"Quantum Computation: Theorist's Dream, Experimenter's
Nightmare"
A quantum computer uses all of the states allowed by the
quantum principle
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.