5 ms·

If you can't retrieve the information "quantum teleported" until something else bridges the gap afterwards, then how can you say that anything was teleported at

by jaekwon 14y ago

If you can't retrieve the information "quantum teleported" until something else bridges the gap afterwards, then how can you say that anything was teleported at all? There would have to be a qualitative or quantitative difference between what was supposedly teleported and the "key" that must be transfered afterwards. Either you can teleport a gigabyte of information and retrieve it with a kilobyte key, or you can teleport a material thing with a "key" comprised of less information needed to encode its molecular structure. Otherwise, teleportation seems to be a misnomer.

srl 14y ago
> Otherwise, teleportation seems to be a misnomer. It is. People call it teleportation because when you attempt to mesh the results you get from experiments involving quantum entanglement with our intuitive model of physics, it looks like something has been teleported instantaneously (the state of the photon or what have you). But in reality, no information is transmitted faster than the speed of light, and there's no good reason that will change.
- greeneggs 14y ago
  People call it teleportation because it moves the state of a system from one place to another, without going through the intermediate positions. I'm not sure where this idea came from that teleportation has to be "instantaneous." From special relativity, we know that the word "instantaneous" isn't even well-defined, except in a fixed reference frame. Basically, quantum teleportation takes the quantum state of a system, measures it. The measurement destroys the system and converts it into classical information that can be sent over a phone line. The information, once received, can be used to reconstruct the initial state. If you think of a classical system, as computer scientists usually do, this is not very surprising. It seems obvious that in principle you could look very carefully at a complicated classical system, a human being for example, write down all of its details down to the positions of the individual atoms as a long classical string, and send this information down a phone line to reconstruct the person at the other end. It is only surprising because quantum teleportation works with quantum systems, and we know that there is no physical way of extracting all the details of a quantum system as a long classical string (Heisenberg's uncertainty principle, etc.).
sp332 14y ago
The point is that you can exactly recreate a particle including its entire quantum wave function (preserving uncertainties) from one place to another without "existing" in between. That's why it counts as teleportation: you get the same object, not a similar object but the same object all they way down to quantum fluctuations, at a distance.
- Symmetry 14y ago
  That actually isn't how this works. Yes, you do have two objects with entangled wavefunctions but that isn't remarkable at all. And the entanglement occurs when the objects are created at the same place, and then they proceed to the measurement points using entirely conventional slower-than-light means. Its only a single bit, the polarization of the photon, that is found to have been determined in both places simultaneously. Its true that this is a new piece of information that arises in both locations simultaneously, but since this is a thing that appears in two places rather than something that moves from place A to place B I'd argue that the term "teleportation" is pretty misleading here. After the measurement the measured particle is hopelessly entangled with the measurement apparatus and through that with the surrounding universe, so the unique link between the two particle is hopelessly broken at that point.
  - sp332 14y ago
    found to have been determined in both places simultaneously. That's not true. You measure one of the qubits, and its state is destroyed, randomly scrambled. When you transmit the information and apply it to the other qubit, it now has the same polarization as the first one had, before. You still don't know what the polarization is though! The entanglement is still intact, and you haven't made a measurement of the system (as a whole) so the wavefunction has not collapsed. http://en.wikipedia.org/wiki/Quantum_teleportation#Remarks http://en.wikipedia.org/wiki/Quantum_teleportation#Remarks
    - Symmetry 14y ago
      Oh, huh, the process described there is distinct from the sort of "Quantum Teleportation" that I was used to. Using an entangled pair and some transmission of classical bits to transmit a qubit across a distance is pretty neat, and this "quantum teleportation" really is quantum teleportation. It's only sort of related to the cryptogrpahic technique I'd previously heard described as "quantum teleportation", but in the absence of evidence as to who grabbed that pair of words first I won't complain.

by jaekwon 14y ago

srl 14y ago

> Otherwise, teleportation seems to be a misnomer. It is. People call it teleportation because when you attempt to mesh the results you get from experiments involving quantum entanglement with our intuitive model of physics, it looks like something has been teleported instantaneously (the state of the photon or what have you). But in reality, no information is transmitted faster than the speed of light, and there's no good reason that will change.

greeneggs 14y ago
People call it teleportation because it moves the state of a system from one place to another, without going through the intermediate positions. I'm not sure where this idea came from that teleportation has to be "instantaneous." From special relativity, we know that the word "instantaneous" isn't even well-defined, except in a fixed reference frame. Basically, quantum teleportation takes the quantum state of a system, measures it. The measurement destroys the system and converts it into classical information that can be sent over a phone line. The information, once received, can be used to reconstruct the initial state. If you think of a classical system, as computer scientists usually do, this is not very surprising. It seems obvious that in principle you could look very carefully at a complicated classical system, a human being for example, write down all of its details down to the positions of the individual atoms as a long classical string, and send this information down a phone line to reconstruct the person at the other end. It is only surprising because quantum teleportation works with quantum systems, and we know that there is no physical way of extracting all the details of a quantum system as a long classical string (Heisenberg's uncertainty principle, etc.).

sp332 14y ago

The point is that you can exactly recreate a particle including its entire quantum wave function (preserving uncertainties) from one place to another without "existing" in between. That's why it counts as teleportation: you get the same object, not a similar object but the same object all they way down to quantum fluctuations, at a distance.

Symmetry 14y ago
That actually isn't how this works. Yes, you do have two objects with entangled wavefunctions but that isn't remarkable at all. And the entanglement occurs when the objects are created at the same place, and then they proceed to the measurement points using entirely conventional slower-than-light means. Its only a single bit, the polarization of the photon, that is found to have been determined in both places simultaneously. Its true that this is a new piece of information that arises in both locations simultaneously, but since this is a thing that appears in two places rather than something that moves from place A to place B I'd argue that the term "teleportation" is pretty misleading here. After the measurement the measured particle is hopelessly entangled with the measurement apparatus and through that with the surrounding universe, so the unique link between the two particle is hopelessly broken at that point.
- sp332 14y ago
  found to have been determined in both places simultaneously. That's not true. You measure one of the qubits, and its state is destroyed, randomly scrambled. When you transmit the information and apply it to the other qubit, it now has the same polarization as the first one had, before. You still don't know what the polarization is though! The entanglement is still intact, and you haven't made a measurement of the system (as a whole) so the wavefunction has not collapsed. http://en.wikipedia.org/wiki/Quantum_teleportation#Remarks http://en.wikipedia.org/wiki/Quantum_teleportation#Remarks
  - Symmetry 14y ago
    Oh, huh, the process described there is distinct from the sort of "Quantum Teleportation" that I was used to. Using an entangled pair and some transmission of classical bits to transmit a qubit across a distance is pretty neat, and this "quantum teleportation" really is quantum teleportation. It's only sort of related to the cryptogrpahic technique I'd previously heard described as "quantum teleportation", but in the absence of evidence as to who grabbed that pair of words first I won't complain.