In-situ triaxial residual magnetic field measurement based on optically-detected electron paramagnetic resonance of spin-polarized potassium

We present an approach that allows detecting all three components of the residual magnetic field inside shielding, based on the electron paramagnetic resonance (EPR) of spin-polarized K atoms. The residual field experienced by spin-polarized K atoms dominates Larmor precession frequency, the smaller the frequency is, the more benefits it has to realize spin-exchange relaxation-free (SERF) regime under a definite spin-exchange rate. The measurements are accomplished based on depopulation optical pumping and optically-detected peak of the K EPR spectrum. The EPR-based cross modulation method is employed to determine all three components of the residual field independently by modulating the frequency of transverse field and the strength of longitudinal field. The weighted averages of all measurements with the corresponding uncertainties are (9.60 ± 0.06) nT, (2.25 ± 0.03) nT, and (2.83 ± 0.17) nT along with the negative directions of their related axes, respectively.