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I read this article about time teleportation. Can someone explain the concept better?

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2 Answers 2

up vote 2 down vote accepted

The preprint is here:

http://arxiv.org/abs/1101.2565

Time teleportation, as described by the Australian ... physicists ... is a meaningless concept that is based on a misinterpretation of what entanglement is and what it is not. The true portion of their statements is well-known and trivial and they vaguely suggest things that are not true, too.

Entanglement is just a correlation between two or many objects - it is a correlation that may affect all properties of the two systems - so it allows two objects to be more completely "identified" than in classical physics. However, it is still a correlation.

In normal situations, the correlation exists between two spatially separated objects. If one of them is measured and we know the result, we have to use the conditional probability - taking the known result of the measurement into account as a condition - to predict the probabilities of the other measurement. We may also run the logic in the opposite order and we obtain exactly the same results.

Of course that the two objects that were measured may also be separated by time-like separations. The correlation will still exist and the predictions will be correlated just like before. Except that the mystery will evaporate. The entanglement is mysterious - at least for someone - exactly because it seems that one particle has to send a signal about its "decision" to the other particle (it sends no signals!) and such a signal would seemingly have to propagate faster than light which conflicts with relativity (no signal is needed for the correlation!).

However, when the two measurements are time-like separated, there is no paradox, not even for the people who think that the entanglement needs "signals". Even if a signal were needed, it could get from one particle to the other particle by a speed that is smaller than the speed of light.

So "teleportation in time" is just the same entanglement minus all the mystery that results from the spacelike separation, and such a "teleportation in time" may be viewed as nothing else than the later particle's behavior according to the correlated properties with the first particle that has decided about both. There is nothing mysterious about these things and it is not useful for anything because such an "entanglement in time" may be replaced by an ordinary storage of information.

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I've not read the paper yet, but I just want to tell that Tim Ralph is a professional physicist, doing real research on quantum information and not some random guy. The paper might be oversold, but I dount it is a "meaningless concept." –  Frédéric Grosshans Jan 21 '11 at 16:51
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Dear Frédéric, in science, it doesn't matter what your name is, what your salary is, and so on. I am looking at the actual stuff that was presented - and nothing else - and it is pure garbage. Whether it's because they couldn't do better or they deliberately turned it into garbage to make it more attractive is a different question. –  Luboš Motl Jan 21 '11 at 18:28

Some more details:

1) In quantum physics, the behavior of objects is not deterministic. A "quantum state" gives you probabilities for the different possible behaviors. "Entanglement" means that the probabilities for two or more objects are correlated. This happens when objects interact with each other.

2) "Teleportation" is a way to transfer a quantum state from one object to another object, which uses entanglement with a third object and also ordinary signaling. You start with two objects that have interacted so they are maximally entangled with each other (maximal entanglement has a precise mathematical meaning). You let the original object (the one whose state you want to teleport) interact with one of these, you measure the results, and then you act on the second entangled object in a way determined by those results. This will recreate, in the second entangled object, the quantum state of the original object.

3) If you have heard of Fourier analysis, you may know that a field can be expressed as a sum of "modes", each with a specific wavelength and direction of movement. Classically you might just say how strong a field mode is (e.g. its Fourier coefficient, if it is a Fourier mode). But when you describe a field according to quantum mechanics, each field mode has to be described by a quantum state, in terms of probabilities. Because of the uncertainty principle, even in the lowest-energy state (the vacuum state, no particles present), each field mode has some probability of being active. Furthermore, the modes will interact (directly or indirectly), and so the probabilities of the different modes won't be independent - their quantum states will be entangled. And you can even imagine using this vacuum entanglement to perform quantum teleportation.

4) There has been some interest in experimentally detecting the entanglement of the vacuum field modes, but it would be difficult because your particle detector (that detects vacuum fluctuations in the field modes) has to undergo constant acceleration for the duration of the experiment. The Australian physicists thought of an alternative way to detect the mode correlations, where the detector remains at rest but is progressively retuned to mimic the effects of accelerated motion. So the behavior of the detector in the future is correlated with the behavior of the detector in the past, when it was responding to different modes. But they are still the same modes whose entanglement might have been detected in an equal-time experiment.

5) "Teleportation in time" is therefore just a form of the ordinary teleportation, in which there is a prominent time delay. In particular, what this paper describes is quantum teleportation via entangled field modes of the vacuum. Remember that for ordinary teleportation to work, you need information about the measurement results at the first step - they tell you what to do to the second entangled object, in order to recreate the original quantum state. So time teleportation also has to start with a measurement, stored somewhere in a conventional fashion (e.g. in a computer memory), and then the future will use that information to recreate the old quantum state by interacting with one of the vacuum field modes.

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+1 Good explained –  Amir Rezaei Jan 21 '11 at 9:06

protected by Qmechanic Mar 17 '13 at 21:45

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