The authors describe an atomic magnetometer, the evanescent wave magnetometer, which uses an evanescent wave to measure the Larmor frequency of Rb atoms near the cell surface. The submicron penetration depth of the probe beam allows the evanescent wave magnetometer to achieve a spatial resolution of several tens of microns or better, and greatly reduces the inhomogeneous broadening due to magnetic field inhomogeneities. Its noise density in the present experiment is ϳ100 pT/ ͱ Hz for

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... between 1 and 10 mHz, and decreases to less than 10 pT/ ͱ Hz as the frequency increases to 25 Hz. Atomic magnetometers determine the magnetic field by measuring the energy separation between the Zeeman levels ͑Larmor frequency͒ of the ground state alkali metal atoms. They have recently attracted much interest due to their potential of achieving ultrahigh sensitivity, 1-3 surpassing that of the superconductor quantum interference devices long considered to be the most sensitive magnetometer. 4 Also of great current interest is the miniaturization of atomic magnetometers, e.g., the development of the chip-scale atomic magnetometer, which has a measurement volume of alkali metal vapor of 1 ϫ 10 −3 cm 3 , a few orders of magnitude smaller than conventional atomic magnetometers, and a sensitivity of 5 pT/ ͱ Hz between 1 and 100 Hz. 5