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 metalligand 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.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) = 2 Omega T-R(m), reaching up to similar to 400 for 0.1Pr:BSO at T = 5 K, and significant improvement of the temperature at which T-m is limited by T-1 (T* = 60 K) compared to other RE ion qubits.