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Delayed ChoiceQuantum Eraser: what about decision?

  1. Nov 27, 2014 #1
    Hello everyone,

    I have read a bit about the Delayed Choice Quantum Eraser, and the fact it could mean there exists a possible timeless retro-causality mechanism.

    So far to me, the results have been paradoxical. But hey, that's QM, right? I will base my questions from a theoretical derivation of the following experiment :

    2000px-Kim_EtAl_Quantum_Eraser.svg.png

    Remember the premises : a photon is emitted by a laser to a double slit (black wall here), the BBO is a Baryum-like material which diffracts the beam into two (from one photon to two, each having double the wavelength of the original), and those two photons are correlated. The prism is used to reroute the photons on proper ways.

    From this experiment, if we detect a wave pattern at D0 then idler photon will opt for D2 or D1 (which of slit A or B was used is not determined), and if we detect a particle pattern at D0 then the idler photon will opt for D4 or D3 (D3 = photon used slit B, D4 = photon used slit A).

    It is known that the decision the idler photon takes is made faster than light, which is the cause for which some physicists suspect a possible retro-causality bond compatible with reversibility of quantum effects in time, but that's not really what bothers me here.

    If we follow the theory, what would happen in the following conditions ?


    We redesign the experiment so that with the help of a switch we can route the photon to a non-decidable solution, just as if it acted as a wave, so to either D1 or D2, or to a decidable route solution, as if we could know if it was slit A or B which was taken, by making it arrive in either D3 or D4.

    This is done by replacing rapidly, and in real time, the beam splitters BSb and BSa for mirrors, such as the Ma or Mb mirrors, if we want a decidable route, or by removing them, to obtain a non decidable route.

    We, of course, modify the experiment so that we delay the idler photon for a time long enough that allows the experimenter to read results from D0 and take actions on the outcome of the experiment before idler photon hits a final detector.

    So, in this modified experiment, when the switch is off, the experiment is basically letting the photon route undetermined.

    But things change when we switch the button on. At this moment, we force the system to opt for a decidable route by forcing the released photon to hit mirrors then either D4 or D3 and then tell us which slit that was opted.

    Questions :

    If we see a wave pattern at D0 and we switch on to force a decidable route (particle pattern) what would happen when idler photon hits D3 or D4 ?

    If we want to be consistent with the theory shouldn't we say that if we decide to switch the button on at the first sight of a lightwave pattern, then the system should resolve in only providing us particles patterns ?

    If we decide to let go off the first three detected patterns of waves, this means taking no action on the route of the system when they are recorded, and to switch the button on only at the fourth sight of a pattern, then shouldn't we see a wave pattern for the first three shots, and then only a particles pattern for all other shots ?

    So you would control the system by the "will of your mind" ? :-/
     
    Last edited: Nov 27, 2014
  2. jcsd
  3. Nov 27, 2014 #2

    bhobba

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    Your 'mind' has got nothing to do with anything. The eraser experiment simply indicates in simple cases decoherence can be reversed.

    Thanks
    Bill
     
    Last edited: Nov 27, 2014
  4. Nov 27, 2014 #3
    My original post is about setting up an experimental frame in which we try to force the photon to behave the opposite of the results we read on D0. I base the experiment on the Quantum Eraser Delayed Choice experiment for that. So, in the case I described, would it be possible to make the results conflict ?

    I know it's kind of long to read, but quoting the last sentence without reading the whole rest won't really help to figure out.
     
  5. Nov 27, 2014 #4

    bhobba

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    Sorry for not being clear.

    I wasn't commenting on your variation, which I admit to not being able to follow, but then again I am not an experimental type. But on that point, if you do attract the attention of an experimental type I think you will find they will want you to tighten up what you want to do considerably - eg 'We modify the experiment so that we can indefinitely idle the idler photon in some kind of loop without breaking the interference. Photon will be released from loop when experimenter decides, and without this decision breaking the interference.' Indefinitely idle the photon in some kind of loop?????????? - that has me well and truly beat. It has the feel of the proposal to use entanglement to send information FTL. That relied on cloning - rolls of the tongue easily doesn't it - clone a system - but it turns out you cant do that.

    I was commenting on your use of 'mind'. Mind has nothing to do with anything either in the delayed choice or, once its fully fleshed out (if it can be) your variation.

    Thanks
    Bill
     
  6. Nov 27, 2014 #5
    All advices are good to take! :) I edited consequently to simplify so that it should be more obvious what the experiment is about.

    It is about deciding if we can have the correlated photons conflict in their behavior, by modifying the experience in real-time, so that you could read wave on D0 and particle on D1 or D2, and what that implies if not.
     
  7. Nov 27, 2014 #6

    bhobba

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    If you are discussing an actual experiment you need to detail exactly what it is. Now, exactly how are you going to 'We, of course, modify the experiment so that we delay the idler photon for a time long enough that allows the experimenter to read results from D0 and take actions on the outcome of the experiment before idler photon hits a final detector.'

    My background is not experimental physics, so I will have to leave it up to other to comment on exactly how you do it, bit you do need to detail it.

    Thanks
    Bill
     
  8. Nov 27, 2014 #7
    In the original experiment the delay between D0 and the other detectors is 8 nano seconds, all I do here is suppose that, in the theoretical experimental frame proposed, we make this delay longer so we have time to react and decide the final route to detectors prior idler photon hits them (D1, D2, D3 or D4).

    Goal here is to know if deciding to take an action can modify the outcome of the result.
     
  9. Nov 27, 2014 #8

    bhobba

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    Yea - but what you don't seem to get is experimental set-ups need to be exact - you just cant hand-wavey say - do this. Its like the FTL thing I mentioned - hand-wavey clone a system - it cant be done.

    Thanks
    Bill
     
  10. Nov 27, 2014 #9
    We have had thought experiments long before we had practical experiments. See this as a thought experiment please, and help me to assess if it is possible that your yet unrealized decision might affect the behavior of photon at D0, or if you are aware of similar experiment.
     
  11. Nov 27, 2014 #10

    bhobba

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    Without looking into further I can say if its unrealised it will have zero effect - the mere act of DECIDING will do nothing.

    I say this from my understanding of QM.

    Thanks
    Bill
     
  12. Nov 27, 2014 #11
    But if you decide to perform the action, to switch the button on in the experiment, and so you do, then can you get to both determine the path of the initial photon and obtain a wave pattern detected at D0? Sounds improbable. So what happens?
     
  13. Nov 27, 2014 #12
    I attach a PDF which is a response from Paul Kwiat, Professor of Physics, in relation to an email I sent him regarding delayed choice quantum eraser experiments back in 2011. You may find it interesting, and possibly useful to your quest in finding answers to your questions.
     

    Attached Files:

  14. Nov 28, 2014 #13

    bhobba

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    That would be impossible - if you know the path then you will not get an interference pattern.

    Thanks
    Bill
     
  15. Nov 28, 2014 #14
    So, in the experience, what if you can change the route of the idler photon AFTER you obtained the result 'wave or particle' at D0?

    If, originally, your experience is designed so that you do not know the path (wave), and you change that condition AFTER the entangled photon has been measured as a wave, then what would happen? Would you get a contradictory result ?

    Thank you Stevie, I have read the answer it has interesting insights about results of the interpretion of the double slit experiment, but is not specific enough on the causality effect which is what I am trying to address here
     
    Last edited: Nov 28, 2014
  16. Nov 28, 2014 #15

    Nugatory

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    That's not how the delayed choice experiment works. Detector D0 is just a detector - it clicks when a particle hits it. The interference pattern is observed by moving D0 to different positions (that's what the little arrow at D0 means) in the field illuminated by the signal photons. The interference pattern only appears when we look at where D0 was positioned at every click. The clicks that happened when D1 or D2 also triggered are distributed according to the interference pattern, with many clicks happening when D0 is at one position, not so many when it's moved a bit sideways, then more when it's moved a bit farther. No such pattern is seen when we look at the clicks when D2 or D3 triggered.

    Thus, there is no possibility of observing any interference pattern until after the fact, when all the photons are done flying around.

    Replacing the beam splitters with mirrors that we can control doesn't change the result.
     
    Last edited: Nov 28, 2014
  17. Nov 28, 2014 #16
    Interesting, I thought that there were specific x areas when D0 is moving for which the detection of a photon would mean wave pattern, as the probability for a photon to be there in a particle pattern would be very low.

    If we can only measure the behavior of the photon a posteriori by having the global picture, then taking a decision during the process would be affecting the results in a normal way (each time we opt for a which-way decidable solution, we have a particle pattern). Then yes, there is no paradox. But is limitation of measuring an interference at a unitary level a limitation of our current know-how, or is it fundamentally due to QM laws?
     
  18. Nov 28, 2014 #17

    Nugatory

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    It's fundamental. Quantum mechanical interference is a statement about probabilities (high probability a particle will land here, not so high that it will land there) and probabilities can only be measured by studying the results of a large number of trials.

    A more prosaic example: You flip a coin once, and it comes up heads. Is it an honest coin with a 50/50 chance of coming up heads or tails? You don't know. Flip it 100 times and get 75 heads, and you can be pretty sure it's not.
     
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