1. Department of Physics, Harvard University, Cambridge, MA 02138, USA.
2. Institute for Quantum Information and Matter and Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA.
3. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
4. Sandia National Laboratories, Albuquerque, NM 87185, USA.
5. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.