Femtotesla Nearly-Quantum-Noise-Limited Pulsed Gradiometer at Earth-Scale Fields

We describe a magnetic gradiometer that operates at finite fields and uses an intense pulsed laser to polarize a 87Rb 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 14fT/Hz1/2, corresponding to gradiometer sensitivity of 70fT/cm/Hz with a 0.2cm baseline, over a broad dynamic range, including Earth's field magnitude and a common-mode rejection ratio higher than 104. 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.