# Magneto-Optical Traps for Cold Atomic Gravimetry: Research Status and Development Trends
## 3 Classification and Application of MOTs
### Rise of Single Beamed MOTs  
To meet the measurement requirements in dynamic field environments, such as shipboard [55; 56], airborne [57], and vehicle-mounted [58; 59], the miniaturized design of CAG is crucial. The key technologies for miniaturization are the integrated design of a laser system [60; 61; 62; 63; 64] and the compact design of the vacuum cavity. The size of a sensor head can be reduced by simplifying the conventional six-beam MOT to a single-beam MOT. The principle is that in the trap region, similar to a six-beam MOT, only a circularly polarized  
Figure 6: Structure of four-beam MOT. Figure 7: Five-beam MOT schematic. beam-expanding laser and mirrors [65] are used to form an optical configuration. The mirrors can be pyramidal or axially conical. In 1996, [PERSON] et al. first proposed this novel and simple design of MOT by using a tetrahedral mirror reflection with only one laser beam, as shown in Figure 8. The number of cold atoms trapped in this MOT at a loading time of 400 ms is about \(1.2\times 10^{7}\)[38]. [PERSON]'s study shows that the polarization contamination of the incident circularly polarized laser is less than 1%. The power loss of the laser due to the reflection of the metal mirror is less than 12%, and the reflectivity of the aluminum mirror is greater than 88%. The experimental results indicated that the confinement characteristics of this single-beam MOT are largely unaffected. In 1996, [PERSON] at the University of Colorado at Boulder proposed a slow-velocity intense source with a loading rate of 70% [66]. In 1998, [PERSON]'s pyramidal MOT design was adapted into an atomic funnel by [PERSON] et al. at the University of Wisconsin [67]. A small hole is opened at the top of the pyramidal mirror. A set of 1/4 waveplates and plane mirrors are placed in opposite directions to form two mid-axis directionally opposing laser beams. The feature of this optical configuration is that cold atoms can be pulled out through the small hole, and then a high flux beam of cold atoms can be formed [68,69]. Loading into a second MOT can improve its efficiency. The simple structure and compact design of this single-beam pyramidal funnel gives it an advantage for loading MOTs in ultra-high vacuum environments. In 2010, [PERSON] et al. [70] realized a compact cold atom gravimeter scheme that achieves the cooling and trapping of atoms using a hollow pyramidal structure. They demonstrated a relative sensitivity to acceleration of gravity of 17 \(\upmu\)Gal at one second. The group trapped about \(4\times 10^{6}\)\({}^{87}Rb\) atoms in a loading time of 360 ms. After an optical molasses phase of 20 ms, the laser was adiabatically turned off and the atoms were allowed to fall. The temperature of the cold atoms was measured by Raman velocimetry [71] to be 2.5 \(\upmu\)k, which is comparable to the temperature obtained by a six-beam MOT. This was followed in 2013 by the development of the \"MiniAtom\" transportable atomic gravimeter with a pyramidal MOT, which loads about \(10^{7}\) atoms in 400 ms. The long-term stability was in the 10-9 range and up to two days of measurements with this type of architecture [72]. The reduction in the beam allows a significant reduction in the size of the sensor head, which can fit into a cylinder 40 cm high and 20 cm in diameter. It saves a lot of space compared to the transportable absolute gravimeter developed by SYRTE with a cylindrical sensor head that is 80 cm high and 50 cm wide [73]. In 2018, the French company Muquans launched a commercial removable absolute gravimeter product. It was based on a hollow pyramidal MOT. This trap was loaded with \(10^{7}\)\({}^{87}Rb\) atoms in 250 ms at an atomic temperature of 2 \(\upmu\)k. The measurement of the absolute gravitational acceleration has been performed continuously with a long-term stability below 1 \(\upmu\)Gal [74]. The gravimeter can be used for both continuous observatory measurements and gravity mapping, and can be set up in less  
Figure 8: Pyramidal MOT structure. than 20 min.