Viewpoint Distinguishing between molecules that look the same

Optical spectroscopy is a remarkably powerful tool that can be used to discriminate between samples that have very similar physical properties. But what do you do if two molecules or samples you want to discriminate between absorb and emit at the same frequencies? In a paper published in Physical Review Letters, Matthias Roth, Jonathan Roslund, and Herschel Rabitz of Princeton University, Laurent Guyon, Véronique Boutou, and Francois Courvoisier at Université Lyon, and JeanPierre Wolf at the University of Geneva demonstrate an approach to this problem based on “coherent control” [1]. They make use of shaped ultrafast lasers and an adaptive algorithm that controls the pulse shapes to distinguish between two different molecules with identical absorption and emission spectra. Their work illustrates the power of using controlled quantum interference to discriminate between similar systems.