Design and performance of an absolute He/Cs magnetometer

We report on the design and performance of a highly sensitive combined He/Cs magnetometer for the absolute measurement of magnetic fields. The magnetometer relies on the magnetometric detection of the free spin precession of nuclear spin polarized He gas by optically pumped cesium magnetometers. We plan to deploy this type of combined magnetometer in an experiment searching for a permanent electric dipole moment of ultracold neutrons at the Paul-Scherrer Institute (Switzerland). A prototype magnetometer was built at the University of Fribourg (Switzerland) and tested at Physikalisch-Technische Bundesanstalt (Berlin, Germany). We demonstrate that the combined magnetometer allows Cramér-Raolimited field determinations with recording times in the range of 10–500 s, measurements above 500 s being limited by the stability of the applied magnetic field. With a 100 s recording time we were able to perform an absolute measurement of a magnetic field of ≈1μT with a standard uncertainty of ΔB ∼ 60 fT, corresponding to ΔB/B < 6×10−8.