1. Department of Physics, University of California, San Diego, La Jolla, CA 92093
2. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139
3. The State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, People’s Republic of China
4. IBM T. J. Watson Research Center, Yorktown Heights, NY 10598