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Delayed Choice Quantum Eraser & The Observer (2000)

  1. Apr 21, 2015 #1
    Hello, I signed up cause I was going through the Quantum Eraser threads, but couldn't find an answer to my question. It's about this version from 1999/2000 of the elaboration on the classic double slit experiment:

    Results seem quite straight forward. Wherever we have "path information" the interference pattern collapses into a particle pattern. Wherever "path information" is unavailable, the interference pattern is retained.

    Hence, von Neumann seems to be vindicated and the act of observation itself collapses reality into a steady state.

    I'd like to know the current status of this experiment among you guys who are in the know (and unlike me aren't retards in everything to do with math and physics). Does the statements of this video hold up to scrutiny or is this interpretation of the experiment already discarded? Maybe the video is even New Age mumbo jumbo from the outset?

    Note that "the observer/consciousness creates reality" feature is the only one I'm interested in. I don't care whether the experiment proves some kind of reverse causality or anything, so please leave that out.

    Also, if anyone feels like replying, it would be great if you could use ordinary English and not mess up the syntax too much. Like stated, I'm a retard and what's worse, I'm gullible too.
    Last edited: Apr 21, 2015
  2. jcsd
  3. Apr 21, 2015 #2


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    Staff: Mentor

    There's no conscious observer in that experiment, just a hunk of electronics called a coincidence counter. The interference pattern doesn't "appear" in the sense of you or me any one else seeing it; all that happens is that D0 triggers concurrently with one of the other detectors more often in some locations than others. There's no reason to think that this setup would behave differently if we're watching it or if we've locked it up in a room and never look at it again.

    The word "observation" is best understood as meaning "interaction with something macroscopic and complex". Being seen by a conscious observer certainly qualifies as an observation, but so do many other interactions - consciousness is not required.
  4. Apr 21, 2015 #3
    Hm... alright, "observation" might be a bad term.

    But is the gist of the experiment still considered correct, i.e that "path information" leads to a particle result and lack of "path information" (when we, due to the experimental setup, cannot know which way the electrons took) leads to an interference pattern?

    In other words, instead of "observation" we could talk about "knowledge", perhaps?

    Or is the result invalid for other reasons, for example instrumental interference, altered properties of the electrons as they move through the different tracks etc?
    Last edited: Apr 21, 2015
  5. Apr 21, 2015 #4
    The amount of which way information available, and the interference that will be observed, are correlated.

    In the original thought experiment by Scully et al. interference is still present, just hidden (with which way info present). It is only after erasure of which way info and correlations of that data to points on the screen that two interference patterns emerge.

    After email discussion with Paul Kwiat about the experiment he said 'In general I rather agree with much of what you said, but not all of it. Starting from the end, it’s definitely true that the fact that a quantum eraser can be done to reveal interference does seem to imply that the particle did not originally go through just one slit. However, I’m not sure that I would agree that the statement that “the particle hasn’t actually traveled both or just one path until we find out which-way info, or erase which-way info”. In my opinion, I think it would be appropriate to say that the particle *has* traveled both paths, at least to the extent that that would be the quantum mechanical description of the state after the slits. In particular, even if there are which-way detectors sitting by both the top and bottom slit, the passage of the photon through the slits—according to strict quantum mechanics—simply entangles the path of the particle with the state of those detectors.'
  6. Apr 21, 2015 #5
    No it doesn't hold up. Neither is consciousness involved. Neither is "information" as such the key. The experiment is a good one, but Inspiring Philosophy has a metaphysical agenda. The DCQE set up uses a particular kind of interaction which destroys the coherence and which can be either be used to create a permanent measurement or else reversed. In the latter case, coherence is restored but the possibility of creating a measurement record is lost. It is complete rubbish to say that it proves that consciousness causes collapse. What it does indicate is that collapse certainly does not occur until after all the detections have been made. Nothing unusual about that: in QM we frequently have to assume a superposition continues at least as long as it is needed in order to record interference effects. Consciousness is not involved. It sits there as a passive recipient of information.
  7. Apr 21, 2015 #6
    Trying to follow you here. So in your mind, if it's not path information that causes the collapse of the wave function, then what is it?
  8. Apr 21, 2015 #7
    As I indicate in my post above, even if there is which-way information, that does not mean 'collapse of the wave function' has occurred.
  9. Apr 21, 2015 #8


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    Last edited: Apr 21, 2015
  10. Apr 21, 2015 #9


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    It is not exactly correct to say that in order to see interference in D0 you just need to erase something. You have to get "interference information" from D1 and D2. Because that is what BSc does - it makes two paths interfere by different phase offset in two of it's outputs.
    And when you relate "interference information" from D1 and D2 with detections at D0 you see complementary interference patterns in subsets.
    Last edited: Apr 21, 2015
  11. Apr 22, 2015 #10
    To ask what causes something is to assume that it does happen! Collapse is actually a superfluous hypothesis, as the appearence of collapse emerges automatically from linear, (Schrodinger) evolution without any actual collapse at all. So, since you ask, I don't think there is a collapse at all. But if the thought of Many Worlds is too disconcerting, you can add wavefunction reduction wherever it can do no harm to the predictions of QM. The obvious place is somewhere after decoherence. It doesn't really matter as long as the records are irreversible.
  12. Apr 26, 2015 #11
    Alright, the reply I wrote today contained links that were regarded as too unscientific or something, and I guess the moderator might have been correct, so I reckon I just let this thread sink into the virtual ocean floor for now.
  13. May 18, 2015 #12
    An 'observation' is anything that produces an irreversible change in our universe's course of history.

    In terms of the double slit experiment, when an electron is not 'observed' it is a wave, and thus it can pass through 2 slits simultaneously. This means that both possibilities coexist in our universe as a superposition, they both contribute 50/50 to our current 'now' reality, because a universe in which nobody will ever be able to know which slit it went through is just one universe, not two. The universe did not have to make a choice. Nothing has changed in a universe where that information is and will never be known, it is one and only one universe reality.

    You might imagine that there could still be 2 different universes, one in which it passed through slit A but nobody will ever know, and another where it passed through slit B but also nobody will ever know. But this is how the quantum works, in practice those 2 imaginary universes would be identical, nothing would be irreversibly changed in them making them ever distinguishable from each other, their futures are identical until the end of times, so nature is economical and merges those 2 options as one single universe reality which is in fact a 50/50 superposition of both.

    On the other hand if the electron passes through slit A or through slit B leaving any irreversible change in the fabric of the universe, even if that change (whether it passed through A or B) is not observed now but it will only become apparent within billions of years and in a very distant place, these are necessarily 2 different realities, two different futures.
    If there is any way in which it will ever be possible to know that once an experiment was conducted on Earth in which the electron passed through slit A and not through B, then the universe needs to make a choice and become that reality and discard the option where it will be possible to know that it passed through slit B and not through A, because that would be a different universe, something irreversible was changed in the course of the experiment changing the future (Many Worlds says that when we put the detector the universe does actually split in 2 versions of itself, one in which the detector will detect slit A and the other where it will detect slit B).

    Anything we do which will allow, now or in the distant future, to know which slit the electron went through does necessarily 'force the universe to make the choice' because they would be 2 different futures, they can not coexist. Anything that has the potential to alter the future is 'an observation'.

    When you think about it, it is actually completely logical, it could not be otherwise. What is actually amazing is that as long as the futures are identical, all the possible options leading to that future do actually coexist in superposition in the form of waves creating an interference among them, a kind of blurred reality consisting in a haze of all the possible realities, all the possible 'nows' which while being different are still completely consistent with that future. In the simple double slit experiment (when unobserved) we have only 2 different realities in superposition with 50/50 contribution, but in real life complex situations all the 'unobserved' phenomena, i.e. those which do not represent any difference in their future histories, the different possible 'nows' can be many and are superposed with different contributions according to their probability of resulting in that specific future.
  14. May 23, 2015 #13
    Superposition is not about future possibilities, it is about the quantum state of a system. Superpositions are two-a-penny; Many Worlds follows logically from superposition. Thus for Many Worlds NOT to be correct, something must happen to all the worlds you don't (and can't) see. It is not immediately obvious why the entire universe instantaneously discarding most of its states when requested to make up its mind should be deemed more economical or logical than the superposition continuing indefinitely.
    Last edited: May 23, 2015
  15. May 23, 2015 #14


    Staff: Mentor

    MW is not about superposition, its an interpretation about decoherence.

    Regarding it being correct, note the key word - interpretation.

  16. May 24, 2015 #15
    I did not say anything about MWI. I specifically omitted the word interpretation because MW is a principle which follows tautologically from the assumption of linear (quantum) evolution.

    I am surprised you think MW is about decoherence too. Many Worlds means many phenomenal worlds, i.e. many observer experiences, these being the states of the observer relative to the states of the system being observed. The relative states are present even before decoherence.

    Decoherence just adds some important details that Everett glossed over. RSF provides the many worlds; it does not directly yield the correct statistics of a mixed state. decoherence does this. The mixed state is an improper one, representing a superposition of the larger system which includes the environment.

    Of course this is an ontic interpretation but the issue here is not the onticity of the wavefunction (or of states) but what sort of reality they could represent if indeed they do at all.

    What you appear to be saying is that decoherence plus many worlds is a viable modern synthesis but still requires an interpretive step. I can't say I agree with this either. It works on its own, why clutter up a good theory with ontic hypotheses? :oldruck: No, seriously, I am a realist by nature, I think MW does provide a good framework for a realist interpretation - not classical direct realism of course, but relative realism. But if MW does indeed work and I'm not simply wrong, then it is a direct consequence of linear QM regardless of my preferred ontology. And it is therefore correct.

    Sorry about the attached file - I can't delete it , duh.

    Attached Files:

  17. May 24, 2015 #16


    Staff: Mentor

    MW and MWI is the same thing - obviously.

    Its pretty obvious it doesn't. But if you have a rigorous mathematical proof, not philosophical waffle, but an actual proof, I am all ears.

    Last edited: May 24, 2015
  18. May 24, 2015 #17
    Of course the formalism does not say anything about interpretations, and no interpretation has been proven to be more correct than others. This does not mean that discussing about interpretations is unproductive b*t.
    Bhobba, so you say qbits do not exist? What is a qbit according to you? If you acknowledge their existence, what do they represent? simply uncertainty? The quantum Zeno effect is just uncertainty? Sorry but don't think so.

    In my view the unobserved electron in the double slit experiment is like a qbit, it is a superposition of both going through slit A and through slit B, therefore causing interference between both. This is because as long as it remains a qbit, the future can never be contingent on our experiment.

    An 'observation' is anything by which that qbit 0+1 turns into an ordinary bit, either a 0 (slit A) or a 1 (slit B). It does not matter whether we 'look' at that now or not. Only whether the information has been changed from qbit to bit. If we have changed one bit of the universe's information from qbit to bit, the future will inevitably depend on what we have done, now or perhaps billions of years from now, it doesn't matter. If the qbit 0+1 turned into a 0, the future will be X. If it turned into a 1, the future will be Y. This is necessarily so, because if both futures would still be identical, then it would still be a qbit 0+1.

    So anything we do by which it will ever be possible to know which path did the electron take necessarily modifies the qbit into an ordinary bit. The future where it can be known that it passed through slit A can not be the same future as that where it can be known that it passed through slit B.

    Of course the state of the universe evolves during the experiment when unobserved, the qbit 0+1 was at the source and at the end of the experiment it's at the detection screen, but it's still a qbit, a superposition of 2 possible events without any possibility to ever know which of the 2 did happen (because they both happened, i.e. an interference pattern was created, meaning that the qbit was not turned into a bit, and that's irreversibly so).

    So, as long as the information defining the future of the universe is not turned into one or the other way, all the possible realities, all the possible 'nows' which while different are completely consistent with a unique future, coexist in a superposition and this superposition of states displays interference among the different possibilities which we perceive as if it was a wave-like behaviour.

