Light Shifts in Free-Precession Alkali Magnetometry at Earth-Scale Fields

We report recent progress on our study of low buffer gas pressure alkali magnetometers for Earth-scale fields in the presence of light shifts. Free-precession magnetometers typically work to avoid noise introduced by light shifts by implementing separate pump and probe periods, where the measurement of free precession is only done during probing periods. For the highest sensitivity free-precession measurements, a linearly polarized, off-resonant probe is used to monitor the evolution of alkali spins in a plane transverse to a total magnetic field. Here, any remnant helicity of the probe can cause a light shift that the electronic state experiences as a fictitious magnetic field, which will enter in quadrature with the total Earth-scale magnetic field being measured and can often be neglected. However for projections of near-resonant light along the measured total field, light shifts add linearly and accompanying noise can occur, similar to that seen in continuously pumped magnetometers (RF, SERF, Bell-Bloom, etc.). Here we explore these and other light shift effects.