Femtotesla nearly quantum-noise-limited pulsed gradiometer at Earth-scale fields

TL;DR

This paper presents a highly sensitive, nearly quantum-noise-limited pulsed atomic gradiometer capable of detecting Earth's magnetic field with femtotesla sensitivity, using a compact setup with advanced noise rejection.

Contribution

It introduces a novel finite fields atomic gradiometer with near quantum-limited performance and high dynamic range suitable for Earth-scale magnetic field measurements.

Findings

Achieved 14 fT/Hz^{1/2} sensitivity over Earth's magnetic field range

Demonstrated a high common-mode rejection ratio exceeding 10^4

Observed nearly quantum-noise-limited behavior in the gradiometer

Abstract

We describe a finite fields magnetic gradiometer using an intense pulsed laser to polarize a $^{87}$Rb atomic ensemble and a compact VCSEL probe laser to detect paramagnetic Faraday rotation in a single multipass cell. We report differential magnetic sensitivity of $14$ fT/Hz$^{1/2}$ over a broad dynamic range including Earth's field magnitude and common-mode rejection ratio higher than $10^4$. We also observe a nearly quantum-noise-limited behaviour of the gradiometer, by comparing the experimental standard deviation of the estimated frequency difference against the Cram\'{e}r-Rao lower bound in the presence of white photon shot-noise, atomic spin noise and atomic diffusion.