Massive 116 GHz Crystal Field Clock Transition in a Tetragonal Molecular Ho(III) Complex*

Molecular lanthanide complexes are promising candidates for development of next-generation quantum technologies. In particular, high-symmetry structures can give rise to well-isolated crystal-field quasi-doublet ground states, i.e., quantum two-level systems that may serve as a basis for spin qubits. More importantly, recent work has shown that the coordination environment around the lanthanide can be tailored to produce an avoided crossing, or clock transition within the ground doublet, where the first-order sensitivity to fluctuations in the local magnetic field is suppressed, leading to significantly enhanced coherence times. Here, we employ single-crystal high-frequency electron paramagnetic resonance (EPR) spectroscopy to interrogate a series of new tetragonal molecular Ho(III) complexes. An axial coordination environment with four-fold symmetry gives rise to a ground state m_J = ±8 crystal-field quasi-doublet with clock transitions as large as 116 GHz. Larger clock transition frequencies result in faster qubit dynamics, reducing the time required for implementing quantum logic operations. Additionally, the second-order sensitivity to magnetic field fluctuations decreases with increasing clock transition frequency, potentially enhancing coherence further.