1. Quantum Architectures and Computation Group, Microsoft Research, Redmond, Washington 98052, USA
2. Department of Computer Science, University College London, Gower Street, London WC1E 6BT, United Kingdom
3. DAMTP, University of Cambridge, Cambridge, United Kingdom
4. Departamento de Análisis Matemático, Universidad Complutense de Madrid, Madrid, Spain
5. Institute for Quantum Information and Matter, Caltech, California 91125, USA
6. IMI, Universidad Complutense de Madrid, Madrid, Spain
7. ICMAT, C/Nicolás Cabrera, Campus de Cantoblanco, 28049 Madrid, Spain