  Self-oscillating atomic magnetometers, in which the precession of atomic
spins in a magnetic field is driven by resonant modulation, offer high
sensitivity and dynamic range. Phase-coherent feedback from the detected signal
to the applied modulation creates a resonant spin maser system, highly
responsive to changes in the background magnetic field. Here we show a system
in which the phase condition for resonant precession is met by digital signal
processing integrated into the maser feedback loop. This system uses a modest
chip-scale laser and mass-produced dual-pass caesium vapour cell and operates
in a 50 microtesla field, making it a suitable technology for portable
measurements of the geophysical magnetic field. We demonstrate a Cramer-Rao
lower bound-limited resolution of 50 fT at 1 s sampling cadence, and a sensor
bandwidth of 10 kHz. This device also represents an important class of atomic
system in which low-latency digital processing forms an integral part of a
coherently-driven quantum system.
