Optical pumping of alkali-metal vapor in the quasi-high-pressure regime

TL;DR

This paper develops a unified theoretical framework for optical pumping of alkali-metal vapor in the quasi-high-pressure regime, improving understanding for atomic magnetometry under realistic buffer gas pressures.

Contribution

It introduces a new model that accurately describes optical pumping when collisional broadening is comparable to hyperfine splitting, extending beyond the high-pressure approximation.

Findings

Light absorption and spin polarization differ significantly from high-pressure predictions.

The model identifies favorable operating conditions in the quasi-high-pressure regime.

Guides improved atomic magnetometry performance under realistic buffer gas pressures.

Abstract

Optical pumping is fundamental to high-precision measurement using thermal alkali-metal atoms in vapor cells. In applications such as atomic magnetometry, buffer gases (e.g., $\mathrm{N}_2$ or $\mathrm{He}$) at specific pressures are introduced to quench fluorescence and mitigate wall relaxation. In the high-pressure limit (e.g., the $\mathrm{N}_2$ pressure $p_{\mathrm{N}_2}> 1$~atm), where collisional broadening exceeds hyperfine splittings of the atoms, optical pumping theory provides a clear description of the angular momentum exchange between photons and atomic spins. However, in many magnetic sensing scenarios, the high-pressure approximation becomes inadequate as its pressure conditions are not strictly satisfied. Consequently, an explicit description of optical pumping under realistic pressures is critical for selecting operating points and enhancing system performance. To address this, we develop a unified theoretical framework of optical pumping in the quasi-high-pressure regime, where collisional broadening is comparable to the ground-state hyperfine splitting. We demonstrate that light absorption, spin polarization, and magnetic-resonance linewidth in this regime differ significantly from those predicted by the high-pressure limit and offer favorable operating conditions. Our study extends conventional modeling and offers critical guidance for atomic magnetometry operating under realistic buffer gas pressures.