  The number of two-qubit gates required to transform deterministically a
three-qubit pure quantum state into another is discussed. We show that any
state can be prepared from a product state using at most three CNOT gates, and
that, starting from the GHZ state, only two suffice. As a consequence, any
three-qubit state can be transformed into any other using at most four CNOT
gates. Generalizations to other two-qubit gates are also discussed.

Quantum Physics