Significance Optically pumped magnetometers (OPMs) are crucial in magnetic- field measurements. Its high sensitivity and low noise allow it to detect weak magnetic- field changes up to the nT level, thus significantly expanding human understanding into the micro- magnetic- field environment. OPMs are beneficial in various areas, including the precise mapping of Earth's magnetic field, the detection of weak biological magnetic signals within biological tissues, quantum computing, and magnetic- material research. The main function of an OPM is to detect magnetic fields using specific polarized pump light and detection light. High- purity pump light, which is typically linearly or circularly polarized, interacts with atomic cells. The selective excitation of pump light causes the arrangement of atomic magnetic moments to align with the polarization direction of light, thus forming a magnetized region. This process is known as optical pumping, which effectively places atoms in specific magnetic quantum states, thereby enhancing their sensitivity to magnetic fields. This allows the detection of light passing through magnetized regions with different polarization states. Under the effect of a magnetic field, the energy levels of atoms undergo Zeeman splitting, which affects the absorption and scattering of detection light. By analyzing and detecting changes in light intensity or polarization rotation, the intensity and direction of the magnetic field can be inferred accurately. The polarization state must be controlled precisely as it determines the manner by which atoms are excited and their response to magnetic fields, thereby ensuring the high sensitivity and measurement accuracy of the equipment. This principle based on atomic magnetic resonance renders the OPM an ideal tool for detecting weak magnetic fields; thus, OPMs are widely used in scientific research and practical applications. Aided by the development of modern technology, researchers have comprehensively investigated vector lights with complex polarization states and gradually applied them to OPMs. In an OPM, particularly the component involving the interaction between vector light and atoms, dichroism and birefringence are two key optical phenomena. Dichroism refers to the different propagation characteristics of light in different polarization directions. This characteristic is used to selectively excite or manipulate atoms of specific energy levels in an OPM. Birefringence refers to the difference in the refractive index between vertically and horizontally polarized light in a medium, which causes a beam to separate when it passes through an atomic medium. By leveraging the properties above and examining the practical application of vector light in OPMs, we can further improve the sensitivity of OPMs. Compared with conventional OPMs, a new type of OPM based on vector light pumping/detection features rapid response, adaptability to more complex environments, and high sensitivity. Owing to the vector characteristics of vector light, this OPM can simultaneously measure multiple components of the magnetic field, thus achieving complete magnetic- field vector measurements without requiring additional optical paths. Therefore, it is more suitable for integration into complex optical systems in fields such as biomedical, material science, and space exploration. Progress This paper summarizes the development history of OPMs, describes their operating principle (Figs. 2 and 3), and explains the energy- level transition of optically pumped polarized alkali metal atoms based on a 87 Rb atomic energy- level diagram (Fig. 4). As per the development history of OPMs, optical polarization is crucial to the entire process (Fig. 5). Vector light is a new type of beam that has emerged in recent years, and its polarization state is distributed in a certain pattern on the cross-section of the beam (Fig. 6). The measurement of vector light in a magnetic field has been investigated extensively (Fig. 7). In terms of light and matter, investigating the dichroism and birefringence in the interaction between vector light and alkali metal atoms is crucial for OPMs. Based on dichroism, when a beam of linearly polarized light is irradiated onto an alkali metal atomic- gas chamber, the absorption and scattering efficiency of light varies owing to the ultrafine structure of the atoms and the energy levels of different magnetic quantum numbers. By adjusting the polarization direction of light, atoms can be more effectively pumped into specific magnetic states, thereby enhancing their response to magnetic fields (Fig. 8). Based on birefringence, vector light is incident in the orthogonal pumping direction, and the relationship between the polarization rotation angle of the detected light and the external magnetic field after the light passes through the gas chamber can be derived (Fig. 9). In practical applications, vector light with complex polarization states can be used as pump/detection light not only for the real-time dynamic detection of the magnetic- field size (Fig. 10) but also for solving the " dead zone" problem in magnetometers, improving spatial resolution and sensitivity (Fig. 11), and achieving miniaturized OPMs. Conclusions and Prospects Comprehensive investigations into quantum optics and the development of micro- and nano- technology will refine the manipulation of vector light, which is expected to enable magnetic- field measurements with higher sensitivities and resolutions. New OPMs based on vector optical pumping/detection have gradually received the attention of researchers. Thus, the application of OPMs in biomedicine, geological exploration, quantum information processing, and other fields will be promoted. Additionally, the miniaturization and integrated design of OPMs render them more portable and will further broaden their practical use. In the future, the development of vector- light technology will significantly improve OPMs.
