A 54.6 GHz Clock Transition in Ho3+ Electron Spin Qubits Assem-bled into a Metal-Organic Framework

A high-symmetry assembly of molecular spin qubits has been achieved in a metal-organic framework (MOF) [Ho(pzdo)4](ClO4)3 (1), where the eight-coordinate Ho3+ nodes are bridged by pyrazine-1,4-dioxide (pzdo) ligands. The approximate square-antiprismatic (D4d) coordination of the Ho3+ ion leads to the stabilization of the mJ = ±4 ground-state doublet due to crystal-field splitting of the J = 8 total angular momentum state. Mixing of the mJ = +4 and mJ = –4 projection states opens a zero-field energy gap () resulting in the spin clock transition (SCT) evi-dent in the EPR spectra of 1. The SCTs are known to protect qubits from the surrounding magnetic noise to first order, thus enhancing the coherence time of the superposition states crucial for quantum information processing. Frequency-dependent EPR studies reveal that the Ho3+ centers in 1 exhibit a high-frequency SCT with SCT = 54.6 GHz, which can be beneficial to minimizing second-order decoherence effects. The angular dependence of the resonance fields at which the SCT is observed maps well onto the lattice symmetry, with two distinct orienta-tions of the molecular anisotropy axes related by the tetragonal space group symmetry. All salient aspects of the magnetic and EPR measure-ments have been captured by a model that uses a new theoretical technique based on a constrained DFT derivation of the effective spin Ham-iltonian. This work demonstrates the possibility of engineering SCTs in ordered arrays of molecular spin qubits, thus paving the way to scaling up molecular systems that are promising for applications in emerging quantum technologies.