1. National Physical Laboratory, Hampton Road, Teddington TW11 0LW, Middlesex, UK
2. School of Mathematics and Physics, University of Surrey, Guildford GU2 7XH, Surrey, UK
3. Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany
4. National Scientific Centre “Institute of Metrology”, UA-61002 Kharkiv, Ukraine
5. Bureau International des Poids et Mesures, Pavillon de Breteuil, 92312 Sèvres, CEDEX, France
6. Federal Institute of Metrology METAS, Lindenweg 50, 3003 Bern-Wabern, Switzerland