1. Kapteyn–Murnane Laboratories, Inc., Boulder, Colorado 80301, USA
2. Research School of Physics and Engineering, The Australian National University, Canberra ACT 2601, Australia
3. Symerio, 91120 Palaiseau, France
4. Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, USA
5. JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309, USA