1. Department of Physics, Brandeis University, Waltham, Massachusetts 02454;
2. Department of Biology and Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02454
3. Department of Electrical Engineering, Columbia University, New York, NY 10027
4. Department of Applied Physics and Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91125