Ultra-sensitive magnetometry based on free precession of nuclear spins

We discuss the design and performance of a very sensitive low-field magnetometer based on the detection of free spin precession of gaseous, nuclear polarized He-3 or Xe-129 samples with a SQUID as magnetic flux detector. The device will be employed to control fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron. Furthermore, with the detection of the free precession of co-located He-3/Xe-129 nuclear spins it can be used as ultra-sensitive probe for non-magnetic spin interactions, since the magnetic dipole interaction (Zeeman-term) drops out. Characteristic spin precession times T-2 (*) of up to 60 h were measured. The achieved signal-to-noise ratio of more than 5000:1 leads to an expected sensitivity level (Cramer-Rao lower bound) of delta Ba parts per thousand 1 fT after an integration time of 220 s and of delta Ba parts per thousand 10(-4) fT after one day. By means of a co-located He-3/Xe-129 magnetometer, noise sources inherent in the magnetometer could be investigated, showing that CRLB is fulfilled, at least down to delta Ba parts per thousand 10(-2) fT. The reason for such a high sensitivity is that free precessing He-3 (Xe-129) nuclear spins are almost completely decoupled from the environment. Therefore, this type of magnetometer is particularly attractive for precision field measurements where long-term stability is required.