1. National Physical Laboratory, Teddington, TW11 0LW, United Kingdom
2. Royal Holloway University of London, Egham, TW20 0EX, United Kingdom
3. Physikalisch-Technische Bundesanstalt, Braunschweig, D-38116, Germany
4. Univ. Grenoble Alpes, Inst. NEEL, Grenoble, F-38042, France
5. Istituto Nazionale di Ricerca Metrologica, Torino, I-10135, Italy