1. Institute for Theoretical Physics, Utrecht University, Princetonplein 5, 3584CC Utrecht, The Netherlands
2. Adaptive Quantum Optics (AQO), MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
3. Department of Physics, Stockholm University, S-10691 Stockholm, Sweden
4. Department of Physics and Astronomy, Uppsala University, SE-75120 Uppsala, Sweden