Entangled two-photon absorption by atoms and molecules: A quantum optics tutorial

Two-photon absorption (TPA) and other nonlinear interactions of molecules with time-frequency-entangled photon pairs (EPP) has been predicted to display a variety fascinating effects. Therefore, their potential use in practical quantum-enhanced molecular spectroscopy requires close examination. This paper presents tutorial style detailed theoretical study one- two-photon by molecules, focusing on how treat the quantum nature light. We review some basic optics theory, then we density-matrix (Liouville) derivation optical response, emphasizing incorporate states light into treatment. For illustration detail TPA created spontaneous parametric down conversion, an emphasis differs from that classical In particular, question much enhancement rate can be achieved using entangled states. The includes known methods results, as well extensions, especially comparison processes occur via far-off-resonant intermediate only those involve off-resonant state virtue dephasing processes. A brief discussion main challenges facing experimental studies is also given.