1. Department of Physics, University of Connecticut, Storrs, Connecticut, 06269, USA
2. Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
3. Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche, 20133 Milano, Italy
4. Dipartimento di Fisica Politecnico, 20133 Milano, Italy
5. Elettra-Sincrotrone Trieste S.C.p.A., 34149 Basovizza Trieste, Italy
6. Department of Physics, Indian Institute of Technology-Madras, Chennai 600036, India
7. Institute of Physics, University of Freiburg, 79104 Freiburg im Breisgau, Germany
8. Department of Physics, Ludwig-Maximilians-Universität Munich, 85748 Garching, Germany
9. Max Planck Institute of Quantum Optics, 85748 Garching, Germany
10. SLAC National Accelerator Laboratory, Menlo Park 94025, CA, USA
11. Department of Applied Physics, Stanford University, Stanford 94305, CA, USA
12. Indian Institute of Science Education and Research, Pune 411008, India