# Magneto-Optical Traps for Cold Atomic Gravimetry: Research Status and Development Trends
## 3 Classification and Application of MOTs
### Extremely Miniaturized MOT Development  
In the pursuit of extreme miniaturization of cold atom sensors, the integration of optical elements with atomic chip techniques [83; 84] for the detection and quantum coherent operation of cold atoms is being extensively investigated. Similar to the principle of a macroscopic pyramidal MOT, a microscopic pyramidal magneto-optical trap (PMOT) can also be used to cool and trap small arrays of atomic clouds on the chip. Planar diffraction gratings fabricated and integrated by photolithography enabled grating the magneto-optical trap (GMOT) [85], further reducing the size of the cold atom sensor platform. Table 1 shows the research on MOTs on chips that has been created in recent years. In 2021, [PERSON] et al. [91; 92] showed that grating chips are compatible with holes. Future chip-level cold atomic beams can be achieved by introducing holes in the center of the chip. In 2022, [PERSON] et al. [93] carried a MOT with dimensions of 370 mm \(\times\) 350 mm \(\times\) 100 mm, a weight of 6.56 kg, and a power of 80 watts on board an unmanned aircraft. Their work paved the way for an unmanned portable cold atom gravimeter. In the same year, the planar inverse Helmholtz coil chip proposed by [PERSON] et al. [96] greatly reduced the power consumption and volume of the GMOT. They pointed towards a new direction of development for integrated cold atom gravimetry and portable precision measurement systems [94]. Combined with techniques such as passive pumping [97], it is possible to develop a cubic centimeter volume cold atom physical package in the future. Integrated chips designed with the GMOT have been successfully used for fundamental physics research [98; 99]. Since the GMOT enables a large optical volume overlap [100], it has been applied in miniaturized cold atom systems for cold atom clocks [101] and slow cold atom beam preparation [102]. It is also widely used in cooling and trapping various \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Year** & **Institution** & **Type** & **Results** & **Reference** \\ \hline
2009 & \
\begin{tabular}{c} Imperial College \\ London Blackett Laboratory \\ \end{tabular} & PMOT &
\begin{tabular}{c} 2000 \({}^{85}Rb\) \\ atoms at a temperature of 100 \(\upmu\)k \\ \end{tabular} & [PERSON] et al. [86; 87] \\ \hline
2009 &
\begin{tabular}{c} University of \\ Strathclyde \\ \end{tabular} & PMOT & \(1.3\times 10^{6}\) atoms & [PERSON] et al. [88] \\ \hline
2013 & \
\begin{tabular}{c} University of Maryland \\ \end{tabular} & PMOT &
\begin{tabular}{c} \(5\times 10^{5}\) \\ atoms at a temperature of 10 \(\upmu\)k \\ \end{tabular} & [PERSON] et al. [89] \\ \hline
2017 & \
\begin{tabular}{c} University of \\ Strathclyde \\ \end{tabular} & GMOT &
\begin{tabular}{c} \(3\times 10^{6}\)atoms at a temperature of 3 \(\upmu\)k \\ in 10 ms \\ \end{tabular} & [PERSON] et al. [90] \\ \hline
2019 & \
\begin{tabular}{c} National Institute of \\ Standards and \\ Technology in \\ Maryland \\ \end{tabular} & GMOT &
\begin{tabular}{c} \(10^{67}Li\) atoms \\ \end{tabular} & [PERSON] et al. [91] \\ \hline
2021 & \
\begin{tabular}{c} University of Maryland \\ \end{tabular} & GMOT &
\begin{tabular}{c} \(10^{7}\)atoms at a temperature of 146 \(\upmu\)k \\ in 0.25s \\ \end{tabular} & [PERSON] et al. [92] \\ \hline
2022 &
\begin{tabular}{c} University of \\ Birmingham \\ \end{tabular} & PMOT & \(2.1\times 10^{7}\) cold atoms & [PERSON] et al. [93] \\ \hline
2022 & \
\begin{tabular}{c} Sandia National \\ Laboratories \\ \end{tabular} & GMOT &
\begin{tabular}{c} 0.17 cm\({}^{3}\) atomic \\ cluster at a temperature of 15 \(\upmu\)k \\ \end{tabular} & [PERSON] et al. [94] \\ \hline
2022 &
\begin{tabular}{c} China Academy of \\ Metrology \\ \end{tabular} & GMOT & \(2.8\times 10^{7}\)\({}^{87}Rb\) atoms & [PERSON] et al. [95] \\ \hline
2022 &
\begin{tabular}{c} China Academy of \\ Metrology \\ \end{tabular} & GMOT & \(10^{6}\)\({}^{87}Rb\) atoms & [PERSON] et al. [96] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Status of research on MOTs on chips.