ua nt - p h / 02 05 11 5 v 1 1 8 M ay 2 00 2 Toffoli or Control - NOT needs little help to do universal quantum computation

What additional gates are needed for a set of classical universal gates to do universal quantum computation? We answer this question by proving that any single-qubit real gate suffices, except those that preserve the computational basis. The result of Gottesman and Knill [3] implies that any quantum circuit involving only the Control-NOT and Hadamard gates can be efficiently simulated by a classical circuit. In contrast, we prove that Control-NOT plus any single-qubit real gate that does not preserve the computational basis and is not Hadamard (or its alike) are universal for quantum computing. Previously only a “generic” gate, namely a rotation by an angle incommensurate with π, is known to be sufficient in both problems, if only one single-qubit gate is added.