1. Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
2. JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309, USA
3. Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, Massachusetts 02138, USA
4. Department of Physics and Astronomy, University of Montana, Missoula, Montana 59812, USA
5. Centre for Cold Matter, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, United Kingdom