[1] Institute of Photonics and Quantum Sciences, Heriot-Watt University, Edinburgh
[2] School of Physics and Astronomy, University of Glasgow, Glasgow
[3] Université Lille, CNRS, UMR 8523, PhLAM-Physique des Lasers Atomes et Molecules, Lille
[4] Institut Universitaire de France (IUF), 1 rue Descartes, Paris Cedex 05