    Of course I know this is just an interpretation and I know of no way to prove it better than any other one. I know the formalism does not give any hint as to whether this interpretation is any more valid than others, but that does not preclude me from having a preferred interpretation.
  19. May 24, 2015 #18
    Read Everett for yourself. If you can find where his argument breaks down or depends on decoherence rather than superposition, post it here. After all, I am not a mathematician and Everett himself was only dipping into the maths, so with your abilities plus sixty years of hindsight you should be able to find the flaws, debunk the interpretation and shut me up together with countless others, experts and amateurs alike. You might regard that as a good day's work.
  20. May 24, 2015 #19


    Staff: Mentor

    I have had bad experiences with claims references support a certain view only to find often its nothing of the sort - so please post a precis of it.

    Now your claim is 'MW is a principle which follows tautologically from the assumption of linear (quantum) evolution.'

    Please explain that principle. To be specific your claim is many worlds is a tautological principle - I claim its an interpretation meaning the assumption of many worlds requires an interpretation. Exactly how you get an assumption from something that doesn't assume it beats me - but I am all ears.

  21. May 24, 2015 #20


    Staff: Mentor

    I have said nothing of the sort.

    Why are you diverting the discussion to irrelevancies?

  22. May 24, 2015 #21
    Because I don't think it's irrelevant. As I said I consider the unobserved electron very much as a qbit, therefore asking about your 'interpretation' of qbits did not seem irrelevant. I you don't believe in superposition, what do qbits represent for you? just uncertainty?
  23. May 24, 2015 #22


    Staff: Mentor

    Where do you get the idea I don't believe in superposition? Its an utterly trivial consequence of the vector space structure of pure states.

    Qbits are quantum systems whose vector space is two dimensional and described by the Bloch sphere:

    If you think the double slit can be described by a Block Sphere I am all ears.

    BTW if its merely that you think something is relevant don't say things like:

  24. May 24, 2015 #23
    Really? Well if you genuinely need a precis of MW sans trappings, I will do exactly that, but not today as I really am dead beat. Perhaps we will then be able to get somewhere.
    I didn't say that MW is tautological in itself, I said it follows tautologically from linear (quantum) evolution. A tautology is a proposition whose negation cannot be true. So when I say MW follows tautologically from linear (quantum) evolution I am just saying that given linearity (the relevant aspect here), MW cannot not be true. You have explained to gerinski that the structure of the vector space - which is linear - implies superposition. I would call superposition a tautological consequence of the properties of the vector space but if you don't like the term tautology we can move on without it. Superposition still follows inexorably from linearity and my claim - which is far more germane to the discussion - is that relative states follow just as inexorably from superposition as superposition follows from linearity. And to apply relative states to observer-states is trivial. The resultant superposition of observer-states is MW, nothing more and nothing less.
    I don't know what you mean by that. As I have now explained, there are no further assumptions to MW beyond linearity. What assumption had you in mind?
  25. May 24, 2015 #24


    Staff: Mentor

    Now you are shifting your claim.

    First you need to define what you mean by relative state.

    BTW superposition is different to linearity. Linearity is a property of a linear operator and states that if something is in superposition it remains in superposition - but first lets pin down what you mean by relative state.

  26. May 24, 2015 #25
    Assume that a device with a pointer is located close to ONE of the paths. After propagating further over a really long distance and time, the wave packet reaches a detection zone. Just before this happens, we can decide to switch the experiment between a setting where the pointer reveals the path information, to one where it can't. Depending on this decision, we se or don't see interference.

    Now, does it matter how "macroscopic" the pointer device is? If it is just an atom or something, then it's plausible that it could give back the energy or whatever that it exchanged with the photon, and no one any the wiser. On the other hand, let's say the device is a photomultiplier tube, and the cascade of electrons should have reached the point of no return long before the wavepacket could reach the detection zone. What then? Can you put the toothpaste back in the tube even at this point? Or should we say that the cascade would be triggered nonlocally - or not triggered at all - depending on the scenario that exists in the detection zone at the last femtosecond? Would it make a difference whether or not the coherence length spans the PMT and the detection zone?
    Last edited: May 24, 2015
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