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Can someone explain the quantum eraser please?

  1. Oct 10, 2011 #1
    A double slit experiment which has two stages: first the experimenter marks through which slit each photon went, without disturbing their movement, and demonstrates that the interference pattern is destroyed. This stage shows that it is the existence of the "which-path" information which causes the destruction of the interference pattern, and not a mechanical disturbance (as thought at the time of the creation of quantum theory). The second stage goes by erasing the "which-path" information, and demonstrating that the interference pattern is recovered. It does not matter whether the erasure procedure is done before or after the detection of the photons.

    So can anyone dispell the mystisism behind this experiment. I thought Richard Feynman explained that "measuring" a quantum process had nothing to do with a councsiess observer. Please try to explain the reasons as simply as possible to a layman.
    Last edited: Oct 10, 2011
  2. jcsd
  3. Oct 10, 2011 #2


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  4. Oct 10, 2011 #3


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    What difference does a "counciess" observer make in the outcome of this experiment?
  5. Oct 11, 2011 #4


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    This is one of the simplest layman explanations I know:
    https://www.physicsforums.com/blog.php?b=7 [Broken]
    Last edited by a moderator: May 5, 2017
  6. Oct 11, 2011 #5
    Lets say you mark photons at the slits with either having vertical (|) or horizontal (-) polarization. On the screen it appears there is no interference, but there is. Think what happens when the phase between (|) and (-) is zero you get a diagonal polarization like this (/). If the phase is pi then you would have the other diagonal (\) and for pi/2 and 3pi/2 you would get circular (0), so the interference pattern would look like this

    \ 0 / 0 \ 0 / 0 \ 0 /

    instead of the usual
    min max min max min max

    Of course you cant see polarization, so you might say the interference was destroyed, but its still there just in a different form. Add a diagonal polarizer before or after the screen and then you will allow through the (/) and block (\) and then you can see the interference was still there. This is the step referred to as destroying the path information, which is a useful way to look at the situation but I think you were looking for a less abstract explanation.
  7. Oct 11, 2011 #6
    Thenewdeal38, I’ll see if I can dispel the mysticism for you at a higher level. That should make it easier to understand. If you don’t know exactly what QM predicts in the quantum eraser experiment, then it makes it much harder to understand. So I’ll tell you this. The results are just what is predicted by QM regardless of which interpretation you prefer. In other words, this experiment changed nothing except to reinforce the theory.

    Since QM is counterintuitive from our classical point of view, it’s hard to come up with an interpretation that reasonably explains QM. So some interpretations have included strange things like consciousness. On top of that, there are several prominent physicists that like the idea of conciseness playing a roll. I think Bohm is one. He likes “quantum consciousness.” And to top it all off, there are movies like what the bleep do we know and the secret.

    Here’s the thing. If you have a theory like this, it really helps if you can come up with a way to test it. A prediction would be nice. There is none. So why bring something as complicated as consciousness into the discussion? It may be fun to think about but it’s not practical. Like every other fringe theory about physics, there’s no way to disprove it. That doesn’t mean it’s not true. It just means that the likelihood is really, really low. So if you have a high tolerance for this kind of thing, then you might want to dig into the quantum eraser a little deeper and see just how far down the rabbit hole goes.
  8. Oct 12, 2011 #7
    So let me get this straight. You shoot a laser through a double slit with a measuring device before the slit that causes the interference pattern to collapse. Then you stick another meausuring device after the slit with some sort of filter that sepparates the photons form the electrons therefore restoring the interference pattern.

    What I dont understand is why would you use the term "erase" to describe a photon polarizing filter and why would you use the term "information" to describe photon electron interaction. Its extremely confusing and I have heard people talk about time travel when refered to this experiment. As if once youve collected the "information" like on a harddrive you then press the "erase" button and magiccly the completley non local to where the inormation is stored interference pattern reamerges just by deleting the information at a diffrent unnatached to the double slit space time source where the "information" is being "held".
  9. Oct 12, 2011 #8


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    Because if you describe the same experiment in terms of electromagnetic waves and their polarisation, you could at best present it as a high school demonstration and it would never be accepted by Phys.Rev. Even if you put it on arXiv noone would ever cite it.
    Publish or perish - under such pressure many people forget about Occam's rule...

    Thread https://www.physicsforums.com/showthread.php?t=521715&highlight=walborn" already discussed issues of confusion and overinterpretation in "quantum eraser" experiment.
    Last edited by a moderator: Apr 26, 2017
  10. Oct 12, 2011 #9


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    The issue is that the entire context of a setup must be considered, and this means that there are spacetime considerations that can appear nonlocal or reverse causal. There is another experiment which demonstrates the same effects:


    "Abstract: Quantum teleportation strikingly underlines the peculiar features of the quantum world. We present an experimental proof of its quantum nature, teleporting an entangled photon with such high quality that the nonlocal quantum correlations with its original partner photon are preserved. This procedure is also known as entanglement swapping. The nonlocality is confirmed by observing a violation of Bell's inequality by 4.5 standard deviations. Thus, by demonstrating quantum nonlocality for photons that never interacted our results directly confirm the quantum nature of teleportation. "

    "Figure 1: Entanglement swapping version of quantum teleportation. Two entangled pairs of photons
    0–1 and 2–3 are produced in the sources I and II respectively. One photon from each pair is
    sent to Alice who subjects them to a Bell-state measurement, projecting them randomly into one
    of four possible entangled states. Alice records the outcome and hands it to Victor. This procedure
    projects photons 0 and 3 into a corresponding entangled state. Bob performs a polarization
    measurement on each photon, choosing freely the polarizer angle and recording the outcomes. He
    hands his results also to Victor, who sorts them into subsets according to Alice’s results, and checks
    each subset for a violation of Bell’s inequality. This will show whether photons 0 and 3 became
    entangled although they never interacted in the past. This procedure can be seen as teleportation
    either of the state of photon 1 to photon 3 or of the state of photon 2 to photon 0. Interestingly, the
    quantum prediction for the observations does not depend on the relative space-time arrangement
    of Alice’s and Bob’s detection events."

    The effect is nonlocal, and further, you can perform the entanglement swapping operation AFTER the nonlocal effect is detected. None of these is anything other than standard QM though. Ditto with the Quantum Eraser. You are free to interpret it in several different ways.
  11. Oct 12, 2011 #10
    First off, I just want to say that there’s a better version of this experiment called the delayed choice quantum eraser. I think that might be what you really want.

    I don't know of anything about electrons in the quantum erasor experiment. If you see that somewhere else, tell me where and I'll take a look.

    Wow, not until this moment did I realize how bad that wikipedia page is. For one thing, it talks about the order of events and it’s flat out wrong. (And I’m not about to try fixing it.) I’ll see if I can explain it myself here for you. So the double slit sets up a nice little problem that demonstrates wave-particle duality. The interference pattern makes it look like photons always go through both slits just like waves. If you block one slit, you lose the pattern. But if you put detectors in both slits, only one dings as if the photon is a particle. This is called the which-way information.

    If you have 2 entangled photons, then you can do it one way to one of them and the other way to the other. This means that you can get both the which-way info and the interference pattern. Now, if you move the wave side of the experiment farther away, then you might think that you get the which-way info first and therefore the interference pattern should get destroyed. Then again, maybe the photons on the wave side don’t know that the which-way info has been captured. So they go right ahead and make the pattern. This means that the which-way info has been erased.

    So that’s the basic setup. To explain this further takes a lot of typing. So I’ll let you do some reading and wait for your follow-up questions. Warning: My explanations tend to involve special relativity.

    So hopefully I have explained the use of “erase” and “information” above enough.

    Yeah! It’s extremely confusing! You have picked a very difficult topic here. But hang in there! You will be amazed by yourself and by what you now understand about the world once you understand this.

