1. Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
2. Interdisciplinary Nanoscience Center and Department of Chemistry, Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark
3. Forschungszentrum Jülich, Institute of Quantum Control (PGI-8), D-52425 Jülich, Germany