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 Lu et al., 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 this light-induced effect, highlight its significance under low-field conditions, and experimentally validate it.