What is quantum computation?

"Information is physical and any processing of information is always perfor med by physical means," an innocent-sounding statement, but its consequence s are profound. When quantum effects become important, for example, at the level of single atoms and photons, the classical theory of computation beco mes fundamentally inadequate. Entirely new modes of computation and informa tion processing become possible. In the last few years there has been an ex plosion of theoretical and experimental research in quantum computation. In this brief review we describe some of these developments.