Hyperfine resolved Optical Pumping for Precision Magnetometry in Earth-scale Fields

For precision atomic magnetometry, inert buffer gas is included in alkali-metal vapor cells to broaden hyperfine transitions. Here we discuss low-buffer gas pressure alkali vapor cells with resolved hyperfine manifolds that also yield high performance magnetometers. We use a probe-assisted depopulation pumping to achieve high polarization in ⁸⁷Rb, optically pumping F = 2 states with narrow linewidth σ₊ light, while tuning a probe beam to depopulate F = 1 states (Δν = 6.8 GHz from F = 2). The probe tuning then also provides F = 2 detection with high optical rotation and low probe broadening; we demonstrate top-bottom gradiometry, within a small cell with single-pass geometry, that yields an Earth’s field free-precession (and separately RF) magnetometer sensitivity below 20 fT/Hz^1/2. We also discuss implications of this technique for gradient tolerance and retaining sensitivity in large Earth-scale fields, as well as future paths to improve sensitivity and heading-error suppression.