OPM can detect magnetic field signals of sub ft order (1ft=10-15T, geomagnetic field size approximately 5*10-4T) [6]. With the improvement of system integration, reduction of power consumption, and enhancement of stability[7], OPM have become one of the most sensitive magnetic field detection methods, widely used in many fields such as basic physics[8], quantum information[9], medical testing[10], and material testing[11]. The OPM measures the size of the magnetic field by measuring the frequency interval between magnon energy levels [1]: The atomic energy level formed by the splitting of a specific atomic energy level under the action of a magnetic field is called a magnon energy level, and the frequency interval between magnon energy levels is proportional to the amplitude of the external magnetic field. Early OPM used atomic gas discharge lamps as system light sources [2] [3]. With the development of laser technology, gas discharge lamps were gradually replaced by lasers. While improving the detection sensitivity of OPM [4], it also promoted the miniaturization of OPM [5]. The current miniaturized OPM are mainly based on the spin-exchange collision relaxation-free (SERF) mechanism, which means that when the relaxation rate of alkali metal atoms is greater than the Zeeman splitting of atomic energy levels, the spin-exchange collision relaxation broadening will disappear, and the magnetic resonance spectrum line will be narrow to be indistinguishable. The narrower the magnetic resonance spectrum line, the higher the measurement sensitivity of the OPM. Princeton University's Happer et al. demonstrated through theoretical calculations that when the atomic density is large enough and the external magnetic field is small enough, the spin-exchange collision relaxation rate between atoms can exceed the Larmor frequency of atomic energy level splitting. By increasing the atomic density and reducing the external magnetic field, narrower magnetic resonance spectral lines can be obtained experimentally [12]. This atomic spectral line narrowing method is particularly suitable for miniaturized OPM under zero field conditions.
The application of SERF mechanism in OPM began in 2002. A collaborative group composed of the Allred team from the University of Washington and the Romalis team from Princeton University [13] detected magnetic deflection caused by the interaction of linearly polarized probe light with potassium atoms under a weak magnetic field in a hot potassium atomic gas chamber filled with a mixture of helium and nitrogen gas (as a buffer gas) with a diameter of 2.5cm. In order to narrow the signal line width as much as possible, the Allred team further adopted the method of filling a buffer gas with a certain pressure in the atomic gas chamber. By frequently colliding with potassium atoms to prevent their diffusion towards the glass bubble wall, the interaction time between potassium atoms and laser was extended. Moreover, the non-radiative collision between the buffer gas and potassium atoms can also quench the spontaneous emission fluorescence of potassium atoms and eliminate optical noise; Secondly, due to the high spin-exchange collision rate between potassium and potassium atoms, which exceeds the Larmor precession frequency of atoms in a weak magnetic field, the first-order (i.e. first-order) contribution of spin-exchange collision to atomic lateral relaxation disappears, leaving only a small second-order (i.e. square) contribution. Through these measures, the Allred team achieved a resonance linewidth of 1.2Hz, combined with the use of 5-layer nested magnetic shielding to reduce magnetic noise, resulting in a sensitivity of 10fT/Hz1/2 for the magnetic field measurement device. On the basis of the miniaturized magnetometer mentioned above, the Romalis team has developed a multi-channel miniaturized magnetic gradient meter, with a magnetic detection sensitivity of 0.54 fT/Hz1/2 (frequency range from 28-45Hz) and a spatial resolution of 2mm [14]. Miniaturized magnetometers can also be used for NMR detection of liquid samples. In 2005, the Romanis team utilized their developed potassium atom miniaturized magnetometer to measure the free induction decay (FID) signal and NMR signal of 129Xe atoms in magnetized pure water after a 0.14T magnetic field pre polarization [15], and proposed the use of a miniaturized magnetometer for three-dimensional imaging of samples [16]. The miniaturized magnetometer developed by the Romanis team mainly determines the magnetic field strength by measuring the deflection angle of the polarization direction of the probe light.
The size of the atomic bubble is only 3*2*1 mm3, with a resonance linewidth of 83 nT (for Rb87 atoms, i.e. 580 Hz), and the main limitation of its linewidth comes from spin breakdown relaxation [18]. In addition, using a laser beam as both pump and probe light simplifies the structure while also reducing system power consumption. Quspin Company in the United States has commercialized SERF based OPM products [19], which have been applied to zero field nuclear magnetic resonance measurements. Kernel Corporation in the United States [20] developed a high-density OPM array (Kernel Flux) and attempted to apply it in brain computer interfaces. In recent years, the gap between China and foreign countries has also been narrowing. Some institutions, such as Peking University [21], Beijing University of Aeronautics and Astronautics [22], Chinese Academy of Sciences [23-25], Southeast University [26] and Zhejiang University of Technology [27], have invested in the research and development of small-scale OPM. In general, domestic OPM have ranked first in the world in terms of research field and performance level. At present, commercial miniaturized OPM mainly determine the magnetic field intensity by directly detecting the transmission intensity of the pump light. This method is called zero field level crossing resonance, also known as the Hanle effect. As early as 1969, Dupont Roc et al. utilized the Hanle effect of the Rb87 atomic ground state to achieve a highly sensitive OPM [17]. Based on the same principle, in 2007, the Kitching team of the National Bureau of Standards in the United States utilized the atomic gas chamber manufactured by the Micromotor System (MEMS) process to achieve a miniaturized magnetometer based on transmitted light intensity detection.
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