Restoring Narrow Linewidth to a Gradient-Broadened Magnetic Resonance by Inhomogeneous Dressing

We study the possibility of counteracting the line-broadening of atomic magnetic resonances due to inhomogeneities of the static magnetic field by means of spatially dependent magnetic dressing, driven by an alternating field that oscillates much faster than the Larmor precession frequency. We demonstrate that an intrinsic resonance linewidth of 25~Hz that has been broadened up to hundreds Hz by a magnetic field gradient, can be recovered by the application of an appropriate inhomogeneous dressing field. The findings of our experiments may have immediate and important implications, because they facilitate the use of atomic magnetometers as robust, high sensitivity detectors in ultra-low-field NMR imaging.