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-
13C]-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
T2∗. 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.
The use of nuclear spins as physical sensing systems is disadvantaged by their low signal responsivity, particularly when compared to sensing techniques based on electron spins. This primarily results from the small nuclear gyromagnetic ratio and the difficulties in achieving high spin polarization. Here we develop a new approach to investigating the response of hyperpolarized molecular nuclear spins to magnetic fields and demonstrate orders-of-magnitude enhanced magnetic responsivity over state-of-the-art proton and Overhauser magnetometers. Using hyperpolarized molecules with proton spins, we report the realization of magnetic amplification in linear and nonlinear types. We further extend this amplification to hyperpolarized scalar-coupled multi-spin molecules and observe substantial magnetic amplification exceeding 10%. Moreover, we observe an anomalous amplification with dispersive frequency dependence that originates from magnetic interference effects. Our work highlights the potential of hyperpolarized molecular nuclear spins for use in a new class of quantum sensors, with promising applications in both applied and fundamental physics, including highly accurate absolute magnetometry and the exploration of axion-nucleon exotic interactions.
We propose laser-based methods for the production of nuclear spin hyperpolarized molecules. Following pulsed, coherent excitation from ground-state molecules in a molecular beam to specific rotational $|J\phantom{\rule{0.16em}{0ex}}{m}_{J}\ensuremath{\rangle}$ states, the resulting hyperfine polarization beating produces desirable nuclear spin
mI population distributions and ${m}_{I}\ensuremath{-}{m}_{J}$ correlations at specific later times. At this point, selective dissociation can be used to remove undesired spin states, or a strong magnetic field can be applied, which later can be removed adiabatically to transfer the population-enhanced nuclear spin configurations to the desired nuclear spin state. Finally, the polarization beating is stopped, and the molecules are stored by cryogenic trapping. We show how these methods can be used to produce the long-lived singlet state of
15N2O and ortho
H2O, which have been proposed as magnetic tracers. We discuss how large production rates between 1 and $100 \ensuremath{\mu}\mathrm{mol}$/s with singlet-state populations between 35% and 100% can be achieved.