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Thanks for the explanation of what "teleportation" means in this context. I guess that word is maybe not the best choice as "quantum teleportation" conjures up

Thanks for the explanation of what "teleportation" means in this context. I guess that word is maybe not the best choice as "quantum teleportation" conjures up all sorts of images in an average person's head, but that does seem to be the case for a lot of quantum stuff. I guess where I still get hung up with entanglement is the idea that "classical bits" are necessary at all in the operation. I get that reading the state of the particle on the receiving side is destructive, so you can only do it once. But how is it that just reading it doesn't give you any information without those "classical bits"? Is it something like "there's 10 different ways to read it, and if I don't use the right way I get out some junk, but if I pick the right way I get a definite signal"? So then the only information you're sending to the receiver side using classical bits is the instruction manual on how to read this particular qubit, without which they can't be certain if they really received something at all? I also get that we might be getting past the point where it's easily explainable without a lot of math jargon. :)

jerf 2y ago
"I guess that word is maybe not the best choice as "quantum teleportation" conjures up all sorts of images in an average person's head, but that does seem to be the case for a lot of quantum stuff." The word teleportation is IMHO a reasonable term for what is occurring, because of the fact that particles are otherwise indistinguishable from each other. If you have some state on an electron, and you perfectly convey that state, entanglement and all, to some other electron, there's a fairly realistic sense that you have "teleported" that electron, because its quantum state and that it is an electron is all there really is to a particle. Particles don't have their own identity. It isn't really realistic to trace "the" path "an" electron takes through the universe. (The recent PBS Space Time about electrons has an illustration that will, in passing, demonstrate this: https://www.youtube.com/watch?v=KXPhaAsnrfs https://www.youtube.com/watch?v=KXPhaAsnrfs , around 13:05.) You can't teleport large systems, but it's not a math restriction, it's just a brute physical restriction; you need a non-trivial machine to transmit something and there's just never going to be an engineering solution to transmitting macroscopic quantities of quantum information somehow. People get really confused about this because Star Trek transporters are magic, straight-up lunatic nonsense magic, but it defines the term for people, and those actually move matter around. Quantum teleportation does not; if you want to teleport matter (protons, electrons, etc) it requires the target already be present at the other end. So to "quantum teleport" a kilogram of water you'd have to have a kilogram of water already at the other end, and you'd still have a kilogram of water at the original source. It's only the quantum state that would move. So it has nothing to do with teleportation in the Star Trek sense. The other key thing to understand about quantum communication is that you basically don't "transmit" any information at all. The payload that comes out of these systems is basically a stream of zeros and ones, where you know that the other side has the same stream, but neither side controls the stream, so the stream is quantum-level random. (Some engineering may need to be done for this, e.g., one side may be inverting all their bits, but this is the end result.) From here you can use this to construct keys that you know are not intercepted, and you do indeed send other conventional communication that is obscured by the quantum bit stream. (You can think one-time pad for this as at least a model, whether or not that's what they do.)
- BobaFloutist 2y ago
  People don't get confused because Star Trek is "magic nonsense", they get confused because "teleport" is a longstanding speculative fiction word that quantum scientists decided to co-op to describe something almost, but not quite, entirely unlike teleportation. It's like if you called something "quantum resurrection" and then making fun of people for asking if it brings people back to life because that's obviously impossible, you buffoon, resurrection is actually when someone's already alive and you check and they're still alive, idiot, stop reading the Bible and start reading more Physical Review.

jerf 2y ago

"I guess that word is maybe not the best choice as "quantum teleportation" conjures up all sorts of images in an average person's head, but that does seem to be the case for a lot of quantum stuff." The word teleportation is IMHO a reasonable term for what is occurring, because of the fact that particles are otherwise indistinguishable from each other. If you have some state on an electron, and you perfectly convey that state, entanglement and all, to some other electron, there's a fairly realistic sense that you have "teleported" that electron, because its quantum state and that it is an electron is all there really is to a particle. Particles don't have their own identity. It isn't really realistic to trace "the" path "an" electron takes through the universe. (The recent PBS Space Time about electrons has an illustration that will, in passing, demonstrate this: https://www.youtube.com/watch?v=KXPhaAsnrfs https://www.youtube.com/watch?v=KXPhaAsnrfs , around 13:05.) You can't teleport large systems, but it's not a math restriction, it's just a brute physical restriction; you need a non-trivial machine to transmit something and there's just never going to be an engineering solution to transmitting macroscopic quantities of quantum information somehow. People get really confused about this because Star Trek transporters are magic, straight-up lunatic nonsense magic, but it defines the term for people, and those actually move matter around. Quantum teleportation does not; if you want to teleport matter (protons, electrons, etc) it requires the target already be present at the other end. So to "quantum teleport" a kilogram of water you'd have to have a kilogram of water already at the other end, and you'd still have a kilogram of water at the original source. It's only the quantum state that would move. So it has nothing to do with teleportation in the Star Trek sense. The other key thing to understand about quantum communication is that you basically don't "transmit" any information at all. The payload that comes out of these systems is basically a stream of zeros and ones, where you know that the other side has the same stream, but neither side controls the stream, so the stream is quantum-level random. (Some engineering may need to be done for this, e.g., one side may be inverting all their bits, but this is the end result.) From here you can use this to construct keys that you know are not intercepted, and you do indeed send other conventional communication that is obscured by the quantum bit stream. (You can think one-time pad for this as at least a model, whether or not that's what they do.)

BobaFloutist 2y ago
People don't get confused because Star Trek is "magic nonsense", they get confused because "teleport" is a longstanding speculative fiction word that quantum scientists decided to co-op to describe something almost, but not quite, entirely unlike teleportation. It's like if you called something "quantum resurrection" and then making fun of people for asking if it brings people back to life because that's obviously impossible, you buffoon, resurrection is actually when someone's already alive and you check and they're still alive, idiot, stop reading the Bible and start reading more Physical Review.