1. Department of Physics, Technical University of Munich, 85748 Garching, Germany
2. Condensed Matter Physics Center (IFIMAC) and Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
3. Department of Physics, University of Regensburg, Regensburg, Germany
4. Fachbereich Physik, Universität Konstanz, D-78457 Konstanz, Germany
5. Munich Center for Quantum Science and Technology (MCQST), D-80799 München, Germany
6. Centre for Quantum Engineering (ZQE), Technical University of Munich, Garching, Germany