Clock transitions are well known in atomic and solid-state systems, but are largely unexplored in molecular liquids. Here we demonstrate a clock-like, nuclear-spin avoided crossing in [1--13C]-fumarate that supports long-lived and directly observable coherences at ultralow magnetic field: a three-spin transition ∣S0α⟩↔∣T+1β⟩ near 400 nT exhibits a shallow crossing with a frequency minimum of 2 Hz. The transition is first-order immune to magnetic field perturbations and displays a lifetime of 25 s, around three times the longest single-spin T2∗. Sensitivity to effective pseudo-fields is also demonstrated, including the internal dipolar field of the sample.
@article{arxiv.2601.07614,
title = {A directly observable, Zeeman-insensitive nuclear spin coherence in solution},
author = {James Eills and Anushka Singh and Amir-Mahyar Teimoori and Irene Marco-Rius and Morgan W. Mitchell and Michael C. D. Tayler},
journal= {arXiv preprint arXiv:2601.07614},
year = {2026}
}