1. Quantum
Measurement Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
2. Joint Quantum Institute, University of Maryland and National Institute of Standards and Technology, Gaithersburg, Maryland 20742, United States
3. School
of Physics and Australian Research Council Centre of Excellence in
Low-Energy Electronics Technologies, UNSW Node, The University of New South Wales, Sydney 2052, Australia