    What you’re describing here about pressing the erase button requires a particular interpretation of QM that is “non-local”. Not all interpretations are non-local. But all interpretations have some counterintuitive part to them that makes them hard to believe. And there’s no experiment to tell you which interpretation is correct. Your harddrive example is a good one. Please tell me where you got that and I can help you with that more (assuming I don’t get some actual work to do).
  12. Oct 12, 2011 #11
    This will show whether photons 0 and 3 became entangled although they never interacted in the past.
    HOW? I thought Alice entangled them when she performed this:
    One photon from each pair is sent to Alice who subjects them to a Bell-state measurement, projecting them randomly into one of four possible entangled states.

    Interestingly, the quantum prediction for the observations does not depend on the relative space-time arrangement of Alice’s and Bob’s detection events." I think thats a false assumption that suggests time travel teleportation. I thought Alice separates already entangled photons and reentangles them with diffrent photons then bob separtes them again. What does this have to do with teleportation or time travel?
  13. Oct 13, 2011 #12


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    Absolutely nothing. Alice sorts photon pairs in different subsets. Within each subset correlations characteristic to "entanglement" are observed.
  14. Oct 13, 2011 #13
    Then why is it considered "non-local"?
  15. Oct 13, 2011 #14


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    Well, I guess one presents open problem in provocative manner to attract attention to it (and to get funding).
  16. Oct 13, 2011 #15


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    The Bell State Measurement (BSM) is on photons 1 and 2, which causes spatially separated (and therefore nonlocal) photons 0 and 3 to become entangled. The BSM can be performed AFTER 0 and 3 are already detected, which means they were entangled BEFORE they were placed into the entangled state by the BSM. That is a reversal of causality.

    And the relevance is that the causality reversal is similar to what happens in an eraser.
  17. Oct 13, 2011 #16
    (DrChinese, please correct me if I’m wrong. I don’t know quantum teleportation too well.)

    Thenewdeal38, although things don’t happen in the order you expect, this does not mean you can communicate with the past. Although QM may allow for traveling backwards through time, it has never been demonstrated. I don’t want to disappoint you but you may need to limit your expectations.

    Just to recap this at a higher level, in every case, there are at least 2 possible explanations for the counterintuitive weirdness that happens in a QM experiment. One is where information is transferred instantly across space. This brakes locality (special relativity). The other is where information takes some strange path through time, which breaks causality. These 2 possibilities are essentially the conclusion of Bell’s theorem. You will find that everyone has their preference. So if someone tells you that something is caused by breaking locality or by breaking causality, then you should understand that it’s just an interpretation.

    When DrChinese tells you that things are happening in reverse, I don’t think he means that it’s breaking causality. I think he means that it appears to break causality. When someone says “non-local,” they don’t necessarily mean that it breaks locality. It just means that there’s a sufficient distance between events. In other words, one event could not have caused the other without breaking special relativity. Ya follow me? Physicists (of which I am not one) have narrower definitions for terms that can make these things even harder to understand.

    (Note to self: Study difference between reality, causality and counterfactual definiteness.)
  18. Oct 13, 2011 #17


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    Pretty much says it all. We don't know exactly what the mechanisms are, and there are several interpretations of the results that are viable. However, none of these follow a classical locally causal model. You always lose at least locality or causality. Or as is often stated (a la Bell): you lose locality or realism. Can't really continue to claim them both.

    In the various experiments, this element is highlighted so that it becomes impossible to maintain them both. And yet, these are straight predictions from Quantum Theory, circa 1935.
  19. Oct 13, 2011 #18
    "Nevertheless, the explanation is actually very simple, provided that you allow the existence of something that you do not see (the analog of Bohmian hidden variables in quantum mechanics)"

    but there aren't hidden variables, so the analogy is bogus.
    Last edited by a moderator: May 5, 2017
  20. Oct 13, 2011 #19
    The hidden variables in the Bohmian interpretation are not like the Einstein's hidden variables from the EPR paradox. The Bohmian hidden variables are pilot waves that go backwards through time to set the angle of photon spin right from where they originate. It's still considered a legitimate interpretation.
  21. Oct 14, 2011 #20
    From what I understand there is a sort of unknown connection between the other (not entangled to each other) digits of two diffrent yet related entangled particles like a sort of super entanglemnt network. It is obviously breaking locality where information is transferred instantly across space. The time reality thing cant be true.
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