Magnetic-field-independent spin-exchange relaxation-free magnetometer

SERF magnetometers based on dense ensembles of alkali-metal spins are precision quantum sensors that hold the record of measured and projected sensitivity to magnetic fields, in the $\mu \text{G}-\text{mG}$ range. At geomagnetic fields however, these sensors quickly lose their magnetic sensitivity due to spin decoherence by random spin-exchange collisions. Here we discover that atoms with nuclear spin $I=1\mathrm{/}2$ can operate in the Spin-Exchange Relaxation Free (SERF) regime even at high magnetic field. We counter-intuitively show that frequent collisions between a dense and optically-inaccessible $(I=1\mathrm{/}2)$ gas with another optically-accessible spin gas ($I>1\mathrm{/}2$) improve the fundamental magnetic sensitivity of the latter. We analyze the performance of a dual-specie potassium and atomic hydrogen magnetometer, and project a fundamental sensitivity of about $10\mathrm{a}\mathrm{T}\sqrt{{\mathrm{c}\mathrm{m}}^3\mathrm{/}\mathrm{H}\mathrm{z}}$ at geomagnetic fields for feasible experimental conditions.