1. National Physical Laboratory 1 , Hampton Road, Teddington TW11 0LW, United Kingdom
2. Physical Measurement Laboratory, National Institute of Standards and Technology 2 , Gaithersburg, Maryland 20899, USA
3. Joint Quantum Institute, University of Maryland 3 , College Park, Maryland 20742, USA
4. Cavendish Laboratory, University of Cambridge 4 , J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom