1. Department of Physics, Columbia University, New York, New York 10027, United States
2. IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, United States
3. Institute for Nanoelectronic Devices and Quantum Computing, Fudan University, Shanghai 200438, P.R. China
4. Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, United States
5. Intel Corporation, Santa Clara, California 95054, United States
6. Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
7. College of Nanoscale Science and Engineering, The State University of New York at Albany, Albany, New York 12203, United States
8. Department of Physics, University of Virginia, Charlottesville, Virginia 22904, United States
9. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States