1. Physical Measurement Laboratory, National Institute of Standards and Technology (NIST) 1 , Gaithersburg, Maryland 20899-8171, USA
2. Department of Physics, National Taiwan University 2 , Taipei 10617, Taiwan
3. Graduate Institute of Applied Physics, National Taiwan University 3 , Taipei 10617, Taiwan
4. Joint Quantum Institute, University of Maryland 4 , College Park, Maryland 20742, USA
5. Electricity Division, Physikalisch-Technische Bundesanstalt (PTB) 5 , 38116 Brunswick, Germany