1. Department of Physics and Arnold Sommerfeld Center for Theoretical Physics (ASC), Ludwig-Maximilians-Universität München, Theresienstr. 37, München D-80333, Germany
2. Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, D-80799 München, Germany
3. ITAMP, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
4. Department of Physics, Harvard University, Cambridge, MA 02138, USA