1. Experimentelle Physik 2, Technische Universität Dortmund, 44221 Dortmund, Germany
2. Department of Physics & Institute for Photonic Quantum Systems (PhoQS), Paderborn University, 33098 Paderborn, Germany
3. Technische Physik, Universität Würzburg, 97074 Würzburg, Germany
4. Institute of Physics, University of Oldenburg, 26129 Oldenburg, Germany