1. Department of Physics and Arnold Sommerfeld Center for Theoretical Physics, Ludwig-Maximilians-Universität München, Munich D-80333, Germany
2. Munich Center for Quantum Science and Technology (MCQST), Munich D-80799, Germany
3. Department of Physics, Harvard University, Cambridge, MA 02138, USA
4. Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien, Vienna 1020, Austria
5. Institute of Theoretical Physics, University of Regensburg, Regensburg D-93053, Germany