1. Foundation for Research and Technology - Hellas, Institute of Electronic Structure & Laser, P.O. Box 1527, GR71110 Heraklion (Crete), Greece
2. Department of Physics, University of Crete, P.O. Box 2208, GR71003 Heraklion (Crete), Greece
3. Max Planck Institute of Quantum Optics, Garching, Germany
4. Department of Physics, Ludwig-Maximilians-Universität München, München, Germany