Real-time stabilization of the alkali-metal transverse axis orientation in nuclear spin comagnetometer by biaxial differential detection

Axis orientation stability has a great effect on the long-term drift performance of nuclear spin comagnetometers that utilize alkali atoms to detect the magnetic moment of nuclear spin ensembles. Previous studies have analyzed the sensitivity influence of the misalignment angle and demonstrated a post-processing calibration method in the transverse polarized comagnetometer, but there is no study on the traditional longitudinally polarized setup. We propose an innovative biaxial differential detection method to monitor the misalignment angle in the longitudinally polarized comagnetometer and then eliminate it on-line. By monitoring the amplitude difference of a reference magnetic field’s projections in a rotated measurement frame, an error signal with good linearity is obtained. Experiments showed that any misalignment angle within (−π/4, π/4) can be successfully compensated to near 0 within about 1.5 s, which is fast enough compared to the drift rate of a practical device. The new detection method is robust and insensitive to many factors, such as direct component drift and scale factor inequality, so the method can be widely used in similar devices that acquire high accurate alignment of the axis orientation, such as the spin exchange relaxation-free magnetometer.