QLC: EAGER: Collaborative Research: Developing Experiment and Theory for Entangled Photon Spectroscopy

Northwestern University, Evanston IL

Investigators

Abstract

This research project is concerned with understanding unusual phenomena that arise when light interacts with molecules. Light can be thought of as tiny bundles of energy known as photons. In most cases, molecules absorb light energy one photon at a time. This absorption can give rise to things we commonly observe, such as color and fluorescence. The focus of this project is the near-simultaneous absorption of two photons by molecules. Photons normally act independently, but there is a quantum effect in which two photons become entangled, and absorption of one photon becomes strongly dependent on the other photon. We can think of the two photons talking to each other. When this happens, the rate of absorption of two photons can be enhanced by many orders of magnitude. Such enhancements in light absorption could advance many technological applications that range from electronic devices to chemical sensing. However, the details of molecule absorption of entangled photons are poorly understood. The few experiments that have been done on entangled photons do not always produce consistent results, even for similar molecules. In this research project, Professors George Schatz of Northwestern University and Theodore Goodson of the University of Michigan are combing experimental laser-based measurements and computational modeling to study the absorption of entangled photons by molecules. Students and postdocs who work on this project are being trained in new experimental and theoretical techniques that are likely to play a role as new technologies are developed based on these unique quantum effects. In addition, Profs. Schatz and Goodson are engaging in active outreach programs related to their research that touches on broad segments of society. In this project, a quantum optical experimental approach and a quantum electronic computational approach are being applied to investigate new properties of organic molecules that are accessible through entangled two-photon absorption (ETPA). The studies take advantage of the quantum entanglement of photons created by the process of spontaneous parametric down conversion and the subsequent excitation of electronic states in molecules. The phenomenon of ETPA has been theoretically predicted to exhibit interesting non-classical effects such as linear rather than quadratic dependence of absorption rate on incident photon flux. In prior work, Prof. Goodson, demonstrated the surprising result that some molecules show significant ETPA cross sections while other nominally similar molecules show no observable ETPA. To go further, this project employs theory and experiments aimed at understanding intermediate states that participate in ETPA. The theory studies involve calculating transition moments that couple excited states, which is an ability that is generally missing from electronic structure theories. The computational aspect of the project also utilizes a new time-dependent density functional theory (TDDFT) approach developed by Prof. Schatz, for calculating energies and transition moments, and ultimately ETPA cross sections. The computational results are being compared with pump-probe entangled time-resolved measurements using a new apparatus, with the goal of sorting out the role of intermediate states in ETPA. The project is also concerned with training graduate students and postdocs, and enabling them to develop careers of their own that explore directions of the quantum world that have not yet been conceived. In addition, the PIs have active outreach programs where the research described in this proposal is presented at levels that range from graduate courses in quantum mechanics to talks to the general public. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.