1. Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, United States
2. IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
3. Department of Chemistry, Michigan State University, East Lansing, Michigan 48864, United States
4. Delft University of Technology, Kavli Institute of Nanoscience, NL-2628 CJ Delft, Netherlands
5. Centre for Quantum Computation and Communication Technology, School of Physics, The University of New South Wales, Sydney 2052, Australia