Crystal Growth for Rare-Earth Optical Quantum Materials

Not all single crystals are created equal, and the tolerances in imperfections in optical materials vary widely by application. The optical excitations of rare-earth ions provide uniquely long coherence times, and their ability to couple to other optical systems gives them an intrinsic advantage when forming integrated systems for quantum information storage, communication, or computing. Historically, rare-earth ions in transparent crystals have been used as laser gain media, where the rare-earth ions are dopants, present in small concentrations, and randomly substituted with optically-inactive host ions. While much is understood about these systems, serious challenges arise when storing quantum information because the variation in coordination environments of the rare-earth ions leads to spectral broadening and crosstalk that make addressing specific states impossible. For high-optical-density quantum applications, the coordination environments must be sufficiently uniform, which has yet to be demonstrated in a robust, air-stable material. We are developing new candidates for Eu³⁺-based quantum information storage, including frameworks, molecular crystals, and dense inorganic phases. These materials display the required ⁵D₀-⁷F₀ transition at 579 nm and have large Eu-Eu spacings larger than 4.0 Å. Our preliminary work leads to a broad set of questions: (1) what is the maximum density of Eu³⁺ permissible in such a material for the individual ions to be equivalent, (2) how close can Eu³⁺ ions be in a solid to retain “atom-like” optical spectra, and (3) given these constraints, how many materials are likely to host the desired set of properties? As a growth challenge, we must tackle complex reactions in aqueous solvents and disentangle effects of macroscopic defects and strain gradients from intrinsic point defects. We will present results on synthesis, optical characterization, and computational modeling that addresses these questions.