  Rare-earth ions are promising solid state systems to build light-matter
interfaces at the quantum level. This relies on their potential to show narrow
optical homogeneous linewidths or, equivalently, long-lived optical quantum
states. In this letter, we report on europium molecular crystals that exhibit
linewidths in the 10s of kHz range, orders of magnitude narrower than other
molecular centers. We harness this property to demonstrate efficient optical
spin initialization, coherent storage of light using an atomic frequency comb,
and optical control of ion-ion interactions towards implementation of quantum
gates. These results illustrate the utility of rare-earth molecular crystals as
a new platform for photonic quantum technologies that combines highly coherent
emitters with the unmatched versatility in composition, structure, and
integration capability of molecular materials.
