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The spin-exchange relaxation-free (SERF) atomic magnetometers (AMs) exhibit ultrahigh sensitivity in ultraweak magnetic field measurement, but their narrow bandwidth constrains the scope of their applications. To increase the bandwidth, magnetic field closed-loop control technology has been introduced into AMs with demonstrated efficacy. However, previous research on closed-loop AMs resulted in signal distortion problems, primarily due to imprecise closed-loop models and control parameters selection without theoretical support. This problem is specifically characterized by the unexpected amplification or reduction of signals at certain frequencies, directly leading to the degradation of the frequency performance of the closed-loop AM, making it unsuitable for applications requiring full-bandwidth measurement. In this study, we proposed a performance-enhanced closed-loop SERF AM, maximizing the bandwidth while ensuring consistent performance across the entire bandwidth. An accurate closed-loop system model was first established. Subsequently, an optimal relationship between the control parameters was proposed according to the eigenvalues of the closed-loop system, and the particle swarm optimization (PSO) algorithm was introduced to determine the optimal control parameters. The improved closed-loop system was constructed in a triaxial SERF AM, achieving an average bandwidth of 410 Hz and a sensitivity of 10 fT/Hz $^{{1}/{2}}$ in three axes, while ensuring consistent performance across the entire bandwidth. With the proposed method, the optimized control parameters can be obtained only by requiring the frequency response of the open-loop AM, which is convenient and useful for closed-loop AM applications.

Miniaturized high-sensitivity atomic magnetometers play a crucial role in biomagnetic measurements. The utilization of multipass cells enhances the sensitivity of atomic magnetometers. This study explores the benefits of employing a multipass cell in spin-exchange relaxation-free (SERF) atomic magnetometers through theoretical and experimental analysis. A signal-to-noise ratio (SNR) model was established by accounting for the optimal detuning frequency of the probe light and low-frequency 1/f noise. This enabled the optimal number of probe light passes through the vapor cell to be determined. Additionally, compact and symmetric reflective optical paths were designed to mitigate polarization gradients in atomic magnetometers with high atomic number density. Experimental results confirmed that the magnetic field sensitivity of the triple-pass configuration (1.8 fT/Hz $^{1/2}$ ) was 1.89 times that of the single-pass configuration (3.4 fT/Hz $^{1/2}$ ), consistent with the theoretical analysis. This study provides valuable theoretical guidance for analyzing the SNR performance of SERF atomic magnetometers and demonstrates the potential of multipass cells in enhancing their magnetic field sensitivity. The utilization of multipass cells presents a viable approach toward the realization of highly sensitive and portable miniaturized SERF atomic magnetometers.

Most magnetic shielding rooms (MSRs) are made of highly permeable permalloy. The limitation of such MSR is its intrinsic magnetic noise, which exceeds the sensitivity of magnetic sensors, impeding the further applications of ultrasensitive magnetic sensors, such as the magnetometers operated in the spin-exchange relaxation free (SERF) regime. In this article, a cubic ferrite magnetic shield with a low magnetic noise and magnetic field is proposed. The demagnetization model and magnetic noise calculation models are given. After optimizing several different coil structures and degaussing parameters, the magnetic field and the magnetic noises inside the cubic ferrite shield were measured. Within a distance of ±10 mm near the center, the residual magnetic field is below 0.5, 1.72, and 1.57 nT at the x-, y- and z- directions. The magnetic noise measurements at the center of the cubic ferrite magnetic shield reveal an averaged magnetic noise of 2.62 fT/Hz−1/2 from 20 to 100 Hz. Compared with the noise spectrum of permalloy shield, the noise level is reduced more than 133%. The results of this scaled-down cubic ferrite magnetic shield can be applied in larger sized shielding rooms. By assembling multiple ferrite panels together, it is possible to create a larger shielding space with a low magnetic field and low magnetic noise.

Elliptically polarized atomic magnetometers (AMs) offer high sensitivity and compactness by using a single laser beam to polarize atoms and detect optical rotation angles. However, their sensitivity advantage over optical absorption-based AMs has not been sufficiently demonstrated. This is because the optical parameters significantly affect the pumping and probing processes, leading to parameter coupling when both processes are optimized simultaneously. Such coupling presents a considerable challenge for achieving simultaneous parameter optimization, ultimately limiting the achievable sensitivity. In this study, we developed a modified input–output model for a magnetometer, based on which an optical parameter decoupling and optimization method is proposed. Our method enables us to obtain the optimal optical parameters simultaneously, including the ellipticity, intensity, and frequency of light. Consequently, a sensitivity of 6 fT/Hz1/2 was achieved using a $4\times 4\times4$ mm3 87Rb vapor cell (an effective volume of 0.027 cm3). This work offers an efficient strategy to achieve higher sensitivity using single-beam miniaturized AMs, contributing significantly to promote biomagnetism research.