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Depiction of the University of Innsbruck experimental setup for achieving quantum teleportation. In the quantum teleportation process, physicists take a photon (or any other quantum-scale particle), transfer its properties (such as its polarization, the direction in which its electric field vibrates) to another photon--even if the two photons are at remote locations. What's important to emphasize is that this scheme doesn't allow physicists to teleport the photon itself--only its properties to another, remote photon.

At the sending station of the quantum teleporter, Alice encodes photon M with a specific state: 45 degrees polarization. This travels towards a beamsplitter. Meanwhile, two additional "entangled" photons are created. The polarization of each photon is in a fuzzy, undetermined state, yet the two photons have a precisely defined interrelationship. Specifically, they must have complementary polarizations. For example, if photon A is later measured to have horizontal (0 degrees) polarization, then the other photon must "collapse" into the complementary state of vertical (90 degrees) polarization.

Entangled photon A arrives at the beamsplitter at the same time as the message photon M. The beam splitter causes each photon to either continue towards detector 1 or change course and travel to detector 2. In 25% of all cases, in which the two photons go off into different detectors, Alice does not know which photon went to which detector. This inability for Alice to distinguish between the two photons causes quantum weirdness to kick in. Just by the very fact that the two photons are now indistinguishable, the message photon M loses its original identity and becomes entangled with A. The polarization value for each photon is now indeterminate, but since they travel towards different detectors Alice knows that the two photons must have complementary polarizations.

Since message particle M must have complementary polarization to particle A, then the other entangled particle B must now attain the same polarization value as M. Therefore, teleportation is successful. Indeed, Bob sees that the polarization value of particle B is 45 degrees: the initial value of the message photon. In the experimental version of this setup executed at the University of Innsbruck, the 45-degree polarization detector would always fire when detector 1 and detector 2 fired. Except in rare instances attributable to background noise, it was never the case that the 135-degree polarization detector fired in coincidence with detectors 1 and 2.

Note that this scheme is intended only for quantum-scale particles, such as photons and atoms. Although no existing laws of physics prevent quantum teleportation from being carried out in humans and automobiles, it is extremely unlikely that this scheme could be carried out in such macroscopic objects, because the uniquely quantum properties (such as entanglement) that make teleportation possible quickly break down as objects scale up to the macroscopic sizes.

Also, quantum teleportation does not allow for faster-than-light communication. Although the teleported particle attains the polarization value instantly, the people at the sending station must convey the fact that teleportation was successful by making a phone call or using some other light speed or sub-light-speed means of communication.

In this setup, teleportation is achieved 25% of the time, corresponding to the percentage of times in which the two photons travel off to two different detectors, a condition which necessitates the two photons to have complementary polarizations. In more advanced schemes, in which Alice has a more elaborate measuring station, teleportation would be achieved more frequently and the detectors would provide information allowing the person at the sending station to give instructions to the receiving station (again, via a phone call) on how to massage the target particle into the desired state.

Illustration by Malcolm Tarlton, American Institute of Physics.


The Innsbruck experiment is reported by Dik Bouwmeester, Jiai-Wei Pan, Klaus Mattle, Manfred Eibl, Harald Weinfurter, and Anton Zeilinger in the 11 December 1997 issue of Nature.

It is described in a nice article by Tony Sudbery of the University of York in the 11 December 1997 issue of Nature.

The idea of quantum teleportation was theoretically predicted by Charles H. Bennett, Gilles Brassard, Claude Crepeau, Richard Jozsa, Asher Peres, and William K. Wootters in the paper, "Teleporting an Unknown Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels," in Physical Review Letters, volume 70, pp. 1895-1899 (29 March 1993).


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