µSQUID-EPR Reveals Topologically Quenched Tunnelling in a Lanthanide Molecule

Preservation of quantum coherence can be achieved by harnessing a system’s topology—encoding information in global geometric properties of quantum states insensitive to local perturbations. The robustness arises as certain geometric properties depend only on the trajectory of a quantum operation and the curvature in the controlled-parameter space, rather than on details of the system’s dynamics, thereby offering a topology-based framework for fault-tolerant quantum computation. To probe a system’s topology, geometric phase interference (Berry phase) has been studied in various optical systems and condensed matter physics, including molecular magnets (MMs). However, its direct detection in 4f-based MMs—prospective qudit (multi-level qubits) platforms—has remained elusive due to large tunnel gaps and environmental coupling. Here, we report a magneto-spectroscopic approach: the μSQUID-EPR technique, to directly resolve tunnel splittings in the ¹⁶⁰ Gd-based MM [ ¹⁶⁰ GdPc₂]⁻ (Pc = phthalocyanine). By irradiating single crystals with microwaves under transverse magnetic fields, we map the spin manifold and report the first direct observation of pronounced oscillations in the tunnel splitting—a signature of quantum phase interference. These oscillations reveal topological quenching of tunnelling and highlight the role of fourth-order transverse anisotropy in shaping the spin orientation space. Our findings uncover a nontrivial topological structure in 4f-MMs, opening possibilities towards holonomic quantum computation.