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Recently I've been reading about advances in quantum mechanics, particularly in the field of quantum teleportation. It's a difficult subject to understand, and for that reason I explicitly don't trust the reporting on it. Many articles,

http://www.livescience.com/7647-teleportation-milestone-achieved.html

by using their protocol, atom-to-atom teleported information can be recovered with perfect accuracy about 90 percent of the time — and that figure can be improved.

http://www.technologyreview.com/view/427910/chinese-physicists-smash-distance-record-for/

Teleportation turns out to be extremely useful. Because teleported information does not travel through the intervening space, it cannot be secretly accessed by an eavesdropper.

appear to be saying that information is being transmitted from one location to another instantaneously. As in, faster than the speed of light. From what I DO understand about relativity, this has the potential to massively violate causality. Is the effect of quantum teleportation being misreported, misunderstood by me, or are they actually transmitting usable information at faster than light speed?

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    To be clearer about the idea of "usable information". I don't think determining the state of your qubit and thus determining the state of the distance qubit is "usable information", since you can't choose what data to send. – John Rhoades Aug 21 '12 at 13:37
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    Isn’t this question much better suited on physics.SE? – Konrad Rudolph Aug 21 '12 at 14:30
  • The actual claim is that they hope to use Quantum teleportation(and I think believe they will be able to) to transmit information. Not that they can do this today. – Chad Aug 21 '12 at 14:37
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    A popular classical analogy for how "information" about a far away object can be acquired instantaneously is a pair of gloves: Imagine you have a pair of gloves, you put the left-handed glove in one box and the right-handed one in another box. Now you send your friend, who lives on another planet, one of the boxes... – Oliver_C Aug 21 '12 at 17:34
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    [continued]... Even though the gloves are two seperate objects, they are actually one "unit". If your friend opens his box and sees a left-handed glove he automatically knows the content of your box is a right-handed glove. He just gained knowledge about a far away object instantly without violating Special Relativity. "Quantum Entanglement", which is the basis for "Quantum Teleportation" is a bit more complicated, but the point is, that two (or more) objects can be "linked" in a special way, so that they form one "unit". – Oliver_C Aug 21 '12 at 17:35
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From the first article you linked:

But because ion A's state is irreversibly tied to ion B's, the measurement also forces B into the complementary state. Depending on which state ion A is found in, the researchers now know precisely what kind of microwave pulse to apply to ion B in order to recover the exact information that had been written to ion A by the original microwave burst. Doing so results in the accurate teleportation of the information.

There is no faster-than-light information transfer. To retrieve the information from the destination, you need the results from the sending operation at the start. While the QuantumTeleportation is "instant" you cannot actually get any information out of the system until a light-speed message arrives at the destination as well. The advantage, is that you can eavesdrop this light-speed message and be unable to get the information, as you don't have access to the entangled particle.

An analogy would be this process: You have some quantum carbon paper. You write an encrypted message on the top sheet, which in turn writes something to the bottom. You then separate the bits of paper. Someone then measures paperA and records the result. They send this result to someone who has paperB (loudly and in public) who then does a measurement to determine the original message.

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  • Would it not be feasible to apply many different wavelengths of "microwave burst" to the ion (presumably hitting the correct one somewhere in the set), receive all the data back, use the decoding standard, and figure out which was the original data (based on which wavelength DOESN'T result in pure gibberish) all without knowing the state of ion A? – asteri Sep 20 '12 at 13:17
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    @Jeff That's not going to work. It would be like trying to brute force a message encrypted with a one-time-pad. You will get back all possible messages, which doesn't tell you anything. – Nick Sep 20 '12 at 13:30
  • There is no faster-than-light information transfer... in quantum physics! (uh, or anywhere else that we know is possible. I just mean that it's a sensible tenet, and likely correct, but there are modern theories that break it) – user11172 Jan 22 '13 at 0:27
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The OP is clearly not asking about the sort of entanglement effects seen in EPR-type experiments, so the gloves analogy is not relevant. He wants to know whether 'spooky action at a distance' is possible, ie whether FTL or even instantaneous communications are possible, ie whether the relativistic conception of a light cone is mistaken.

A Cambridge University page says in relevant part:

Once considered impossible, in 1993 a team of scientists calculated that teleportation could work in principle using quantum laws. Quantum teleportation harnesses the ‘entanglement’ law to transmit particle-sized bites of information across potentially vast distances in an instant.

However, that page does not further identify the 1993 'team of scientists', and I was unable to find anything further on them by Googling. I suspect that this is poor reporting, and that causality remains inviolate. I cannot see any claims to the contrary in the paper linked to on that Cambridge page.

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