Measurement of the geomagnetic field using parametric resonances in optically pumped He 4

A single-axis vector magnetometer which can be used to measure changes in geomagnetic field components has been constructed using a parametric resonance in the optically pumped 2 3 S 1 level of He 4 . The principle of operation for the n=1, p=1 parametric resonance is described. This resonance has a peak amplitude-to-noise ratio of 1.2 \times 10^{4} in a 0.5 Hz noise bandwidth and a 14.9 \times 10^{-4} G linewidth. The performance of the instrument is demonstrated by observing changes in the earth's field and 1.0μG calibration signals. Sensitivity is established to be 1.0 \times 10^{-7} G in a 5 Hz noise bandwidth.