1. Elettra Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, 34149 Basovizza (TS), Italy.
2. Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
3. Department of Physics, University of Trento, Via Sommarive 14, Povo (TN), Italy.
4. IOM-CNR, Strada Statale 14, km 163.5, in Area Science Park, I-34012 Basovizza (TS), Italy.
5. JILA and Department of Physics, University of Colorado, Boulder, CO 80309, USA.