1. Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
2. Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
3. Low Temperature Laboratory, Department of Applied Physics, Aalto University, P.O. Box 13500, FI-00076 AALTO, Finland