Seeing the color of entangled photons in molecular systems

Spectroscopy is the study of how matter absorbs and emits light and other radiation. It allows scientists to study the structure of atoms and molecules, including the energy levels of their electrons. Classical optical spectroscopy relies on the way particles of light called photons interact with matter. These classical spectroscopy techniques include one-photon absorption (OPA) and two-photon absorption (TPA).