  When a low flux of time-frequency-entangled photon pairs (EPP) illuminates a
two-photon transition, the rate of two-photon absorption (TPA) can be enhanced
considerably by the quantum nature of photon number correlations and frequency
correlations. We present a quantum-theoretic derivation of entangled TPA (ETPA)
and calculate an upper bound on the amount of quantum enhancement that is
possible in such systems. The derived bounds indicate that in order to observe
ETPA the experiments would need to operate at a combination of significantly
higher rates of EPP illumination, molecular concentrations, and conventional
TPA cross sections than are achieved in typical experiments.
