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.
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