Ultrasensitive spin exchange relaxation-free (SERF) comagnetometers have been widely applied in various new physics experiments and show great potential for inertial navigation. However, the spatially nonuniform spin polarization of alkali metal (AM) and noble gas (NG) atoms results in an effective magnetic field gradient on the order of 100 nT/cm which is much larger than that is produced by the coils and remanence on the magnitude of 1 nT/cm, thereby reducing the suppression ability of the magnetic noise and limiting the scale factor and noise floor of the comagnetometers. In this study, we establish a model of the effective magnetic field gradient caused by uneven spin polarization and analyze the influencing factors. Our results indicate that optimizing the density ratio of hybrid AM atoms can suppress the effective magnetic field gradient. Nevertheless, the available pump light power imposes a limitation on the density ratio. To further address this issue, we propose a balanced light-assisted hybrid spin exchange optical pumping (HSEOP) method. Our experimental results confirm the effectiveness of the mentioned approach, with 67% suppression of the nuclear transverse relaxation rate and a nearly threefold increase in the scale factor. Furthermore, the optimal density ratio in experimental applications can be reduced from 200 to below 80, thereby overcoming the limitation of laser power in SERF comagnetometers and providing the possibility for wider applications. Finally, the suppression factor of magnetic noise is enhanced by 89% by optimizing the density ratio and the balanced light-assisted HSEOP method. This research sheds light on the fundamental physics experiments and potential for inertial navigation.
Ultrasensitive spin exchange relaxation-free (SERF) comagnetometers have been widely applied in various new physics experiments and show great potential for inertial navigation. However, the spatially nonuniform spin polarization of alkali metal (AM) and noble gas (NG) atoms results in an effective magnetic field gradient on the order of 100 nT/cm which is much larger than that is produced by the coils and remanence on the magnitude of 1 nT/cm, thereby reducing the suppression ability of the magnetic noise and limiting the scale factor and noise floor of the comagnetometers. In this study, we establish a model of the effective magnetic field gradient caused by uneven spin polarization and analyze the influencing factors. Our results indicate that optimizing the density ratio of hybrid AM atoms can suppress the effective magnetic field gradient. Nevertheless, the available pump light power imposes a limitation on the density ratio. To further address this issue, we propose a balanced light-assisted hybrid spin exchange optical pumping (HSEOP) method. Our experimental results confirm the effectiveness of the mentioned approach, with 67% suppression of the nuclear transverse relaxation rate and a nearly threefold increase in the scale factor. Furthermore, the optimal density ratio in experimental applications can be reduced from 200 to below 80, thereby overcoming the limitation of laser power in SERF comagnetometers and providing the possibility for wider applications. Finally, the suppression factor of magnetic noise is enhanced by 89% by optimizing the density ratio and the balanced light-assisted HSEOP method. This research sheds light on the fundamental physics experiments and potential for inertial navigation.
We investigate the effects of pump laser power density on the hybrid optically pumped comagnetometer operated in the spin-exchange relaxation-free (SERF) regime. The analytic steady-state output model for the comagnetometer considering two alkali metal species and one nuclear species is presented for the first time. And the effects of pump laser power density on the rotation sensitivity, suppression of low-frequency magnetic noise and long-term stability of the comagnetometer are studied experimentally. The results indicate that when the product of pumping rate and density ratio of pumped atom to probed atom is equal to the spin relaxation rate of the probed atom, the maximum response and highest sensitivity of the comagnetometer are achieved. However, the suppression of low-frequency magnetic noise and long-term stability improve with the increasing of pump laser power density due to the increasing of nuclear spin polarization. Our focus is to optimize the performance of the comagnetometer for rotation sensing, but the theory and method presented here are relevant to all applications of the hybrid optical pumping technique.
Spin exchange relaxation free (SERF) comagnetometers, which use coupled alkali metal and noble gas spins, are promising for precision measurements due to their potential for miniaturization and high sensitivity. In these systems, alkali atoms are optically pumped with circularly polarized light, and their polarization is transferred to noble gas nuclei via spin-exchange collisions. However, spatial inhomogeneities in alkali polarization induced by factors such as optical absorption, gas diffusion, and temperature gradients generate effective magnetic field gradients for the noble gas spins. These gradients accelerate spin relaxation and degrade overall sensor performance. This work introduces a pump beam reflection compensation scheme to improve alkali polarization uniformity in a SERF comagnetometer. By retro reflecting the pump beam back along its original optical path with reversed helicity, alkali atoms are repolarized from the opposite direction. This counter propagating beam compensates polarization gradients through reverse optical pumping, thereby homogenizing the effective field experienced by the noble gas spins. The modified pumping configuration enhances spatial uniformity while maintaining a simple and compact sensor architecture. A revised polarization rate model incorporating the retro reflected beam is presented. Simulations comparing single beam, 90% reflection, and total reflection configurations confirm a marked reduction in alkali polarization gradients for reflection based designs. Experimentally, the transverse relaxation time of the noble gas spins is extended by 77.66%, indicating significantly suppressed gradient induced decoherence. The proposed method supports the development of miniaturized, high performance SERF comagnetometers.
