Nuclear spin spectra for ground and excited multiplets of Eu-based magnetic molecule

Achieving quantum light-spin interfaces in molecular magnets is an exciting prospect with potential applications for nanophotonics and quantum information science technologies. While a selected set of magnetic molecules exhibit optical transitions, the molecular electronic or nuclear spin states tend to strongly couple to surrounding environment in an uncontrollable fashion. This hampers the usage of the molecular spin states for platforms for coherent spin-photon coupling. Recently, super-narrow homogeneous optical linewidths have been experimentally achieved by considering nuclear spin states in an Eu-based molecular crystal [Serrano et al., Nature 603, 241 (2022)] with evidence of their coherent optical control. Here we investigate electronic ground-state and excited-state multiplets, nuclear spin spectra, and optical properties for the Eu-based magnetic molecule by employing ab-initio multireference quantum chemistry methods including spin-orbit coupling. We construct an effective nuclear spin Hamiltonian from first principles and compare the results to the experiment. Furthermore, we explore effects ligand substitution and the mechanisms of decoherence.