1. Bureau International des Poids et Mesures (BIPM), Pavillon de Breteuil, 92312 Sèvres, France.
2. Physics Department, East China Normal University (ECNU), Shanghai 200062, China.
3. National Institute of Standards and Technology (NIST), 325 Broadway, Boulder, CO 80305, USA.
4. OFS Laboratories, 700 Mountain Avenue, Murray Hill, NJ 07974, USA.