Alkali metal-noble gas comagnetometers operated in the regime of spin-exchange relaxation-free (SERF) have been widely used in fundamental physics tests and rotation sensing as gyroscope. Spin-exchange relaxation is an important systematic bias of the comagnetometer, which could limit sensitivity significantly. However, the suppression effect of the spin-exchange relaxation has not been examined quantitatively so far. Herein, we systematically investigate the relaxation of the alkali metal atoms and find that the spin-exchange relaxation is not completely eliminated under the normal operating condition of the K-Rb-21Ne comagnetometer. In addition to the optical pumping rate, the spin-exchange relaxation still plays a dominant role in the broadening of electron Zeeman resonances due to the large electron magnetic field. This is the first demonstration that the alkali metal atoms are operated in the near-SERF regime in the alkali metal-noble gas comagnetometer with a large election magnetic field, which will shed light on the way to reduce the relaxation of the alkali metal atoms and enhance the sensitivity of the comagnetometer.
The spin-exchange relaxation-free (SERF) comagnetometers, owing to their ultra-high sensitivity, are widely applied in fields such as inertial and magnetic field measurements, frontier physics research, etc. This technology relies on pump light to achieve the spin polarization of alkali metal atoms. During this process, the pump light is absorbed and thus attenuated by the alkali metal atoms. In this study, an absorption decay model for elliptically polarized light was solved by the fourth-order Runge-Kutta method, breaking through the applicability limitations of conventional analytical solutions for arbitrary polarized light. The effects of pump light polarization states and power on optical pumping rate, electron polarization, and measurement sensitivity were further investigated. It was found that there is a contradiction in achieving low polarization gradient and high scale coefficient by adjusting light intensity, increasing the pump light intensity can reduce the polarization gradient but will decrease the scale coefficient. Elliptically polarized light can realize low polarization gradient while enhancing the magnetic field response. This research establishes a theoretical foundation for optimizing the selection of pump light parameters in quantum sensors.
Abstract Atomic spin comagnetometers based on the spin-exchange relaxation-free (SERF) effect offer significant potential for inertial sensing and support the development of long-endurance, high-precision navigation technologies. However, limited nuclear spin polarization constrains the long-term stability of these systems. Here, a flat-top optical pumping configuration is introduced for SERF comagnetometers, expanding the uniform-intensity region of the beam to approximately 62 % , which enhances atomic polarization by around 19 % and improves magnetic noise suppression by 32 % . Experimental results demonstrate notable improvements in both sensitivity and long-term stability, highlighting the effectiveness of flat-top beams in overcoming atomic polarization uniformity limitations and advancing SERF co-magnetometer performance.
Ultrasensitive atomic comagnetometers have proven to be valuable tools for precise measurement and search for new physics. Besides, comagnetometers have potential for inertial navigation and quantum information. Despite their great potential, sensitivity improvement has been limited by different types of noise. To address this issue, the sources and magnitudes of noises are comprehensively analyzed and a suppression method is proposed for the most critical noise source, optical non-orthogonality. The optical non-orthogonality leads to critical transverse pumping effect, thus transverse light shift, and transverse effective magnetic noise. A direct correlation between the optical non-orthogonality and the electronic spin transverse relaxation rate is found, and a method for suppression of the optical non-orthogonality is proposed accordingly. Finally, the optical non-orthogonal noise is suppressed by 3.1 dB. Besides, an increase of 5.2 dB in the scale factor is achieved. With the combined effect of noise suppression and scale factor enhancement, the resulting inertial rotation sensitivity of 2.4×10-8rad/s/Hz1/2in the frequency range from 1 to 3 Hz is among the most sensitive comagnetometers, which opens up promising opportunities for new physics measurements, such as the search for ultralight dark matter and the fifth force.