We report on the operation of co-located 129 Xe and 131 Xe nuclear spin masers with an external feedback scheme, and discuss the use of 131 Xe as a comagnetometer in measurements of the 129 Xe spin precession frequency. By applying a correction based on the observed change in the 131 Xe frequency, the frequency instability due to magnetic field and cell temperature drifts are eliminated by two orders of magnitude. The frequency precision of 6.2 µHz is obtained for a 10 4 s averaging time, suggesting the possibility of future improvement to ≈ 1 nHz by improving the signal-to-noise ratio of the observation.

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Abstract. Uncertainty in the frequency precision of the planned experiment to search for a 129 Xe atomic electric dipole moment is dominated by drifts in the frequency shift due to contact interaction of 129 Xe with polarized Rb valence electrons. In order to suppress the frequency shift, a double-cell geometry has been adopted for the confinement of 129 Xe gas. A new process has been identified to take part in the optical detection of spin precession. The parameters controlling the oscillation of the maser in this new double-cell arrangement were optimized. As a result, the frequency shift has been reduced by a factor of 10 or more from the former single-cell geometry.