Sub-Ensemble Isolation in SERF Magnetometry Enabled by Micrometer-Scale Polarization Control

Conventional understanding of a spin-exchange relaxation-free (SERF) atom ensemble pertains to the common perception that the rapid exchange of atom state finally results in uniform time evolution of the whole ensemble. However, in this study, we demonstrate that by manipulating pumping polarization in a micrometer level, misalignment between the time evolution of different sub-ensembles can be created within a single SERF ensemble with unprecedented independency. A novel pumping system consisting of a miniaturized ⁸⁷Rb vapor cell and a space-variant polarization metasurface is developed for the proof of concept. Our method induces position-dependent atomic anisotropy in both pumping and absorption into the thermal atomic ensemble. By constructing calculated Zeeman-sublevel populations in the SERF regime, distinct sensing channels are generated with 0.22 V/nT average scale factor, which is comparable with a single channel generated by a single SERF ensemble. The average crosstalk ratios between adjacent channels are measured up to 32 dB, through the excitation of the fictitious magnetic field (3.5 nT, 30 Hz), which is measured as 20 dB without sub-ensemble isolation in the same experimental condition. Our work demonstrates unprecedented crosstalk reduction in single-cell multichannel SERF magnetometry, which holds new promise for applications including high-spatial resolution neural biomagnetism mapping and portable magnetism measurement devices.