Setting Experimental Bounds on Entangled Two-Photon Absorption Cross Sections

Entangled two-photon absorption (E2PA) is a process in which entangled photon pairs are used instead of a pulsed laser to excite a two-photon transition. The correlation of these photons in time and space may improve the excitation efficiency. In this work, we develop experimental apparatuses that enable sensitive measurements of E2PA via both transmittance and fluorescence-based schemes. We show strong evidence that E2PA cross sections are several orders of magnitude lower than many prior reports claimed. We set upper bounds on the cross sections for 8 independent samples of two-photon absorbers in room-temperature liquids.