Light-induced frequency shift and relaxation of ground-state ³He via metastability-exchange collisions

Metastability-exchange collisions (MECs) play a central role in metastability-exchange optical pumping (MEOP) of ^3He, enabling the transfer of polarization from the metastable state to the ground state and the optical detection of nuclear magnetic resonance. Leveraging MECs, optically pumped ^3He nuclear magnetometers have been developed since the early development of MEOP. However, the MEC-induced frequency shift can bias the inferred magnetic field and thereby limit accuracy, whereas MEC-induced relaxation shortens the spin-coherence time and reduces the signal-to-noise ratio, thereby limiting sensitivity. Characterizing these two effects is therefore important for improving both the accuracy and sensitivity of ^3He magnetometry. In this work, we show that light-induced frequency shifts and relaxation in the metastable state can be transferred to ground-state ^3He nuclear spins through MECs. Light shifts are well-known atomic-physics effects and have also been considered in optically detected ^3He magnetometry (Phys. Rev. Research 7 043106 (2025)). The focus of the present work is therefore not the identification of the light shift itself, but the quantitative transfer of light-induced frequency shifts and relaxation from metastable atoms to ground-state nuclear spins through MECs. We develop a theoretical model to characterize these light-induced effects, highlight their significance under low-field conditions, and experimentally validate the model.