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Temporal Extension of Quantum Uncertainty (A Theoretical Quantum Fortune Teller)

  1. Jun 6, 2010 #1
    Based on the results of every variation of the two-slit experiment so far, the presence or absence of the interference pattern is based on whether or not which-path information can be known or not. The Delayed Choice Quantum Eraser experiment
    puts an additional twist on this in that the dispersion pattern (additive or interference) is created before the which-path information is revealed (or lost). It does this by virtue of the fact that it uses two entangled photons; the “signaler” photon is directed at the screen where the dispersion pattern is created while the “idler” photon is directed through additional apparatus and eventually either reveals the which-path information or loses it based on what happens when the photon encounters some beam splitters in the path. (See link above) The important point here however is that the dispersion pattern is created before the which-path information is revealed or lost.

    So as a thought experiment, what if you were to increase the distances between the various beam splitters and mirrors so that it took the idler photon an hour or two before it finally arrived at a detector? Realistically this is impossible but theoretically you could build a giant version of the experiment in space and put many light-minutes of distance between the various apparatus. Meanwhile you place the screen where the dispersion pattern forms much closer to the point where the entangled photons are created. In this way you would have a dispersion pattern formed well before the idler photons had arrived at their final destination. The results of quantum experiments, including the real world version of this same experiment, suggest that the presence or absence of an interference pattern will correctly predict at which detectors the various idler photons will arrive, either at a detector which reveals which-path information or at one that does not. Actually this is already what happens but since light moves so fast the “prediction” happens only a fraction of a second before the idler photon confirms it. (Technically since this experiment loses some which-path information and reveals some, you would get both patterns super-imposed but that is not relevant to where I am going with this idea.)

    What I am proposing is this, if you can create some version of the experiment similar to this where there are two entangled quantum particles, and one particle is sent to the detector screen to create (or not) the interference pattern, while the other is somehow contained for an indefinite period of time (along with the which-path information), the presence or absence of the interference pattern should exactly correlate with what happens to the contained particle, regardless if, whatever happens to it, happens an hour later, or a week later.

    So for example what if the above mentioned experiment were modified so that the idler photon instead of travelling through the various apparatus and hitting one of the 4 detectors, were instead manipulated into one of 2 enclosed containers (one for each of the two slits) and left reflecting in an infinite loop using mirrors?

    Essentially, which container it was in would reveal the which-path information. In addition, I propose that both containers be constructed in such a way that the photons could be channeled out a common path (in which case you wouldn’t know which container it had been in) or an individual path (in which case you would). My theory is that as long as you did not find out which container the photons had been in (in other words released them on the common path which does not reveal which container they were in and likewise does not reveal the which-path information of the original photon) then you will get an interference pattern. This should be true even if you released the photons an hour or a week later. I can think of nothing in the existing experimental evidence to contradict this assertion.

    But here is the paradox, if you get an interference pattern, this means that you will not get which-path information. But if you have those photons contained indefinitely, and they contain the which-path information, then the temptation to try and get that information after already getting an interference pattern would be great. In other words, once the presence of an interference pattern predicts that you cannot get the which-path information, how could you not be tempted to try and get it anyway knowing that the contained photons still have it? But because I believe the quantum particles will not predict wrongly, you will never get an interference pattern and be able to obtain the which-path information. So it may be that if you are prone to give into that temptation, the signaler photons would never create the interference pattern to begin with. They cannot be “tricked” so to speak. I believe that you would have to absolutely resolve not to even try and obtain the which-path information if you get an interference pattern. Keep reading to see why I say so.

    First I think it would be important to establish that the entire setup is capable of creating an interference pattern to begin with, to prove that something about holding the which-path information in limbo for an extended time period would not in itself erase the interference pattern. Perhaps setup the whole experiment to run by computer control and come back later to see the results. Again, my assertion is that you will never get an interference pattern if the which-path information is discovered, even if it is discovered a week later. So by running the experiment automated, with no-one around to mess with it, and configuring it to say wait an hour before releasing the captured particles on the common path so that you lose the which-path information, you should get an interference pattern.

    So far I have not proposed anything that contradicts current experimental evidence, and in fact I don’t intend to. But if experiment bears out the theories I am proposing here then there are some rather important implications. For example, if you established that the experimental setup could indeed create an interference pattern as proposed above (and based on current experimental results I don’t see why it wouldn’t) then what would happen if you intentionally decided to retrieve the which-path information every time you got an interference pattern? I assert that your intention would in itself guarantee that you never got one. Blasphemy I know but there you go.

    So what about this thought experiment? Suppose you got a volunteer who knew nothing at all about the experiment and you had them come in and hit one of two buttons. One button releases the photons on the common path and thereby loses the which-path information, and the other button releases the photons on the individual path which reveals the which-path information. Will the presence or absence of the interference pattern correctly predict the choice of a human being in advance? I believe that it will. Current experimental evidence suggests that it will. If true this has huge implications. And it is these implications that are the destination of this post.

