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Quantum eraser experiment - another thought experiment

  1. Mar 17, 2013 #1
    First of all, I am new to this forum but I already appreciate the great efforts invested here and the quality of many of the discussions!! This is all pretty neat!

    My mind has just been blown by reading about the "Quantum Eraser" experiment..
    It shows that it is the availability of the "which-path" information (which slit has the particle gone through?) that makes the particle wave collapse, and not the existence of the detectors..

    How far can we push this? Or (metaphorically) how far is nature ready to go to prevent us from accessing both the position and the momentum information at the same time.

    So if we do the double slit experiment in the version where there is a detector that tells you which slit each particle went though.. We do the experiment in a closed room where no-one is inside. And we record the outcome of the detector in the RAM of a PC.. We also record whatever shows on the screen in a video footage.
    Then, before getting into the room, we unplug the PC so that all the detector information is irreversibly lost.. and we finally play the video footage.
    What will we see on the recorded video? A two band pattern? Or an interference pattern?

    If it is the former, then where do we draw the line between this RAM experiment and the Quantum Eraser experiment?
  2. jcsd
  3. Mar 17, 2013 #2
    we see two bands as there is no interference because the detector got which-way info.

    the pc, video etc is irrelevant

  4. Mar 18, 2013 #3
    Wrong San K, Even if the detectors get the which-way info, if this information is not delivered in a way in which(someone) you can know the information, it will still produce the interference pattern. It is not the detectors that change the outcome of the experiment.
  5. Mar 18, 2013 #4
    I read of an experiment (similar to your example above) where the results produced 2 bands because the 'which path' info was recorded. When the recording was 'erased' (the ram was destroyed) the 2 bands that were recorded instantly (i'm assuming quantum entanglement) changed to a interference pattern.
  6. Mar 18, 2013 #5


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    Correct! As a help to the poster opening this thread: It should be stressed that what is erased in the quantum eraser experiment are usually some markers (like polarization) which would give information if one performed a measurement. However, these are "reversible" markers, so you can get rid of them again. However, whether these are present at the detector (or detectors in more complicated versions like delayed choice) is the question that matters.

    When you perform a measurement, this is an irreversible interaction. Nothing you will do afterwards will influence what has been detected.

    So I assume you have some reference to the actual publication? I somehow doubt that, because what you mention is exactly what is quoted on fringe and crackpot sites, but has nothing to do with what is happening in experiments. Please stop promoting crackpot stuff. These forums are not the right place for that.
  7. Mar 19, 2013 #6
    Cthugha, I am not saying I believe it, but I will try and find something on it.

    If the 'which path' info is not recorded, then it remain an interference pattern correct?
    because I know the detectors can stay on, but if the path is not recorded it remains an interference pattern.

    Were there ever any experiments where you can view (live) the 'which path' information but it never gets recorded(on something)?
  8. Mar 19, 2013 #7


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    First, as a disclaimer to people just reading this topic: all these patterns are just visible in coincidence counts, not in the signals at either side alone.

    No. The interesting question is whether there has been some measurement of the which-path information. Whether the results of this measurement are recorded, thrown away or not looked at at all, does not matter.

    This is why the delayed choice experiments rely on reversible interactions which do not constitute a measurement. One sets a marker by e.g. placing a polarizer close to one slit. This does not constitute a measurement and can be reverted before a measurement is performed. What is erased is the possibility to detect which-way information if one performed a measurement, which is very different from actually making a measurement and throwing away the result.

    Well, viewing the pattern is some way of recording it.
  9. Mar 19, 2013 #8
    Cthugha, how do you account for detectors being left on but the wire(signal) so that it can be recorded or looked at is broken(blocked) results in a interference pattern?

    This contradicts your statement "The interesting question is whether there has been some measurement of the which-path information. Whether the results of this measurement are recorded, thrown away or not looked at at all, does not matter."

