1. London Centre for Nanotechnology, UCL, London WC1H 0AH, U.K.
2. Department of Electronic & Electrical Engineering, UCL, London WC1E 7JE, U.K.
3. Department of Physics, IIT Madras, Chennai 600036, India
4. Center for Quantum Information, Communication and Computing, IIT Madras, Chennai 600036, India
5. Department of Physics and Astronomy, UCL, London WC1E 6BT, U.K.
6. Department of Chemistry, UCL, 20 Gordon Street, London WC1H 0AJ, U.K.