1. Department of Electrical Engineering and
Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
2. Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States
3. IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, United States
4. ICFO-Institut
de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain