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from the rest-of-the-person-will-teleport-along-shortly... dept.
Physicists at the University of Geneva (UNIGE) ( http://cms.unige.ch/gap/quantum/wiki/research:quantum_communication:teleportation ) have succeeded in teleporting the quantum state of a photon to a crystal over 25 kilometres of optical fibre. The experiment, carried out in the laboratory of Professor Nicolas Gisin, constitutes a first, and simply pulverises the previous record of 6 kilometres achieved ten years ago by the same UNIGE team. Passing from light into matter, using teleportation of a photon to a crystal, shows that, in quantum physics, it is not the composition of a particle which is important, but rather its state, since this can exist and persist outside such extreme differences as those which distinguish light from matter.
http://www.sciencedaily.com/releases/2014/09/140921145007.htm
(Score: 2) by wonkey_monkey on Thursday September 25 2014, @12:09PM
I assume you meant "defined" where you've used "predefined" here.
Off the top of my head, I'd guess the reason you can't do that is that you can't actually measure whether or not a particle is in a superposition. Can a real physicist chip in here? ;)
Since the local particles are entangled, you'll always find them in the same state no matter when you measure them, even if you do it before the distant particle has been measured.
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(Score: 2) by q.kontinuum on Thursday September 25 2014, @12:46PM
That's what I was thinking initially: How do we know the particles from the original article are in a superposition and not just have the same internal state already? Apparently, Bells theorem is a way to detect this difference. So, in other words, my conclusion is:
I think there is an error somewhere in my considerations because otherwise I'd think others would have thought of this before. But I can't spot the error. Probably it is related to my lack of deep understanding of "the Bell thing", but if that's the case I will come back to my original question and ask, what is the reason to assume that particles ever have the superposition rather than already having the final state, with us just being unable to see it?
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(Score: 2) by wonkey_monkey on Thursday September 25 2014, @05:15PM
It's not. It provides a way to test whether or not spin, in general, is a hidden variable or a quantum one - and it's a quantum one, always - but it's not a test you can apply to a single photon (or pair) for a yes/no answer.
systemd is Roko's Basilisk
(Score: 2) by q.kontinuum on Friday September 26 2014, @07:18AM
I don't want to focus too much on the Bells theorem. But I expect that there is some base for the assumption that the superposition actually exists, so particles behave somehow different depending on if they were in a superposition or if their state was defined and just not known to us; otherwise the whole theory of quantum mechanic and superposition could be axed by Occams razor.
My whole idea is, if you have three entangled particles in a superposition, and you explicitly collapse the wave function for one of them, the wave function of the remaining two should also collapse. If the superposition has any practical meaning at all (and I suppose it does, afaik that's the whole point of e.g. quantum computers), it should be possible to detect/test this without knowing anything about the third particle.
As I understood the explanation, the Bell theorem should prove the superposition on a statistical base, not for a single photon. (According to the link [scientificamerican.com] I posted, there is an experiment with a probability of 50% for a certain outcome when the state is predefined, and 55% probability when the particles are in a superposition. So, with the 10^6 particle pairs I mentioned in my previous post, there are 1 million experiments which should be enough to get a halfway certain answer to the question if the probability was 50 or 55 percent, i.e. if the particles where in a superposition or not. If this is not certain enough, increase the number of particles.)
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(Score: 2) by wonkey_monkey on Friday September 26 2014, @12:32PM
My whole idea is, if you have three entangled particles in a superposition, and you explicitly collapse the wave function for one of them, the wave function of the remaining two should also collapse.
Yes, it will, but measuring one of the other two photons also collapses the wavefunction. There's no information to be gleaned.
As I understood the explanation, the Bell theorem should prove the superposition on a statistical base, not for a single photon.
It proves it for all photons, everywhere, always - it's not true for some groups of photons and untrue for others.
According to the link I posted, there is an experiment with a probability of 50% for a certain outcome when the state is predefined, and 55% probability when the particles are in a superposition.
"Being in a predefined state" is not the opposite of "being in a superposition." When you say "when the state is predefined" - well, it never is, that's what the Bell theorem's experiments tell us. You can only "force" a photon into a definite (not predefinite) state along a single axis. As soon as you do, the state along all other axes is undefined (albeit probabilistically correlated with the original measurement, depending on the new axis). Measure along the same axis again, and you'll get the same measurement. Measure along a nearby axis, and you'll probably get the same measurement, but maybe not. And as soon as you make that second measurement, spin along the original axis becomes undefined (albeit probabilistically correlated to the new axis in the same manner as before).
"Being in a predefined state" was one way of explaining the quantum behaviour of certain particle properties, but the Bell theorem experiments proved it not to be true.
systemd is Roko's Basilisk