    This experiment and its prediction are not a prediction of the behavior of the human being, nor of any other real world event. It is a prediction of the fate of the which-path information, nothing else. The ability to use this to predict real world events depends on the ability to correlate those real world events to the fate of that information. In the above thought experiment the fate of the information, either revealed or lost, is directly correlated to the choice of the human being so it appears to be a prediction of the choice of that human being. But as long as you can correlate the real world events strongly with the fate if the which-path information, you should be able to get equally strong correlation of the prediction.

    So for example you may even be able to predict the outcome of an election. If you set it up so that a computer would release the contained photons on the common path (and lose the which-path information) if candidate A wins, then you could simply run the experiment and see if you get an interference pattern. Again the predictive value is only as good as the correlation between the real world event and the fate of the which-path data. So an interference pattern is not a prediction that candidate A will win, it is a prediction that the which-path data will not be recovered. Other unforeseen circumstances could easily come into play such as a computer glitch or malfunction of some part of the experimental apparatus. As long as the which-path information is lost, the interference pattern has correctly predicted the outcome.

    The fewer the elements involved in an event the easier something like this would be to use to predict it. If you had a horse race with 10 horses you could setup one experiment for each horse, correlate the loss of the which-path information with that specific horse winning and then run them all and see which one gives an interference pattern. Keep in mind that in order for the correlation to be effective you must actually make sure that the mechanisms you put in place actually do release the photons on the common path when the event actually occurs. If not the predictive value is meaningless in regard to the event itself and only meaningful in regard to the which-path data.

    I know all of this will seriously rub up against some strong objections with some people, but in truth I cannot see where it contradicts what we already know about quantum behavior and it does in fact follow from the experiments that have already been done. (Well most of it anyway, the ability to correlate real world events with the loss of the which-path information is more or less untested and may not work out quite as simply as I have proposed it.) But in truth the weirdness of it is not all that much different than any of the other weirdness of quantum behavior discovered up until now, and it is in fact the same weirdness just taken to a new extreme. How do the quantum particles “know” about the entire experimental setup so as to change their behavior accordingly? Well the answer to that isn’t actually necessary in order to make use of that uncanny ability. Since the Delayed Choice Quantum Eraser experiment has essentially demonstrated that the “knowing” extends into the future, there should be no reason to suspect there is a limit to how far into the future it extends.
  2. jcsd
  3. Jun 6, 2010 #2


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    Stop! This is plain wrong. This is not an experiment where it matters which comes earlier. In DCQE experiments you see two-photon interference. This is very different from the common single-photon interference seen in usual double slit experiments. The most important point is that it occurs just in the coincidence counts of simultaneous detections of signal and idler and never directly on the screen. The dispersion pattern is also not really created before the which-path information is revealed or lost. The idler photon just gives you information so you can pick a certain subset of signal photons that shows an interference pattern (if there si no which way information) or it does not (if there is which way information).

    This also shows why your thought experiments do not work. There is NEVER any interference pattern in DCQE experiments unless you detect signal and idler and do coincidence counting.

    There are several dozen threads about DCQE experiments on this forum. Maybe you would like to look for some of them. In some the experiment is pretty well explained in detail.
  4. Jun 6, 2010 #3
    Right, this is why I said this:
    (Technically since this experiment loses some which-path information and reveals some, you would get both patterns super-imposed but that is not relevant to where I am going with this idea.)
    Nevertheless it will not affect the experiment as I have described it. There are no beam splitters nor coincidence counters in the experiment I described. In DCQE the pattern is in effect created before the which-path information is revealed or lost by virtue of the fact that the signaler photon travels a shorter distance than the idler. So while it is true that an actual interference pattern is not formed unless you remove the "noise" of the signaler photons that correspond with idlers that revealed which-path information, nevertheless the signaler photon has already encountered the dispersion pattern screen prior to the idler arriving at the detector.

    Here is a quote from the wiki page:

    “However, what makes this experiment possibly astonishing is that, unlike in the classic double-slit experiment, the choice of whether to preserve or erase the which-path information of the idler need not be made until after the position of the signal photon has already been measured by D0.”

    And another:

    "The results from Kim, et al. have shown that whether the idler photon is detected at a detector that preserves its which-path information (D3 or D4) or a detector that erases its which-path information (D1 or D2) determines whether interference is seen at D0, even though the idler photon is not observed until after the signal photon arrives at D0 due to the shorter optical path for the latter.”

    As I mentioned, in regard to the experiment I am describing this is the truly important part of the DCQE experiment. The formation or not of a visible interference pattern is unimportant.

    Either you have misunderstood me or I have misunderstood you because I fail to see where anything you pointed out about DCQE affects the results of the experiment I am describing. I will look for some of these other threads you mentioned.
    Last edited: Jun 6, 2010
  5. Jun 6, 2010 #4


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    Again, you have some really severe misunderstanding about how DCQE experiments work. The reason why you need coincidence counting is NOT filtering out noise. In any DCQE experiment the phase of the signal photons is pretty undefined. Only the phase of the entangled two-photon state is well defined. This means:
    1) The signal beam behaves like incoherent light and will not form an interference pattern under experimental conditions which maintain entanglement.
    2) The two-photon interference pattern is in fact a superposition of two interference patterns shifted by pi. If you have two small movable detectors at the signal and idler screens, you can leave the idler side at one position and move the signal side around. In the coincidence counts ou will see a signal-position dependent interference pattern. Now move the idler position detector a bit and do the same again. You get another interference pattern exactly out of phase with the first one. You can see these in the common well known DCQE experiments (for example the one by Kim). There you have two detectors where which-way information is not present and both show shifted interference patterns. The superposition of them is no pattern at all. Accordingly - even with no noise present - you will not see any interference pattern unless you do coincidence counting and therefore destroy the photon on the other side. Afterwards there is no possibility to retrieve any which-way information. The photon is gone.

    This is - as common on wikipedia - a pretty bad wording. You can choose afterwards whether you will be able to find a subset showing interference or not.

    The same applies here. You again choose a subset which may show interference or not.

    If you see an interference pattern, this means you have already done coincidence counting with the idler (thus destroying the idler) and there is no way to get which-way information again. If the idler photons are still contained somewher, you do not have any information about which subset to pick at the signal side to get an interference pattern.
  6. Jun 6, 2010 #5
    The reason you need coincidence counting is so that you know which idler goes with which signaler. Otherwise there are no meaningful dispersion patterns to speak of. Is this not so? Where is my severe misunderstanding exactly?

    You have not addressed the ONLY relevant point about any of this; does not the idler arrive at its detector after the signaler? This is the ONLY point that is relevant to my thought experiment. What exactly can you find to object to regarding the thought experiment where you recreate a version of DCQE over a vast area of space so that you establish unequivocally that the idler arrives at its detector at a later point in time then the signaler?

    My thought experiment involves no coincidence counting as I have already mentioned. But I do possibly see something here, are you saying that two containers would be necessary for EVERY idler photon? The thought experiment was that all of the idlers would be channeled into whichever container (one for each slit) while all of the signalers would be directed at the dispersion screen. The idlers have not reached their destination and will not do so until you release them, meanwhile the signalers have already formed whatever dispersion pattern they are going to. All you need to do to determine which-path information is see which container the idler went in to. But if it is a problem that all of the idlers are mixed up so that you cannot tell which specific idler goes with which specific signaler than yes there is a problem with the thought experiment.
  7. Jun 6, 2010 #6


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    Yes and no. Coincidence counting allows you to know which signal goes with which idler, so that you can get a dispersion pattern composed of all the signal photons relating to the idlers detected at one single position. In fact you have infinitely many signal dispersion patterns. A slightly different one at each idler detection position.

    I do not understand your point. Which one arrives earlier or later does not matter at all. It is a funny thing to keep one of them in the air to invoke the illusion of changing the past, but this is not how it works.

    That is a problem for your experiment. I will explain below.

    Ok, the dispersion pattern will always be the same, regardless of what you do. It will be similar to the superposition of two single-slit diffraction pattern. Whatever you do to the idlers, whether you keep or destroy which-way info: this pattern is always the same. There is never an interference pattern present at this screen and its shape does not give you any information about what is happening on the other side. There is never an interference pattern without coincidence counting. See comments below for more details.

    Do you agree that in a single photon interference pattern the visibility of the pattern gets better if the phase of the light wave used is well defined and there is no interference pattern if the light is completely incoherent? The light in one of the two arms of entangled beams is extremely incoherent. The phase is pretty random. Accordingly there will be interference patterns for each subset of photons with some certain value, but the superposition of all of them will result in no interference pattern at all. The idea of DCQE is to retrieve the interference pattern by correlating signals to the corresponding idlers AND PICKING IDLERS WITH WELL DEFINED PHASE. This is why you always need at least one position sensitive detector AND coincidence counting in DCQE experiments. If you just take all idlers and have no which way information present, this does not help you at all in getting an interference pattern. DCQE experiments are more like an interesting kind of filtering process. Due to the fixed phase relationship of a two photon state, detecting a photon at one certain position at the idler side, tells you (due to the fixed phase relationship) where the corresponding signal photon will most likely be detected (or was detected - it does not matter).

    This means that just destroying which-way information is not enough to get any interference pattern. You have to perform a measurement on the idler giving you phase information. Then you can get an interference pattern in coincidence counting. However, this is always an and not THE interference pattern as there are many of them depending on the phase you choose to measure at the idler side.

    Besides that: It would indeed be complicated to correlate specific idlers to specific signals when the idlers are kept somewhere for very long times. Usually the arrival time is the marker to correlate signal and idler.
  8. Jun 6, 2010 #7


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    As Cthugha says, you need coincidence counting to make sense of the patterns. Without counting coincidences, you just see a random sequence on one side and a disperion pattern on the other. There is no interference pattern at all visible.

    The paradox you have attempted to model is a variation on the EPR paradox. You anticipate that the idler will act a particular way given what you see from the signal photon (or vice versa). But nature does not actually act that way. The photons will "conspire" to fool you.
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