  Excitation energy transfer is a photophysical process upon which many
chemical and biological phenomena are built. From natural small systems to
synthetic multichromophoric macromolecules, energy transfer deals with the
process of migration of electronic excitation energy from an excited donor to
an acceptor. Although this phenomenon has been extensively studied in the past,
the rapid evolution of quantum-enabled technologies has motivated the question
on whether nonclassical sources of light, such as entangled photon pairs, may
provide us with a better control (or enhancement) of energy transfer at the
nanoscale. In this work, we provide a comprehensive study of the joint
excitation of two non-interacting two-level atoms by time-frequency correlated
photon pairs -- whose central frequencies are not resonant with the individual
particles -- generated by means of spontaneous parametric down conversion
(SPDC). We demonstrate that while strong frequency anti-correlation between
photons guarantees a large two-photon excitation (TPE) probability, photons
bearing a sine cardinal spectral shape exhibit a $\sim$3.8 times larger TPE
signal than photons with a Gaussian spectrum. More importantly, we find that
suppression of time-ordered excitation pathways does not substantially modify
the TPE probability for two-photon states with a Gaussian spectral shape;
whereas photons with a sine cardinal spectrum exhibit the strongest TPE signals
when two-photon excitation pathways are not suppressed. Our results not only
help elucidating the role of time-frequency correlations in resonance energy
transfer with SPDC photons, but also provide valuable information regarding the
optimal source to be used in its experimental implementation.
