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The transmission of quantum information over long distances will allow new forms of data security, based on quantum cryptography. These new technologies rely for security on the quantum "uncertainty principle" and on the long distance transmission of "quantum entanglement." A new type of telecommunications device called the "quantum repeater" can allow the faithful transmission of quantum information over worldwide distances, in spite of the inevitably severe losses while propagating along optical fibers. In a quantum repeater, information is stored in the quantum state of a semiconductor electron spin, while complementary entangled information is transmitted as a photon down the optical fiber. This long-range entanglement permits the execution of the teleportation algorithm, which accurately transmits a quantum state over long distances. The quantum repeater is an excellent stepping stone to larger quantum information processors, since teleportation requires only three quantum logic gates. This paper reviews the experimental status of semiconductor quantum repeaters, including the spin resonance transistor logic gates, and the experimental detection of single photons in a manner that preserves their spin information.
Date of Publication: May 2003