  The nature of quantum computation is discussed. It is argued that, in terms
of the amount of information manipulated in a given time, quantum and classical
computation are equally efficient. Quantum superposition does not permit
quantum computers to ``perform many computations simultaneously'' except in a
highly qualified and to some extent misleading sense. Quantum computation is
therefore not well described by interpretations of quantum mechanics which
invoke the concept of vast numbers of parallel universes. Rather, entanglement
makes available types of computation process which, while not exponentially
larger than classical ones, are unavailable to classical systems. The essence
of quantum computation is that it uses entanglement to generate and manipulate
a physical representation of the correlations between logical entities, without
the need to completely represent the logical entities themselves.
