Determination of Residual Magnetic Field Based on Optically-Detected Free Spin Precession of Hyperpolarized 21Ne

We report an efficient approach that allows inferring the amplitude and direction of magnetic field, based on optically pumped ²¹ Ne/Cs-Rb magnetometer. The feature that characterizes free spin precession (FSP) of hyperpolarized ²¹ Ne in external magnetic field provides feasibility for measuring residual field inside magnetic shielding. The measurements are carried out in a ²¹ Ne cell cohabiting with Cs and Rb vapors, which demonstrate that the hyperpolarized ²¹ Ne starts freely precessing at Larmor frequency around ambient magnetic field in pump-free state with the advantage of eliminating ac stark shift. Through the Larmor precession and corresponding spectral analysis, we experimentally obtain the weighted average values with related estimated uncertainties: the residual field components in the $x$ -, $y$ -, and $z$ -axes are (1.41 ± 0.06), (2.41 ± 0.06), and (1.86 ± 0.08) nT, with negative, positive, and negative directions along their corresponding axes, respectively. This research offers foundation for further studying the hyperfine splitting under near-zero field unaffected by Zeeman effect.