1. Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91125, United States
2. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, United States
3. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
4. Department of Physics, Portland State University, Portland, Oregon 97207, United States