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Teleportation back in time?

  1. Nov 10, 2015 #1
    I can never understand why students of QM speak about "simultaneous measurement" or "collapse of the wave function".

    We all know that you can pick any relativistic reference frame from which to observe an experiment, so what's "simultaneous" to one observer may not be "simultaneous" to another.
    It's easy to imagine an experiment where entangled particles A and B are measured "simultaneously" for me, "A before B" for you in a passing spaceship, and "B before A" for someone going the other way. The results must be the same; that's a constraint that could be useful in our analyses; so why do we pretend it doesn't exist?

    As for "collapse of the wave function", this troublesome notion simply fades away if you stop thinking that the future hasn't happened yet.
    We all know that we're embedded in spacetime, so why not lay out a quantum interaction on a space-time surface? There is no "collapse" then. The events that happen to the particle - its emission at one point in spacetime, its detection at another - are constraints on its wave equation.
    Our everyday notion that the future "hasn't happened yet" leads us to imagine that the end of the particle's journey hasn't happened yet, and that's why we imagine the wave function spreading out; and that's why its "collapse" confuses us.
    If you take yourself "out of time" and imagine that the detection of the particle in the future is as real as its emission in the past, then there's no collapse to explain.

    Sorry if I rant a bit.

    Anyway, that leads to a question. Based on what I just said, I expect that quantum teleportation is not only superluminal, but it travels backwards in time.
    I expect that you can measure the teleported state well in advance of the moment when you make the Bell measurement on the particle whose state you want to teleport. In fact, I expect that you can measure the teleported state immediately after the creation of the entangled pair.
    That may sound like seeing into the future, or sending information back into the past; but I believe you can't set up a "time paradox" because you don't have the two classical bits that will result later from the Bell measurement.

    Question: has anybody tested this arrangement experimentally?

    I'm new to QM and I apologise if I'm asking something obvious. Please enlighten me.

  2. jcsd
  3. Nov 10, 2015 #2


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    If you haven't already studied Ballentine's textbook ("QM -- A Modern Development"), I think you might like it. Much of the nonsense surrounding "collapse" is discarded, and he shows how time-ordering is necessary when working with non-commuting observables, leading to a modification of the classical rules for joint and conditional probabilities.

    IMHO, you won't get anywhere useful if you abandon causality. Modern Quantum Field Theory (which is experimentally verified to very high precision) relies on the use of causal quantum fields.
  4. Nov 10, 2015 #3


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    There have been a number of experiments in which "backwards in time" effects are present. Please keep in mind these are interpretation dependent. So the effect lies in the eye of the beholder. As an example, you can entangle particles after they have been observed and no longer exist:


    A seemingly paradoxical situation arises — as suggested by Peres [4] — when Alice’s Bellstate analysis is delayed long after Bob’s measurements. This seems paradoxical, because Alice’s measurement projects photons 0 and 3 into an entangled state after they have been measured. Nevertheless, quantum mechanics predicts the same correlations. Remarkably, Alice is even free to choose the kind of measurement she wants to perform on photons 1 and 2. Instead of a Bell-state measurement she could also measure the polarizations of these photons individually. Thus depending on Alice’s later measurement, Bob’s earlier results either indicate that photons 0 and 3 were entangled or photons 0 and 1 and photons 2 and 3. This means that the physical interpretation of his results depends on Alice’s later decision.

    "Such a delayed-choice experiment was performed by including two 10 m optical fiber delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate [sic] that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

    Figure 1: Shows diagram of setup.
    "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. ... This procedure
    projects photons 0 and 3 into a corresponding entangled state. [Bob] 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. Interestingly, the quantum prediction for the observations does not depend on the relative space-time arrangement Alice’s and Bob’s detection events. "

    Figure 3: Shows Fidelity, and Fidelity with Delayed Choice. [Note the words "delayed choice"]
    "The square dots represent the fidelity for the case that Alice’s and Bob’s events are space-like separated, thus no classical information transfer between Alice and Bob can influence the results. The circular dot is the fidelity for the case, that Alice’s detections are delayed by 50 ns with respect to Bob’s detections. This means, that Alice’s measurement projects photon 0 and 3 in an entangled state, at a time after they have already been registered."
  5. Nov 11, 2015 #4
    Could it be that the result of the measurement by Bob is in fact in a superposition of states, until the results of both measurements are brought together? You could also see this as that Bob is in (at least) two universes at the same time...? The results have to be decohered by bringing the data together? Data can only be brought together at at most the speed of light! So causality is not violated! If there has been no sub-luminal communication yet, then it makes no sense to talk about causality!
  6. Nov 11, 2015 #5


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    It is true that all of the data must be brought together at sub-luminal speeds to make sense of the results. It is equally true that the decision to entangle the photons can be made after the photons have ceased to exist, and that can be in all reference frames (because they are co-located).

    Again, this experiment is open to multiple interpretations.
  7. Nov 11, 2015 #6
    Thank you ! That's just what I was looking for !
    The effect can go "backwards in time", as I expected. My question is answered.

    Which begs the question; why don't the educators give students an intuitive understanding of QM? So long as people describe these effects as "paradoxical", we are not doing our jobs.

  8. Nov 11, 2015 #7
    No, I don't think so. These concepts (superposition, alternate universes, collapsing) are artefacts created by our assumption that the future hasn't happened yet.

    If you look at the experiment from "outside of time", without thinking of the future as indeterminate, it becomes straightforward. The measurement made at t2 imposes constraints on the entirety of the wave function, past and future, guaranteeing Bob's result at the earlier time t1. There is no need to postulate a second universe!

    The interesting thing about QM is how it prevents you setting up a "time paradox" by various means.

    Last edited: Nov 11, 2015
  9. Nov 11, 2015 #8
    In fact (but I am no expert on it, far from), I think that this is somewhat too firmly put. There is no way of knowing if entanglement 'goes back in time' if you don't measure it! There had been some misunderstanding that there had to be some 'collapse' due to a 'conscious observer', however this notion has become outdated (though still popular in pseudoscientific circles). If I have got it correctly, decoherence is the more recent explanation: maybe decoherence between all the observed measurements (eq. 'watching them') is necessary to constitute the correlation between them. When you don't look at the results, there is no way of knowing that if theory doesn't help you out! (but I may be entirely amiss at this one)
    Last edited: Nov 11, 2015
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