1. Department of Physics, Umeå University, SE-901 87 Umeå, Sweden
2. Measurement Science and Technology, RISE Research Institutes of Sweden, SE-501 15 Borås, Sweden
3. Physikalisch-Technische Bundesanstalt (PTB), Abbestr 2-12, 10587 Berlin, Germany
4. Istituto Nazionale di Ricerca Metrologica (INRiM), Strada delle Cacce 91, 10135 Turin, Italy
5. Conservatoire National des Arts et Métiers, Laboratoire Commun de Métrologie (LNE-CNAM), 1 Rue Gaston Boissier, 75015 Paris, France
6. Laboratoire National de Métrologie et d’Essais (LNE), 1 Rue Gaston Boissier, 75015 Paris, France