Real-time vector field tracking with a cold-atom magnetometer

We demonstrate a fast three-axis optical magnetometer using cold, optically trapped 87Rb gas as a sensor. By near-resonant Faraday rotation we record the free-induction decay of a spin polarized ensemble following optical to obtain the three field components and one gradient component. A single measurement achieves shot-noise limited sub-nT sensitivity in 1 ms, with transverse spatial resolution of ∼20 μm. We make a detailed analysis of the shot-noise-limited sensitivity.