Evanescent wave magnetometer

We describe a new type of atomic magnetometer, the evanescent wave magnetometer, which uses evanescent wave to measure the Larmor frequency of Rb atoms within a distance $\sim 10^{-5}\rm \, cm$ from the cell surface. The evanescent wave magnetometer has an extraordinarily small measurement volume of alkali metal vapor, which in our experiment is $5\times 10^{-7}\,\rm cm^3$. It is less susceptible to field inhomogeneities than conventional atomic magnetometers, and consequently suited for measuring or mapping fields with a large spatial gradient. Under our experimental conditions the minimum detectable magnetic field change is 0.4 nT. The sensitivity of the evanescent wave magnetometer is $20\,\rm pT\, Hz^{-1/2}$ between 0.5 and 50 Hz. Its spin shot-noise limited sensitivity is estimated to be $\rm 1.8\,pT\,Hz^{-1/2}$ for a measurement time of 1 second when the spin coherence time is limited by spin exchange collisions.