    So would the detectors being left on but not being able to see the results be considered a measurement?
  10. Mar 19, 2013 #9


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    Huh? There is no experiment showing that the wire being connected or broken making a difference. A photon hitting some detector is a measurement. Some microscopic absorption event is enough. Whether this event is carried away using a wire, shown on a screen or broadcasted by CNN does not have anything at all to do with the act of measuring.

    Where is the contradiction? If you have any peer reviewed publication or at least a credible source stating that connecting wires makes a difference, please post it. It is just strange that those claims only show up on crackpot sites without even making any attempt to show such an experiment.

    Of course. As long as the detector still absorbs photons, you can even switch it off or replace it by a slice of bread. That would still be considered a measurement.
  11. Mar 19, 2013 #10
    It requires a measurement to determine which slit the particle goes through. And a measurement occurs with an interaction. As soon as there is an interaction the wave function collapses and there is no more interference pattern. So it does not matter whether the interaction is recorded or not, or if or when it is saved or erased on computer or not, or observed by an consciouis observer or not. It's the interaction with the detector that collapses the wavefunction and ruins the interference pattern.
  12. Mar 20, 2013 #11
    Cthugha you should be ashamed of yourself! As a science advisor in this forum you should not be preaching your bias of certain ideas. You should be showing the facts and let students decide themselves what it means.

    What am I talking about? Based on your replies given to me and others around this forum, you claim the measuring device (detector) interferes and causes the wave to collapse.

    It’s funny how when you go read articles on Wikipedia like “Wheeler's delayed choice experiment” and “Quantum eraser experiment” the Wikipedia writers/researchers do not agree with you. This is how they word it:

    Wheeler's delayed choice experiment
    “Several implementations of the experiment 1984-2007 showed that the act of observation ultimately determines whether the photon will behave as a particle or wave, verifying the unintuitive results of the thought experiment.”

    Quantum eraser experiment
    “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.”

    If it was so obvious that it’s the detectors causing this then they would clearly indicate this. ( because Wikipedia science writers are materialists just like you) They would say “the detectors are interfering with the wave causing it to collapse ” but they don’t.

    The “Delayed choice quantum eraser” clearly demonstrates that it is NOT the detectors themselves that cause the collapse of the wave causing an interference pattern. It is the possibility of ‘knowing’ the which path information.

    So shame on you for preaching your bias views just to give your ego a cookie, or slice of bread for that matter.
  13. Mar 20, 2013 #12
    Cthugha is correct. Instead of quoting Wiki you should site your unorthodox source. The standard interpretation as I understand it is exactly what Cthugha said. The polarizers that are used to 'mark' the photons are a reversible unitary interaction which does not provide any classical information. The interference pattern only goes away if a projective measurement is made. That is how the information is recorded.

    There are possible loopholes and other interpretations, but I have not heard what you say from anywhere but the Kakus of the world.
  14. Mar 20, 2013 #13


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    I do not think so.

    These forums are devoted to discussions of credible, peer reviewed mainstream physics (also called: the facts). If you consider the mainstream opinion as biased and have a good reason for that, feel free to submit to publish rebuttals to these papers.

    A measuring device is not necessarily a detector. It can as well be scattering off some air molecules or - as I wrote earlier - a slice of bread. I do not know, though, what you mean by "the measuring device (detector) interferes"

    Well, there are reasons, why wiki is not a good source. A lot of articles are just crap. Pop sci articles are even worse. However, the passages you picked are not that bad. They do not even disagree with me. Let me comment on that.

    This is not at odds with what I said. Observation does not mean placing someone there and having a look at the results. Observation is equivalent to measurement.

    This is again not at odds with what I say. It even supports my point. I do not know what you interpret into that, but the existence of which-path information (as compared to some chance to gather which way info or the possibility to get some if one performed a measurement) requires a measurement.
    edit: Just to make my point more clear: The existence of which path information is equivalent to some irreversible interaction having occured. You can tackle this from the entropy point of view if you like. Existence of some information does not mean that someone knows it or even just has access to it.

    They said "the act of observation ultimately determines whether the photon will behave as a particle or wave", which is the same conclusion.

