1. Department of Computer Science, University of Bristol, Merchant Venturers Building, Bristol BS8 1UB, UK
2. Centre for Quantum Computation, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
3. SFB 343, Facultät für Mathematik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany