1. Department of Electronic and Electrical Engineering University College London London WC1E 7JE UK
2. Department of Physics University of Warwick Coventry CV4 7AL UK
3. Center for Quantum Devices Niels Bohr Institute University of Copenhagen Universitetsparken 5 2100 Copenhagen Denmark
4. London Centre for Nanotechnology University College London London WC1H 0AH UK
5. State Key Laboratory of Functional Materials for Informatics Shanghai Institute of Microsystem and Information Technology Chinese Academy of Sciences Shanghai 200050 China