1. Physical Measurement Laboratory, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899-8171, USA
2. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
3. Department of Electronics and Telecommunications, Politecnico di Torino, Torino 10129, Italy
4. Istituto Nazionale di Ricerca Metrologica, Torino 10135, Italy
5. Joint Quantum Institute, University of Maryland, College Park, Maryland 20742, USA