1. Department of Physics, Harvard University, 17 Oxford Street, Cambridge, MA 02138, USA
2. Harvard-MIT Division of Health, Science, and Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
3. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
4. Department of Biological Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
5. Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 36-834, Cambridge, MA 02139. Fax: 617-324-3534; Tel: 617-253-1583