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 ζSOC ≫ VCF 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 Aiso = 1800 MHz for Pr4+, coherent manipulation of the spin with QM = 2ΩRTm, reaching up to ∼400 for 0.1Pr:BSO at T = 5 K, and significant improvement of the temperature at which Tm is limited by T1 (T* = 60 K) compared to other RE ion qubits.稀土（Rare Earth, RE）离子的晶体场（Crystal Field, CF）调控已被用于抑制量子比特的退相干，并提升单分子磁体的有效势垒。此前晶体场调控的研究多聚焦于对称性对动态磁性质的影响。本文通过调控稀土的氧化态提升了晶体场作用的强度。Pr⁴⁺中增强的4f金属-配体共价性，使得晶体场能量尺度与Pr⁴⁺的自旋轨道耦合形成竞争，从而打破了用于描述三价稀土离子的离子极限ζ_SOC ≫ V_CF的范式，转向中间耦合（Intermediate Coupling, IC）区间。我们研究了Pr⁴⁺掺杂的钙钛矿氧化物晶格体系（BaSnO₃与BaZrO₃），这类体系具备可淬灭轨道角动量的中间耦合特性，因此Pr⁴⁺的单离子自旋轨道耦合态可通过化学手段进行调控。我们证实，Pr⁴⁺具有高达1800 MHz的各向同性超精细相互作用A_iso；以QM=2Ω_R T_m实现的自旋相干操控，在T=5 K下的0.1Pr:BSO样品中可达约400；且相较于其他稀土离子量子比特，其自旋相干时间由纵向弛豫T₁限制的临界温度T*提升至60 K，性能得到显著改善。