Setting bounds on two-photon absorption cross-sections in common fluorophores with entangled photon pair excitation

Excitation with entangled photon pairs may lead to an increase in the efficiency of two-photon absorption at low photon flux. The corresponding process, entangled two-photon absorption (E2PA), has been investigated in numerous theoretical and experimental studies. However, significant ambiguity and inconsistency remain in the literature about the absolute values of E2PA cross-sections. Here, we use a fluorescence-based registration scheme to experimentally determine upper bounds on the cross-sections for six fluorophores. These bounds are up to four orders of magnitude lower than the smallest published cross-section. For two samples that have been studied by others, Rhodamine 6G and 9R-S, we measure upper bounds four and five orders of magnitude lower than the previously reported cross-sections.