Accurate vector measurement of residual magnetic fields with a Rb-Xe nuclear-spin magnetometer.

We demonstrate a frequency-referenced vector measurement of weak residual magnetic fields using a hybrid 87Rb-129Xe magnetometer operated in a near-zero-field environment. The magnetic-field magnitude is obtained from the Larmor precession frequency of hyperpolarized 129Xe nuclear spins via free-induction decay (FID) detection, while a three-axis magnetic-field reversal technique is employed to reconstruct the full vector field and overcome the intrinsic scalar nature of FID-based magnetometry. Experiments in a magnetically shielded environment achieve sub-10 pT uncertainty, with a linear response extending from near-zero fields to several hundred nanotesla. This approach provides a robust and practical route for accurate vector magnetic-field characterization.