"Quantum Computation: Theorist's Dream, Experimenter's
A quantum computer uses all of the states allowed by the
of superposition -- all of the Hilbert space -- to process information.
The resulting sophisticated interference experiment allows a quantum computer
-- at least in theory -- to act as a massively parallel supercomputer,
even though it is employing a single quantum processor. There are important
applications where this quantum parallelism can lead to an exponential speedup.
Decoherence -- a process thought to be responsible for
the transition from
quantum to classical in other contexts -- is the principal enemy of quantum
computation. Several intriguing laboratory implementations of quantum
computers have been nevertheless proposed, and exploratory experiments
in this area are under way.