  Entangled photons exhibit non-classical light-matter interactions that create
new opportunities in materials and molecular science. For example, in entangled
two-photon absorption, the intensity-dependence scales linearly as if only one
photon was present. The entangled two-photon absorption cross section
approaches but does not match the one-photon absorption cross section. The
entangled two photon cross section also does not follow classical two photon
molecular design motifs. Questions such as these seed the rich but nascent
field of entangled light-matter interactions. In this perspective, we use the
experimental developments in entangled photon spectroscopy to outline the
current status of the field. Now that the fundamental tools are outlined, it is
time to start the exploration of how materials, molecules, and devices can
control or utilize interactions with entangled photons.
