Femtotesla Direct Magnetic Gradiometer Using a Single Multipass Cell

We describe a direct gradiometer using optical pumping with opposite circular polarization in two Rb-87 atomic ensembles within a single multipass cell. A far-detuned probe laser undergoes a near-zero paramagnetic Faraday rotation due to the intrinsic subtraction of two contributions exceeding 3.5 rad from the highly polarized ensembles. We develop analysis methods for the direct gradiometer signal and measure a gradiometer sensitivity of 10.1 fT/cm root Hz. We also demonstrate that our multipass design, in addition to increasing the optical depth, provides a fundamental advantage due to the significantly reduced effect of atomic diffusion on the spin-noise time-correlation, in excellent agreement with the theoretical estimate.