Optical pumping enhancement of a free-induction-decay magnetometer

TL;DR

This paper demonstrates a method to enhance spin polarization in a free-induction-decay magnetometer by synchronizing optical pumping with a pulsed magnetic field, improving sensitivity in the presence of transverse fields.

Contribution

The study introduces a novel pulsed magnetic field technique synchronized with optical pumping to improve magnetometer performance under transverse magnetic fields.

Findings

Achieved a noise floor sensitivity of 238 fT/√Hz at 50 μT.

Demonstrated minimal residual magnetic field during detection.

Sensitivity closely matches the Cramér-Rao lower bound.

Abstract

Spin preparation prior to a free-induction-decay (FID) measurement can be adversely affected by transverse bias fields, particularly in the geophysical field range. A strategy that enhances the spin polarization accumulated before readout is demonstrated, by synchronizing optical pumping with a magnetic field pulse that supersedes any transverse fields by over two order of magnitude. The pulsed magnetic field is generated along the optical pumping axis using a compact electromagnetic coil pair encompassing a micro-electromechanical systems (MEMS) vapor cell. The coils also resistively heat the cesium (Cs) vapor to the optimal atomic density without spurious magnetic field contributions as they are rapidly demagnetized to approximately zero field during spin readout. The demagnetization process is analyzed electronically, and directly with a FID measurement, to confirm that the residual magnetic field is minimal during detection. The sensitivity performance of this technique is compared to existing optical pumping modalities across a wide magnetic field range. A noise floor sensitivity of $238\,\mathrm{fT/\surd{Hz}}$ was achieved in a field of approximately $\mathrm{50\,\mu{T}}$, in close agreement with the Cram\'{e}r-Rao lower bound (CRLB) predicted noise density of $258\,\mathrm{fT/\surd{Hz}}$.