  Entangled photon pairs have been promised to deliver a substantial quantum
advantage for two-photon absorption spectroscopy. However, recent work has
challenged the previously reported magnitude of quantum enhancement in
two-photon absorption. Here, we present an experimental comparison of
sum-frequency generation and molecular absorption, each driven by isolated
photon pairs. We establish an upper bound on the enhancement for
entangled-two-photon absorption in Rhodamine-6G, which lies well below
previously reported values.
