1. Department of Computer Science, University College London, 66–72 Gower Street, London WC1E 6EA, UK
2. INL—International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga, 4715-330 Braga, Portugal
3. Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK