Quantum computers, if realized, could perform certain calculations that are beyond the potential of classical computers [ 11. The basic elements for quantum computers are qubits. These can exist in a general superposition of two well-defined quantum states. Qubits are made to interact; the so-called controlled-not gate is a particularly useful example that can entangle the information between qubits. The quantum state of the whole quantum computer is transformed by unitary transformations; any of those transformations can be obtained by a succession of single qubit operations and controlled-not actions. If realized with sufficient size, a quantum computer can perform certain operations (e.g. the factorization of a very large number) that cannot be performed by a conventional computer. For a computation, the quantum computer is first prepared in a well-defined state, transformations are performed whereby no interaction is allowed with uncontrolled degrees of freedom outside the system, and finally the resulting state is measured. The measurement constitutes a collapse onto classical variables that destroys the quantum state. Error correction is possible during computation for low error probabilities.