1. Department of Physics–Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA.
2. Howard Hughes Medical Institute, Cornell University, Ithaca, NY 14853, USA.
3. Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, PA 16801, USA.
4. Department of Physics, Pennsylvania State University, University Park, PA 16801, USA.