1. ICFOInstitut de Ciencies Fotoniques, Mediterranean Technology Park, 08860, Castelldefels (Barcelona), Spain, Applied Optics Group, MESA+ Institute for NanoTechnology, University of Twente, P.O. Box 217, 7500AE Enschede, the Netherlands, and FOM Institute for Atomic and Molecular Physics (AMOLF), Kruislaan 407, 1098 SJ Amsterdam, the Netherlands