Information is stored, transmitted and processed always by physical means. Thus the concept of information and computation can be properly formulated only in the context of a physical theory and the study of information processing requires experimentation . It is clear that if computers are to become much smaller in the future, their description must be given by quantum mechanics. Somewhat more surprising is the fact that quantum information processing can be qualitatively different and much more powerful than its classical analogue. In the following we will explain why.