We describe a magnetic gradiometer that operates at finite fields and uses an intense pulsed laser to polarize a ${}^{87}$$\mathrm{Rb}$ atomic ensemble and a compact vertical-cavity surface-emitting laser probe laser to detect paramagnetic Faraday rotation in a single multipass cell. We report differential magnetic sensitivity of $14{\text{fT/Hz}}^{1/2}$, corresponding to gradiometer sensitivity of $70\text{fT/cm}/\sqrt{\mathrm{Hz}}$ with a $0.2\mathrm{cm}$ baseline, over a broad dynamic range, including Earth’s field magnitude and a common-mode rejection ratio higher than ${10}^4$. We also observe a nearly quantum-noise-limited behavior of the gradiometer by comparing the experimental standard deviation of the estimated frequency difference against the Cramér-Rao lower bound in the presence of white photon shot-noise, atomic spin noise, and diffusion.

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• Magnetometry
• Quantum sensing
• Alkali metals
• Atomic gases
• Devices