Pulsed ⁸⁷Rb vector magnetometer using a fast-rotating field

We present a vector geomagnetic magnetometer based on application of an external rotating magnetic field to a scalar atomic magnetometer. Using two channels in a gradiometer mode, it can provide simultaneous measurements of total field gradient with a sensitivity of 50 fT/Hz^1/2, as well as two polar angles relative to the plane of magnetic field rotation with resolutions of 8 nrad/Hz^1/2 (8 ppb, 400 fT/Hz^1/2) at 50 μT. Their noise spectrums are flat down to 1 Hz and 0.1 Hz, respectively. Crucially, it avoids several metrological difficulties associated with vector magnetometers and gradiometers. We use a field rotation rate faster than the spin relaxation rate and show that it eliminates a class of systematic effects associated with heading errors in alkali-metal scalar magnetometers. We investigate several other systematic effects, such as Berry's phase frequency shift and the effects of eddy currents in nearby conductors. We also derive fundamental limits on sensitivity of such sensor and show that the vector sensitivity can approach the sensitivity of scalar atomic magnetometers.