    This is plain wrong. Whether or not we have which path information decides what state the system will collapse to, not whether it collapses. Whether you see an interference pattern or not: collapse (or its equivalent in interpretations without collapse) happens in both cases. The "disappearance" of an interference pattern is NOT what collapse is about. There is an interference pattern or a simple slit pattern depending on whether the measurement gives position information or momentum/relative phase information. However, there is no interference pattern "collapsing" or jumping into existence and disappearing again.

    If it was different, this would open up the path for retrocausality. This is not the case. However, one should distinguish two different delayed choice settings.

    Experiments like the one by Jacques et al. (Science 315, 966 (2007)) aim for spatial separation of the "choice" whether which-way-info can be gathered and the position where a classical hidden variable determining such a choice must exist. In that special case that just means that one can introduce or remove a which-way info marker after a photon has entered a Mach-Zehnder-interferometer. These experiments primarily aim at disproving a certain class of hidden variable experiments. Here, the interesting thing is that the interference pattern is indeed directly approachable (not visible as it is a Hong-Ou-Mandel interference of many single photon events).

    The other kind of experiment which tends to draw crackpots is the postselection or ancilla-assisted kind of measurement like the standard version by Kim and Scully (Phys. Rev. Lett. 84, 1–5 (2000)). These have been motivated by an interest in complementarity. Here interference patterns are there or not, depending on whether a photon ends up on a detector that allows to gather which-way info or not. These experiments are often misrepresented as people do not make clear that interference patterns only occur in coincidence counts. No past detections are altered by later detections on the other side. In fact, these experiments just rely on a quantum optics technique called postselection, which is basically picking a subset of events conditioned on some other event. It is not surprising that the subset of events, for which no which way-info is present will show an interference pattern, while the subset of events for which such info is present, does not. However, that does not mean that the total pattern changes at the time of correlating (as the crackpot camp likes to claim). One is just able to "sort" the results using postselection.

    If you disagree with the papers above, feel free to post some publications supporting your point.

    edit: If you think that my posts are inappropriate, biased, misleading contain wrong statements or are against forum rules, you are free to report them and tell the mentors about that. It is not the science advisors who work on reported posts.
    Last edited: Mar 21, 2013
  15. Mar 21, 2013 #14


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    That is pretty far over the line of the courtesy normally given at this site. Not sure what is bringing this on, but of course Cthugha is entirely correct.

    To be clear, the rule for interference in these cases: if it is possible, in principle, to know which slit the particle goes through, there is NO interference and there will just be the 2 characteristic bands.

    To see how this works in practice, imagine we have a polarizing filter in front of each slit, and they are oriented orthogonally (perpendicular) relative to each other. The particle (photon) must go through one or the other, but not both. In that case, a typical detection screen does NOT record which slit was traversed. But it is possible, in principle, to learn that information. Therefore, according to the rule above, there is no interference AND that information is not available to any conscious observer. QED. It has nothing to do with a conscious observer, the recording of the information, etc.

    And to obtain an interference pattern, all one needs to do is to slowly re-orient the relative angle between the polarizers until they are parallel. As the angle difference approaches 0 degrees, progressively more interference is present. That is because it is no longer possible to learn the which path information.
  16. Apr 7, 2013 #15
  17. Apr 7, 2013 #16


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    This video is a perfectly reasonable set of statements about physics and has NOTHING to do with the nonsense that micky_gta has been posting.

    I think micky_gta thinks that this prof making the point that we don't have all the answers in some way excuses his own ignorance and unwillingness to listen to the answers he is being given.
  18. Apr 7, 2013 #17


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    He gives the limit of modern measurement capabilities, compares that to the Planck scale and says that we know little about the physics in between. That is perfectly correct. However, it is also pretty much as unrelated to this thread as it can possibly get.
  19. Apr 8, 2013 #18
    No, I'm just trying to discredit all you materialists. That's all.
  20. Apr 8, 2013 #19
    No, he clearly states that it is not a measuring problem.
    It is not a problem based on our technological limits of how we can measure something.
  21. Apr 8, 2013 #20

    Doc Al

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