This paper aims to review the technology, developments, and applications of spin exchange relaxation free (SERF) atomic magnetometers. The generalized comparisons between the SERF magnetometer and the other types of weak magnetic sensors are elaborated. Further, the fundamental composition and operational principle of the SERF atomic magnetometer are elucidated. The development history and recent technological developments with respect to several aspects, including the alkali metal vapor source, atom heating method, pump and probe lasers, and magnetic shielding system, are presented. The classification and respective advantages and disadvantages of these technologies are described in detail. Finally, the existing and potential applications, as well as the prospective future development trends are presented.

We describe a dual-axis atomic spin gyroscope based on an alkali metal-noble gas comagnetometer. Alkali metal vapor is optically pumped, and then the noble gas is hyperpolarized along the z axis. When sensing a transverse rotation, the polarized noble gas will be induced to precess and produce an effective magnetic field in the x – y plane for alkali metals to precess under. Operating in the spin-exchange relaxation-free regime, alkali atoms are modulated by the z axis magnetic field and serve as an integrated in-situ dual-axis magnetometer to detect the gyroscopic precession in the x and y axes simultaneously, using a single probe beam. By using the parametric modulation technique, the low frequency drift is effectively suppressed and a bias instability of less than 0.05 deg/h has been achieved in our dual-axis atomic spin gyroscope.

Atomic co-magnetometers can be utilized for high-precision angular velocity sensing or fundamental physics tests. The sensitivity of a co-magnetometer determines the angle random walk of an angular velocity sensor and the detection limit for a fundamental physics test. A high-sensitivity K-Rb-²¹Ne co-magnetometer, which is utilized for angular velocity sensing, is presented in this paper. A new type of spin relaxation of Rb atom spins, which can broaden the zero-field magnetic resonance lines of the co-magnetometer, is discovered. Further studies show that the spin relaxation of Rb atoms is caused by a high Rb electron magnetization field. With this discovery, the total relaxation rate of Rb atoms is optimized to improve the sensitivity of the co-magnetometer. Moreover, its sensitivity is optimized by suppressing various noises. Especially, to suppress laser-related noises, the co-magnetometer is designed such that the sensitive axis of the co-magnetometer can be fixed to the direction in which the projection input of the earth’s rotation is 0. This is called a rotating co-magnetometer. A magnetic field sensitivity of 1.0 fT/Hz^−1/2@5 Hz, which is equal to an angular velocity sensitivity of 2.1 × 10⁻⁸ rad s⁻¹ Hz^−1/2@5 Hz, is demonstrated using a spherical vapour cell with a diameter of 14 mm.

We investigate the real-time closed-loop control of the compensation point in a $\mathrm{K}\text{−}\mathrm{Rb}{\text{−}}^{21}\mathrm{Ne}$ comagnetometer operated in the spin-exchange relaxation-free regime. By locking the electron resonance, the alkali-metal electrons are free from the fluctuations of the longitudinal ambient magnetic field and nuclear magnetization, which could improve the systematic stability, enlarge the linear measuring range, and suppress the cross-talk error of the comagnetometer. The demonstration of closed-loop control of the magnetic field in the single nuclear species comagnetometer is of great significance for rotation sensing such as in gyroscopes and other high-precision metrology applications of the comagnetometer.

Spin-exchange relaxation-free gyroscope (SERFG) is a promising rotation sensor, and the bias error is one of its most important characteristics. It is identified that rotation measurement noise induced by thermal magnetization fluctuations inherent in the ferrite shield in a SERFG limits its bias stability. The root cause of this error lies in the low-frequency magnetic field sensitivity dominated by the nuclear spin relaxation rate. In this paper, the steady state response model of a SERFG is modified considering the non-negligible nuclear relaxation rate and the pressure-dependent magnetic field sensitivity is experimentally studied. Our results show that reducing nuclear spin relaxation is an effective method to suppress the bias error induced by thermal magnetization fluctuations. Finally, in case of Pneon=1 atm, it is estimated that a bias stability limit of 5×10-4 deg/h is set by the lowest thermal magnetization noise of 4.4f-1/2 fT ever reported in a ferrite shield.

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.