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

这个研究项目涉及到理解光与分子相互作用时产生的不寻常现象。光可以被认为是被称为光子的微小能量束。在大多数情况下，分子一次吸收一个光子的光能。这种吸收可以产生我们通常观察到的东西，比如颜色和荧光。这个项目的重点是分子几乎同时吸收两个光子。光子通常是独立作用的，但有一种量子效应，其中两个光子纠缠在一起，一个光子的吸收变得强烈依赖于另一个光子。我们可以想象这两个光子在互相交谈。当这种情况发生时，两个光子的吸收率可以提高许多数量级。光吸收的这种增强可以推进从电子设备到化学传感的许多技术应用。然而，分子吸收纠缠光子的细节却知之甚少。为数不多的关于纠缠光子的实验并不总是产生一致的结果，即使是对于相似的分子。在这个研究项目中，西北大学的乔治·沙茨教授和密歇根大学的西奥多·古德森教授正在结合实验激光测量和计算模型来研究分子对纠缠光子的吸收。参与该项目的学生和博士后正在接受新的实验和理论技术的培训，这些技术很可能在基于这些独特量子效应的新技术开发中发挥作用。此外，教授。沙茨和古德森正在积极开展与他们的研究相关的外展项目，这些项目涉及社会的广泛领域。在这个项目中，量子光学实验方法和量子电子计算方法被应用于研究通过纠缠双光子吸收（ETPA）可获得的有机分子的新性质。这些研究利用了自发参数下转换过程和分子中电子态的激发所产生的光子的量子纠缠。从理论上预测了ETPA现象会表现出有趣的非经典效应，如吸收率与入射光子通量的线性关系而不是二次关系。在之前的工作中，古德森教授展示了令人惊讶的结果，一些分子显示出显著的ETPA横截面，而其他名义上相似的分子没有显示出可观察到的ETPA。更进一步，该项目采用理论和实验，旨在了解参与经济贸易促进计划的中间状态。理论研究包括计算激发态耦合的跃迁矩，这是电子结构理论通常缺少的能力。该项目的计算方面还采用了Schatz教授开发的一种新的时变密度泛函理论（TDDFT）方法，用于计算能量和过渡矩，并最终计算ETPA截面。利用一种新的仪器将计算结果与泵浦-探针纠缠的时间分辨测量结果进行比较，目的是理清中间态在ETPA中的作用。该项目还涉及培养研究生和博士后，使他们能够发展自己的事业，探索量子世界尚未设想的方向。此外，pi有积极的外展计划，在这些计划中，本提案中描述的研究将在从量子力学研究生课程到公众讲座的各个层面上进行展示。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.