Fig. 1. Principle of optical pumping atomic magnetometer. (a) Larmor precession occurs when the spin magnetic moment of an electron is deflected by an external magnetic field; (b) alkali metal atom pumping polarization process; (c) the nucleus undergoes Larmor precession in the static magnetic field B.

Fig. 2. Miniaturization and application of optical pumping atomic magnetometer. (a) Photograph of the used vapor cell^[20]; (b) photograph of the sensor module, including the main optical components, atomic vapor cell, outer housing, rf coil, electrical wires and optical fibers^[20]; (c) microscope image of the micrometer Rb cell^[46]; (d) magnetic field distribution measured with the HEDscan system in six different subjects^[46]; (e) photograph of the setup inside the MSR, used for both OPM MEG and cryoMEG recordings^[46]

Fig. 3. Schematic diagrams of the CPTM theory and apparatus. (a) Three-level atomic system with Λ configuration; (b) schematic diagram of two-photo transitions in the hyperfine structure of D1 line of 87Rb atoms; (c) CPT resonance measurement apparatus

Fig. 4. CPT magnetometer development research. (a) CPTM sensor probe of the National Space Science Center, Chinese Academy of Sciences^［62］; (b) its CPTM electronics box^[62]; (c) magnetometer experimental facility at China Jiliang University^[63]; (d) experimental facility for measuring CPT resonance linewidth in 87Rb gas chamber at Beihang University^［64］; (e) CPT signal obtained by VCSEL^[64]

Fig. 5. Miniaturization and chip-scale design. (a) Schematic of the magnetic sensor^[40]; (b) photograph of the magnetic sensor^[40]; (c) optical image of the substrate^[66]; (d) the micro-fabricated vapor cell^[66]; (e) flexible printed circuit board frame^[66]; (f) the printed circuit board with a socket used for interfacing the signal from the physics package^[67]; (g) microfabricated vapor cells with the radius size of optical cavity about 2.5 mm^[66]; (h) microminiature CPT atomic magnetometer probe^[66]; (i) iicro atomic vapor cell^[66]; (j) magnetometer performance test^[66]

Fig. 6. Measurement of vector magnetic fields. (a) Diagram of the experimental set-up^[70]; (b) experimental setup of vector CPT magnetometer^[70]; (c) self-fabricated Helmholtz coils on a 3D-printed frame^[17]; (d) experimental setup to study the vector magnetic field effects on CPT^[71]; (e) experimental schematic of the vector magnetometer using a SCPT resonance in a feedback compensation system^[72]

Fig. 7. The coupled dark state magnetometer developed by the Austrian Academy of Sciences established the CDSM flight model for the low Earth orbit China Seismo-Electromagnetic Satellite (CSES) mission^[75]

Fig. 8. Schematic diagrams of the SERF magnetometer theory and apparatus. (a) Working principle of SERF atomic magnetometer; (b) schematic diagram of basic experimental device of double beam SERF atomic magnetometer

Fig. 9. Study of long relaxation time. (a) Experimental setup of SERF-like magnetometry in room-temperature environment^[84]; (b) relaxation of atomic spins change with different repumping scheme^[84]; (c) signal amplitude under different temperatures^[84]; (d) rero-field resonance linewidth as a function of the incident power at different temperatures^[84]; (e) normalized frequency response curves under different incident optical powers at 160 °C^[85]

Fig. 10. Three-axis magnetic field compensation technique. (a)(b) Schematic diagram of the proposed experimental scheme^[89]; (c) the practical dual-cell experimental scheme of the three-axis OPM^[89]; (d) experimental implementation of the ⁸⁷Rb magnetometer based on the pump-probe scheme^[90]; (e) experimental implementation of the ⁸⁷Rb magnetometer based on the pump-probe scheme^[91]

Fig. 11. MicroSERF zero field vector magnetometer^[98]

Fig. 12. Schematic diagram of the NMOR magnetometer apparatus

Fig. 13. Biological weak magnetic signal detection. (a) Spontaneous alpha rhythm signal^[109]; (b) the overall arrangement of the practical unshielded MEG system^[109]; (c) auditory evoked fields detected unshielded in Earth's field with a portable first-order gradiometer^[110]; (d) a picture of the in-the-field MEG recording apparatus with a subject^[110]; (e) schematic representation of the NPOM modular system^[111]