1. IBM Research—Almaden, San Jose, California 95120, USA
2. The Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA
3. École supérieure de physique et de chimie industrielles de la Ville de Paris, Paris 75005, France
4. Department of Electrical and Electronic Engineering, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland