Enabling Clock Transitions in Divalent Lanthanide Complexes with 4fⁿ5d¹ Configurations

The ability to create and control coherence in a quantum object, or "qubit", is key to the development of new concepts in quantum information science (QIS), including sensing, communication, and computing. Fundamentally, interrogating and manipulating quantum phenomena are at direct odds with sustaining coherence, as interactions between a qubit and its environment lead to decoherence and thus a loss of information. As such, long coherence times can often come at the expense of control over initialization and read-out, scalability, and entanglement. Our research uses synthetic chemistry to map the established physics of trapped ions onto molecular architectures, thereby effectively replacing ion traps with highly tunable, atomically precise ligand shells. Coherence times in electron-spin molecular qubits are improved at avoided energy-level crossings where the dependence of the transition frequency on the magnetic field vanishes. As such, the transverse relaxation time (<i>T</i>₂) is less sensitive to magnetic noise, engendering resistance to decoherence from near-neighbor magnetic sites and other nuclear spins in the molecule. Organometallic systems are particularly desirable because ligand field interactions in metallocenes can be adjusted by changing ring size and ring substituents to control both metal–C_ring distances and the electron withdrawing or donating character of the ligand itself. This presentation describes a general strategy for identifying molecules with high-frequency clock transitions based on systems wherein a d-electron is coupled to a crystal-field singlet state of an f-configuration, resulting in a minimally anisotropic ground state with strong hyperfine coupling. Using this approach, a 9.834 GHz clock transition was identified in a molecular Pr complex, [K(crypt)][Cp’₃Pr^II], leading to <i>T</i>₂ enhancements up to threefold relative to other transitions in the spectrum. This result indicates the promise of the design principles outlined here for further development of f-element systems for quantum information applications.