slit experiments the polarizers are placed before the slits. Would it make any difference if they were placed right after the slits? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 11, 2013, 12:53pm UTC](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570/77 "2013-02-11T12:53:38Z")
> [@Dog80](#): > > Some questions about the double slit experiment: > > 1. When we are sending electrons one by one, it appears as if the same electron passed through both slits at the same time. But how many hits are registered on the screen? Is it just one or two? Or we do not have the technology to detect single electrons so the need to make several iterations of the experiment to get the interference pattern? One. > [@](#): > > 1. In all those double slit experiments the polarizers are placed before the slits. Would it make any difference if they were placed right after the slits? I assume by “polarizer” you mean the quarter wave plates in the _s_ beam. Then no, it shouldn’t matter. (There’s possibly a _small_ difference in the number of photons transmitted depending on if they are polarized parallel to the slits or perpendicular. That could lead to a small difference if the order of the quarter wave plates and the slits were swapped. But at the level of this discussion, it’s not important.) [Previous page](https://boards.straightdope.com/t/please-explain-the-delayed-erasure-form-of-the-quantum-erasure-experiment/649570.md?page=3)