Dynamics of hyperpolarized nuclear spins at Zeeman-insensitive points
Avoided crossing (LAC) transitions are central to spin-based precision measurements since they provide strong observability and field-perturbation insensitivity, but while observed commonly in isolated-atom or engineered solid-state platforms, such transitions are unexploited in molecules in liquids. Here we report a solution-state, nuclear-spin LAC transition in hyperpolarized [1-$^{13}$C]-fumarate. The singlet-triplet LAC, occurring near 400 nT produces pronounced magnetization oscillations at approximately 2 Hz with a coherence time of 25 s, three times longer than the longest single-spin $T_2^*$. In the regime of high spin concentration and high polarization the sample's internal dipolar field couples measurably to the transition, providing a direct probe of nonlinear spin dynamics. These results establish nuclear-spin LACs as a platform for quantum sensing in liquids.