1. Department of Physics, Harvard University, Cambridge, MA 02138, USA.
2. Harvard-MIT Center for Ultracold Atoms, Cambridge, MA 02138, USA.
3. Department of Physics, Korea University, Seongbuk-gu, Seoul 02841, South Korea.
4. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
5. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.