Searches for Lorentz violation in 3 He/ 129 Xe clock comparison experiments

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 3 He or 129 Xe samples with a SQUID as magnetic flux detector. Characteristic spin precession times T_2^∗ of up to 115 h were measured in low magnetic fields (about 1 μT) and in the regime of motional narrowing. With the detection of the free precession of co-located 3 He/ 129 Xe nuclear spins (clock comparison), the device can be used as ultra-sensitive probe for non-magnetic spin interactions, since the magnetic dipole interaction (Zeeman-term) drops out in the weighted frequency difference, i.e., Δ ω = ω He − γ He / γ Xe · ω Xe . We report on searches for Lorentz violating signatures by monitoring the Larmor frequencies of co-located 3 He/ 129 Xe spin samples as the laboratory reference frame rotates with respect to distant stars (sidereal modulation).