It is my understanding that particle/antiparticle pairs that are created are action-at-a-distance linked so that to have a state-change (spin direction, momentum?, whatever) in one would cause a corresponding state change in the other to keep with conservation laws.
Is this correct?
Secondly, once such a state change has happened, are the particles no longer “linked” in this way or are they permanently bound to each other across time and space?
Thirdly: If an electron “jumps” energy states in its shells/cloud in an atom by absorbing a photon, and this photon was one of our “linked” photons, and we “changed” the other photon (say, messed with its polarization), in what way would this affect the electron, or would it at all? Similarly, would “being absorbed” by an electron affect our seperate but linked photon in some way?
As far as I can recall (years away from Q-theory courses) they are still anti-pairs and thus are bound. If this isn’t true in every case, I’m sure I’ll be corrected.
The anti-pairs are pairs in energy and state. If one of the pairs is altered (e.g.; absorbing/emitting energy) they aren’t anti-pairs any more. (To the best of my recollection.)
Does this mean that once the wave function has collapsed they are no longer paired particles? I read somewhere that no information could be transmitted using this linked nature; is this why? I believe a poster here had once informed me that this was the case, but I had never heard this before in any of the “popular” accounts of QM. They never implied otherwise, either, just sort of left the whole thing undiscussed, which is a shame because it has recently caused a bit of thought in my head (which isn’t worth bringing up in GQ I suppose).
Information cannot be trannsmitted by using these linked photons. If you measure one there is a 50/50 chance its spin will be up measured in a particular direction. If I measure the other one I will get the same result. If you measure a bunch of them you will get a random sequence of up and down. I would get the same sequence, but this cannot be used to transmit information, because each sequence is random.
Scientific American usually has an article about some facet of this every year or so. From what I remember of the last one I read, once one of the linked pair collapses, both do. At that point they’re no longer linked.
Yes, pretty much. Be careful with your language, though. There is no “action-at-a-distance”, and the particles aren’t “linked”, per se. They have several possible states they can be in, but must obey a conservation rule, so when one particle is forced to choose a state, the other instantaneously chooses the corresponding state required by conservation. This means that wave functions are not inherently local, as was previously assumed, but it doesn’t violate relativity. It’s pretty counter-intuitive, though, and it would be nice if we could think of a better formalism that did away with that aspect of things.
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No, not permanently bound.
See above. The “link” merely affects the choice of states. Each particle on average still occupies states with the correct probabilitity distribution. Once the state choice is made, there is no correlation between the particles’ wave functions.
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Interacting with the electron will force the photons to be in a particular spin state, for example, as the excited electron will have some total angular momentum determined by the angular momentum of the photon. So the photons both have to collapse into a specific orientation when one of them interacts with the electron. The electron just sees a photon in a particular spin state.
And, Drake, you can’t transmit information faster than light speed using these pairs, but you can do quantum computation with entangled quantum systems. This may be what you’re thinking of…
Giraffe:
2600 has a pretty good layman-level discussion of quantum entaglement computing in its latest issue. So hie thee to a good bookstore (Barnes & Noble carries them) and buy it, because the article ain’t online.
Sub atomic particles whose wavefunctions are entangled are called correlated pairs not particle/antiparticle pairs. The latter relate to an entirely different phenomenon. If a particle and it’s anti particle interact they annihilate each other.
I stand corrected. Information can be transmitted by a carrier particle(wave?). What I should have said is information cannot be transmitted faster than light by this or any other means.