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In the process of 'teleporting' a photon with a pair of entangled electrons, how is the information transferred between the two entangled particles? Could it be that the information is travelling at the speed of light, if not, faster?

My question lies in the fact of whether or not the information passed between two entangled particles is instantaneous. I would assume that the speed of light cannot be trumped, but I'm curious to know if the particle separation contributes the information to consider a set speed.

Norbert Schuch
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A. Rudzinski
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    Hi. I think in principle there is not information transfer, in the sense that the system is described by a wavefunction having both properties... so I mean, when one of the two subsystems get's in one state the other, theoretically must be at the same time in the other state. – Constantine Black Sep 06 '17 at 18:05
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    From this point of view the subsystems that are space apart communicate instantaneously. But that doesn't mean faster than light communications since, to see in what state each of the sub systems is you must make a measurement and send a message to the other subsystem via a channel of non- faster than light communication. – Constantine Black Sep 06 '17 at 18:09
  • @ConstantineBlack That sounds like an answer. – garyp Sep 06 '17 at 18:12
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    Possible duplicate of Do quantum particles communicate? – Norbert Schuch Sep 06 '17 at 22:02

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I think in principle there is not information transfer, in the sense that the system is described by a wavefunction having both properties... so I mean, when one of the two subsystems get's in one state the other, theoretically must be at the same time in the other state.

From this point of view the subsystems that are space apart communicate instantaneously. But that doesn't mean faster than light communications since, to see in what state each of the sub systems is you must make a measurement and send a message to the other subsystem via a channel of non- faster than light communication.

Constantine Black
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We don't know. More specifically, the answer to your question is interpretation-dependent.

In objective-collapse interpretations, for example, you do have an explicit collapse of the wavefunction of the first particle to be measured, followed by an instantaneous (?) collapse of the entangled partner, with all the special-relativity problems brought about by the unspecified temporal ordering of the two measurements if their separation is spacelike. To try and make sense of that, you need to allow the particles to 'communicate' their states superluminally; that said, the randomness of the collapse 'protects' the superluminal communication and makes it inaccessible to us.

If you don't like it, then well, that's just how that interpretation looks like, so you can turn to the epistemological comforts of, say, the Many-Worlds Interpretation, which will just tell you that if you measure one of the parties of an entangled state, all you do is become a part of what is now a tripartite entangled state, and the information transfer... what information transfer?

Ultimately, Bell's theorem (and the subsequent observations of Bell-inequality violations in experiment) tells us that, if nature is 'real' at some underlying level, then that reality needs to be nonlocal in some sense. While we have good grasps on certain bounds that that nonlocality needs to obey (like the no-communication theorem) we still don't understand the nature of that nonlocality as well as we would like. And frankly, anybody that tells you that we understand it is either fooling you or fooling themselves.

Emilio Pisanty
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  • Could I add as a comment about your last paragraph: the non-locality in some sense, is related interpretationally with the 4d spacetime as we perceive it, then we could say that an interpretation that takes as real-existing physically all the inner spaces(Hilbert spaces) remains local in the sense that there is a very definite description of the inner properties of the entire system, so there is no need for information transfer, it' s the system that obeys a certain geometrical relation that changes, just not in 4d spacetime. I don't know if that makes any sense to you. Thanks. – Constantine Black Sep 06 '17 at 19:42
  • You can keep the local but have to drop the real. Which also explains why not even in principle Heisenbergs uncertainty relations can be violated. (Think in this terms, if even measurements which were not performed would have a defined value, it seems quite unatural why you can not select an ensemble with two complementary observables with sharp defined values) – lalala Sep 06 '17 at 20:28
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Theoretically quantum entanglement is instantaneous, because the entangled particles are not two independent particles that "need to communicate", but more like one particle that exists in two places at the same time. So what happens to it, happens to it obviously in both places without a delay. While this poses a simultaneity problem in special relativity, it does not violate the restriction that nothing can move faster than light. This restriction applies to objects moving with a certain speed, but in quantum entanglement nothing actually moves and there is no particular speed that would be specifically faster than the speed of light.

Experimentally it is impossible to measure if anything is truly instantaneous, because of the precision of the measuring instruments. All we can do is to establish the lower limit. This limit for quantum entanglement has been set as at least 10,000 times faster than light. Just google for "quantum entanglement 10000 times speed of light" and you'll get multiple references.

safesphere
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