  It has been proposed that entangled two-photon absorption (E2PA) can be
observed with up to 10 orders of magnitude lower photon flux than its classical
counterpart. However, there is a significant controversy regarding the
magnitude of this quantum enhancement in excitation efficiency. We investigated
the fluorescence signals from Rhodamine 6G and LDS798 excited with a CW laser
or an entangled photon pair source at 1060 nm. We observed a signal that
originates from hot-band absorption (HBA), which is one-photon absorption from
thermally-populated vibrational levels of the ground electronic state. This
mechanism, which has not been previously discussed in the context of E2PA,
produces a signal with a linear power dependence, as would be expected for
entangled two-photon excited fluorescence (E2PEF). For the typical conditions
under which E2PEF measurements are performed, contributions from the HBA
process could lead to a several orders-of-magnitude overestimate of the quantum
advantage for excitation efficiency.
