Non-perturbative suppression of spin polarization noise in SERF comagnetometers

Spin-Exchange Relaxation-Free (SERF) comagnetometers hold considerable promise for fundamental physics research and inertial navigation. Spin polarization noise, which arises primarily from misalignment between the pump and probe beams, constitutes a major factor limiting comagnetometers’ sensitivity. Existing suppression methods are hindered by indirect optimization criteria that do not accurately reflect noise suppression performance and often disrupt the steady-state regime. To accurately characterize comagnetometer behavior under misalignment, we systematically reconstructed its coordinate system and established a quantitative relationship between spin polarization noise and misalignment using an electron–nuclear spin dynamics model. Theoretical modeling demonstrates that the efficient suppression of the spin polarization noise coupling coefficient can be achieved effectively via the directional alignment of the probe beam. Due to the inherently non-perturbative nature of spin polarization measurements conducted with the probe beam, this adjustment strategy process sustains uninterrupted operation within the steady-state regime. Furthermore, the quantitative assessment of the noise coupling coefficient is achieved by examining the comagnetometer's response to modulated pump power—a methodology that similarly preserves the steady-state integrity of the comagnetometer. Experimentally, the method reduces spin polarization noise to an equivalent sensitivity of 4.0 × 10⁻⁸°/s/Hz^1/2 at 1 Hz, representing a 24-fold improvement over mechanical alignment and a factor of 43 lower than the state-of-the-art sensitivity. This work provides a technical foundation for achieving unprecedented sensitivity levels in comagnetometers and extends their scope of application. The method is broadly applicable to misalignment suppression in dual-beam atomic magnetometers and other thermal atomic sensors.