Judd-Ofelt analysis and coherent spectroscopy of europium-based molecular complexes for quantum information applications

Abstract: Rare-earth-ion based molecular crystals are emerging as a promising platform for quantum information applications, combining long optical coherence times with the chemical tunability of coordination chemistry. Here we present a comprehensive optical characterisation of twelve Eu$<sup>{3+}$-based molecular complexes, with a focus on the coherent $$^{{5}\text{D}_{0}} \longrightarrow \ <sup>{7}\text{F}_{0}$$ transition at $\sim$580 nm. Judd-Ofelt analyses are performed across all complexes, providing insights into ligand-field environments, local point-group symmetries, and the resulting branching ratios of the coherent transition. We find that lower point-group symmetry (C$<em>1$, C$_2$) yields higher branching ratios of the $$^{5}\text{D}</em>{0} \longrightarrow \ <sup>{7}\text{F}_{0}$$ transition, exceeding 1\% in several complexes. Optical inhomogeneous linewidths measured at 4.2 K reveal narrow linewidths for some complexes of below ten GHz, indicative of a highly ordered crystalline environment. Optical coherence measurements of one of the complexes, 5-Eu, yields an optical coherence time of $$T_{\text{2}}^{\text{opt}} = 2.3~μ\mathrm{s}$$ at 4.2 K, comparable to the best-performing molecular systems reported to date. Systematic characterisation and quantitative comparison will be key to find structure-property relations guiding rational design of optimised complexes for quantum information applications.