1. Department of Quantum Technology, National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom
2. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
3. Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
4. Nanoscale Device Characterization Division, Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA