1. Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
2. Department of Biophysics, University of Michigan, Ann Arbor, Michigan 48109, United States
3. Physics Department, University of Trento, via Sommarive, 14 I-38123 Trento, Italy
4. INFN-TIFPA, Trento Institute for Fundamental Physics and Applications, I-38123 Trento, Italy