Ultrasensitive 3He magnetometer for measurements of high magnetic fields

We describe a ³He magnetometer capable to measure high magnetic fields (B> 0.1 T) with a relative accuracy of better than 10^-12. Our approach is based on the measurement of the free induction decay of gaseous, nuclear spin polarized ³He following a resonant radio frequency pulse excitation. The measurement sensitivity can be attributed to the long coherent spin precession time T₂ ^? being of order minutes which is achieved for spherical sample cells in the regime of “motional narrowing” where the disturbing influence of field inhomogeneities is strongly suppressed. The ³He gas is spin polarized in situ using a new, non-standard variant of the metastability exchange optical pumping. We show that miniaturization helps to increase T₂ ^? further and that the measurement sensitivity is not significantly affected by temporal field fluctuations of order 10^-4.