1. London Centre for Nanotechnology, University College London, London WC1H 0AH, U.K.
2. Centre for Quantum Computation and Communication Technology, School of Physics, The University of Sydney, Sydney, NSW 2006, Australia
3. Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, U.K.
4. Department of Materials, Imperial College of London, London SW7 2AZ, U.K.
5. IHP − Leibniz-Institut für Innovative Mikroelektronik, Frankfurt (Oder) 15236, Germany
6. Department of Physics and Astronomy, University College London, London WC1E 6BT, U.K.
7. Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, U.K.