Increased Crystal Field Drives Intermediate Coupling and Minimizes Decoherence in Tetravalent Praseodymium Qubits

Crystal field (CF) control of rare-earth (RE) ions has been employed to minimize decoherence in qubits and to enhance the effective barrier of single-molecule magnets. The CF approach has been focused on the effects of symmetry on dynamic magnetic properties. Herein, the magnitude of the CF is increased via control of the RE oxidation state. The enhanced 4f metal-ligand covalency in Pr⁴⁺ gives rise to CF energy scales that compete with the spin-orbit coupling of Pr⁴⁺ and thereby shifts the paradigm from the ionic ζ_SOC ≫ V_CF limit, used to describe trivalent RE-ion, to an intermediate coupling (IC) regime. We examine Pr⁴⁺-doped perovskite oxide lattices (BaSnO₃ and BaZrO₃). These systems are defined by IC which quenches orbital angular momentum. Therefore, the single-ion spin-orbit coupled states in Pr⁴⁺ can be chemically tuned. We demonstrate a relatively large hyperfine interaction of A_iso = 1800 MHz for Pr⁴⁺, coherent manipulation of the spin with Q_M = 2Ω_RT_m, reaching up to ∼400 for 0.1Pr:BSO at T = 5 K, and significant improvement of the temperature at which T_m is limited by T₁ (T* = 60 K) compared to other RE ion qubits.