A common theme of this book is that quantum computers are much dierent than classical computers (such as your personal computer), because they require quantum mechanics for their working (and classical computers do not). To understand just how dierent a quantum computer is from a classical

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computer, it is necessary to understand just how dierent quantum mechanics is from classical mechanics. I could just tell you they are dierent and get right to work on the story of the quantum computer, but then you would just have to take my word for this dierence. However, I don’t want you to just take my word for it. I don’t want you to just believe quantum mechanics is dierent than classical mechanics; I want you to know it. So my goal in this chapter is to show you that the quantum theory of Bohr and Heisenberg and Schrödinger is really dierent (and much stranger) from the classical theory of Kepler and Newton and Maxwell.