1. Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Vienna A-1090, Austria
2. Institute for Quantum Optics and Quantum Information (IQOQI) Innsbruck, Austrian Academy of Sciences, Innsbruck A-6020, Austria
3. Institute for Theoretical Physics, School of Mathematics, Computer Science and Physics, University of Innsbruck, Innsbruck A-6020, Austria
4. Institute for Quantum Optics and Quantum Information (IQOQI) Vienna, Austrian Academy of Sciences, Vienna A-1090, Austria