He³-Ne²¹ Ramsey Comagnetometer with Sub-nHz Frequency Resolution

Nuclear spin comagnetometers offer exceptional precision in measurements of spin energy levels and exhibit long-term stability, making them powerful tools for probing spin-dependent physics beyond the standard model as well as for inertial rotation sensing. We describe a new He³-Ne²¹ Ramsey comagnetometer operating with an in situ Rb⁸⁷ magnetometer for initialization and sensing of nuclear spins. During free precession of nuclear spins we turn off all lasers and introduce a microwave field to suppress back action from Rb atoms. We demonstrate that scalar and dipolar interactions between nuclear spins can be eliminated via control of the polarized sample geometry. These improvements result in a bias-free measurement with a frequency sensitivity of 0.6 nHz after six hours of integration.