1. Institute of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
2. Laboratory for Micro-
and Nanotechnology, Paul Scherrer Institute, CH-5232 Villigen
PSI, Switzerland
3. Center for Quantum Devices
and Nano-Science Center, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark