1. Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089, USA
2. Mork Family Department of Chemical Engineering and Material Science, University of Southern California, Los Angeles, California 90089, USA
3. Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, USA
4. IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA