A high-sensitivity laser-pumped M_x-magnetometer

We discuss the design and performance of a laser-pumped cesium vapor magnetometer in the $M_x$-configuration. The device will be implemented for the control and stabilization of fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron. We have determined the intrinsic sensitivity of the device to be 21 fT with a 1 s integration time, limited by low frequency noise of the laser power. In the shot noise limit the magnetometer can reach a sensitivity of 7 fT with a 1 s integration time. We have used the device to study the magnetic field fluctuations in a three layer mu-metal shield for integration times in the range of 2-100 seconds. The lowest values of the fluctuations are traced back to the stability of the power supply used to generate the field.