Computational Study of Entangled Two-Photon Absorption in Organic Dyes

Abstract Entangled two-photon absorption (ETPA) offers advantages for quantum spectroscopy, including linear scaling with photon flux and reduced photodamage, yet distinguishing it from competing one-photon processes remains experimentally challenging. We present a predictive framework based on time-dependent density functional theory (TD-DFT) that clarifies the mechanisms shaping ETPA spectra in organic chromophores. Using rhodamine 6G and zinc tetraphenylporphyrin, we show that nondegenerate photon pairs enhance ETPA intensity when one photon approaches resonance with a one-photon absorption (OPA) transition. We clarify that this enhancement is a near-resonant, sum-over-states effect shared with classical two-photon absorption rather than a quantum advantage; entanglement instead acts through the joint spectral amplitude as a correlation-induced weighting that redistributes intensity and reduces to classical TPA in the appropriate limit. Despite the typical TD-DFT blue shift (∼0.5 eV), the method predicts spectral trends over energy ranges beyond those accessible by spontaneous parametric down-conversion, guiding experimental design.