1. Blackett Laboratory, Imperial College London, London SW7 2AZ, United Kingdom
2. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA
3. Department of Physics, Boston University, Boston, MA, 02215, USA
4. Institut quantique & Département de physique, Université de Sherbrooke, Québec J1K 2R1, Canada