High-Sensitivity Atomic Magnetometer Unaffected by Spin-Exchange Relaxation

Alkali-metal magnetometers compete with SQUID detectors as the most sensitive magnetic field sensors. Their sensitivity is limited by relaxation due to spin-exchange collisions. We demonstrate a K magnetometer in which spin-exchange relaxation is completely eliminated by operating at high K density and low magnetic field. Direct measurements of the signal-to-noise ratio give a magnetometer sensitivity of $10\mathrm{f}\mathrm{T}\mathrm{H}{\mathrm{z}}^{\mathrm{-}\mathrm{1}\mathrm{/}\mathrm{2}}$, limited by magnetic noise produced by Johnson currents in the magnetic shields. We extend a previous theoretical analysis of spin exchange in low magnetic fields to arbitrary spin polarizations and estimate the shot-noise limit of the magnetometer to be $2\times {10}^{-18}\mathrm{T}\mathrm{H}{\mathrm{z}}^{\mathrm{-}\mathrm{1}\mathrm{/}\mathrm{2}}$.