1. Institut für Quantenoptik, Leibniz Universität Hannover, 30167 Hannover, Germany.
2. Istituto Nazionale di Ottica (INO), Consiglio Nazionale delle Ricerche (CNR), and European Laboratory for Non-Linear Spectroscopy (LENS), 50125 Firenze, Italy.
3. Institut für Theoretische Physik, Leibniz Universität Hannover, 30167 Hannover, Germany.
4. Department of Theoretical Physics, The University of the Basque Country, 48080 Bilbao, Spain.
5. Center for Quantum Optics (QUANTOP), Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Århus C, Denmark.