Perhaps the strangest feature of quantum mechanics is the possibility of "entanglement" between two spatially separated particles. If two particles are entangled, they act in some respects as a single entity and can exhibit behavior that defies explanation within the paradigm of classical physics. An example of such behavior is seen in the phenomenon known as quantum teleportation, illustrated in this picture.
The picture shows a particle of light, or photon, entering from the lower left; its state of polarization denoted by . The experimenters, conventionally named Alice and Bob, do not know the state but would like to teleport it into another photon some distance away. This "receptor photon," shown entering Bob's apparatus, is one member of an entangled pair, the other member having been sent to Alice. To effect the teleportation, Alice performs a comparison measurement on her two photons - the measurement has exactly four possible outcomes - and communicates the result to Bob, who then performs an appropriate transformation on the receptor photon. At the end of the procedure, the state has been destroyed on Alice's side but has re-emerged on Bob's side. Interestingly, the teleportation succeeds despite the fact that one cannot, even in principle, ascertain the polarization state of a single photon by measuring it. Thus in carrying out the procedure neither Alice nor Bob ever learns what state they are teleporting.
Quantum teleportation was proposed by William Wootters, Professor of Physics, and his collaborators in 1993. In the past year, realizations of teleportation have been successfully carried out in two laboratories, one in Innsbruck and one in Rome. Each of these realizations is less ambitious than the original proposal. For example, in the Innsbruck experiment Alice checks for only one of the four possible outcomes and the teleportation succeeds only when this outcome occurs. Full implementation of the original proposal will require novel experimental techniques that are still being developed. More information can be found in a report by Graham P. Collins in Physics Today, Feb. 1998, pp. 18-21. The cover picture was constructed by Kevin O'Connor `00. The Science Executive Committee wishes to express its gratitude to the extensive efforts of all the science departmental executive assistants in preparing contributions for this publication, and to Alice J. Seeley for assembling this material in its final form.
Editor: Dr. Bryce Babcock
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