Topological suppression of quantum tunnelling in a lanthanide single-ion molecular magnet

Quantum coherence can be preserved by exploiting topology, encoding information in global geometric properties that resist local perturbations. These properties depend on the trajectory of quantum operations and curvature in parameter space, offering a topology-based route to fault-tolerant quantum computation. While geometric phase interference (Berry phase) is widely studied to probe a system’s topology, its direct detection in 4f-based molecular magnets—promising qudit platforms—has remained elusive. We present a magneto-spectroscopic μSQUID-EPR approach to resolve tunnel splittings in the Gd-based molecular magnet [$^{160}$GdPc₂]⁻ (Pc = phthalocyanine). By irradiating single crystals with microwaves under transverse magnetic fields, we map the spin (S = 7/2) manifold and observe pronounced oscillations in tunnel splitting—a hallmark of quantum phase interference. These oscillations reveal topological quenching and higher-order anisotropy, underscoring the role of topology in 4f systems and opening pathways toward holonomic quantum computation.