  Entangled photon pairs are predicted to linearize and increase the efficiency
of two-photon absorption, allowing continuous wave laser diodes to drive
ultrafast time-resolved spectroscopy and nonlinear processes. Despite a range
of theoretical studies and experimental measurements, inconsistencies persist
about the value of the entanglement enhanced interaction cross section. A
spectrometer is constructed that can temporally and spectrally characterize the
entangled photon state before, during, and after any potential two-photon
excitation event. For the molecule Rhodamine 6G, which has a virtual state
pathway, any entangled two-photon interaction is found to be equal to or lower
than classical, single photon scattering events. This result can account for
the discrepancies between the wide variety of entangled two-photon absorption
cross sections reported from different measurement techniques. The reported
instrumentation can unambiguously separate classical and entangled effects and
therefore is of importance for the growing field of nonlinear and multiphoton
entangled spectroscopy.
