Picotesla magnetometry with coherent dark states

We investigate the experimental sensitivity limit of a scalar optical magnetometer based on coherent population trapping on the D-2 line of a thermal cesium vapor. We find the expected strong dependence on averaging time, with a detection limit below 4 pT for integration times longer than 1 s, limited by slow drifts of the cell temperature and of the applied test magnetic-field itself. A detailed noise analysis shows that for shorter averaging times the demonstrated limit of about 12 pT/root Hz is dominated by the frequency noise of the laser source. The magnetometer can be operated for real-time detection in flux densities up to at least milliteslas and is robust enough for application outside the laser laboratory.