ความคิดเห็นที่ 10
Experiments
The first experimental demonstration of entanglement was carried out at the University of Innsbruck in 1997, by the group of Anton Zeilinger. Their setup allowed to teleport the polarization quantum state of photons across an optical table. As source of entanglement they used parametric down-conversion. A slightly earlier experiment in Rome, by the group of DeMartini, showed the principle of teleportation, however it involved only two particles, instead of three: one carrying the state to be teleported, and two entangled ones, which provide the "quantum channel" and must be initially independent of the one carrying the state.
In 2004, another research group at the University of Innsbruck (Austria) and a group at NIST (Boulder, Colorado) demonstrated the teleportation of atoms: The electronic quantum state of a calcium (Innsbruck) or beryllium (Boulder) ion was teleported to another one, with all the ions being held in a linear ion trap.
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Star Trek beaming
That the term teleportation reminds of the beaming process in the Star Trek TV series might have been intentional. After all, the physicists coining the term might have thought, it is as if the particle would be brought there. But this analogue is prone to miss the point: Only the information about the quantum state is brought there, the particle to take up the state must already be present.
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Entanglement swapping
If a state being teleported is itself entangled with another state, the entanglement is teleported with it. To illustrate: If Alice has a particle which is entangled with a particle owned by Carol, and she teleports it to Bob, then afterwards, Bob's particle is entangled with Carol's.
A more symmetric way to describe the situation is the following: Alice has one particle, Bob two, and Carol one. Alice's particle and Bob's first particle are entangled, and so are Bob's second and Carol's particle:
Alice -:-:-:-:- Bob1 Bob2 -:-:-:-:- Carol
Now, Bob performs a Bell measurement on his two particles, which projects them into a Bell state, i.e. they are now entangled. But, more spectacularly, Alice's and Carol's particles are now entangled as well, although the two never met:
Alice Bob1 -:- Bob2 Carol
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\-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-/
This effect allows (at least in theory) to build a quantum repeater (see there). Breaking news: [[1]] demonstrates one node of a quantum repeater.
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Izawath  
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22 ก.ย. 48 02:23:26
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