# Development of Compact and Robust Physical System for Strontium Optical Lattice Clock
## 2 The Compact and Robust Physical System  
The upper floor of the physical system comprised a laser wavelength control system that included a structurally reinforced commercial wavemeter (High-Finesse, WS-7), a laser mode switcher, and a computer. The laser wavelength control system and the power supply for the vacuum pump were mounted on a liquid-cold plate. This plate could conduct heat away from the devices and act as a structural component, as shown in Figure 1. To display more details inside, the shells of the system are not shown. The laser wavelength control system could record the wavelengths of all the lasers in the optical clock system in real-time, and enable automatic laser frequency adjustment. The physical package was located on the lower floor of the physical system. This physical package was highly compact and structurally designed and had dimensions of 280 mm \(\times\) 588 mm \(\times\) 415 mm, as shown in Figure 2. Two ion-gettering pumps were used to maintain the vacuum of the chamber, each consisting of a 100 L/s getter pump and a 5 L/s ion pump. Compared with high-speed ion pumps, this ion-gettering pump had a smaller volume and weight but allowed relatively high pumping speeds due to the presence of the getter. In order to reduce the influence of the background pressure on the atomic cooling and detection, we set up a differential pumping stage between the atomic oven and the MOT region. We employed a titanium alloy tube with a length of 160 mm and an inner diameter of 6 mm, which covers the section from the beginning of the Zeeman slower to the front of the MOT vacuum chamber center region. This effectively preserved the high vacuum condition of the MOT vacuum chamber and prevented the degradation of the vacuum environment by the atomic oven operating at a high temperature. When the optical clock operated normally (with an oven temperature of up to 400 \({}^{\circ}\)C), the pressure in the MOT vacuum chamber was under 1.3 \(\times\) 10\({}^{-9}\) Pa. Furthermore, the system's status under power-off conditions was also tested to simulate optical clocks operating outside the laboratory. Because the getter pumps could sustain the chamber vacuum during the power-off period, after a week of power failure, the chamber pressure could be returned to the normal level within approximately 6 h once power was restored. Figure 1: Physical system, design drawing (**a**), and real picture (**b**). Figure 2: (**a**) Physics package design. (**b**) Top view of the MOT vacuum chamber. (**c**) Cross-sectional view of the MOT vacuum chamber, facing the atomic oven. A high-efficiency oven based on heating inside the vacuum chamber was developed to reduce thermal dispersion from the heating wire to the environment. Furthermore, a multilayer heat shield made of polished aluminum was placed around the heating filament perimeter to curb the heat loss caused by radiation. Through optimization, the power consumption of this oven was below 14 W during the experiment. To obtain a collimated atomic beam, approximately 50 stainless steel capillary collimators were positioned at the outlet of the oven. Each capillary had a length of 8 mm and a diameter of 0.2 mm, resulting in an emitted atomic beam with a divergence angle of approximately 25 mrad. However, this physical collimation could not ensure sufficient atoms entering the MOT vacuum chamber. Therefore, a two-dimensional transverse cooling vacuum chamber with four perpendicular CF 16 view plates was installed to further reduce the atomic beam divergence angle. There was another chamber beside the former vacuum chamber for detecting the fluorescence signals, allowing the primary cooling laser frequency (461 nm) to be locked onto the atomic transition \({}^{1}\)S\({}_{0}\)\(\rightarrow\)\({}^{1}\)P\({}_{1}\). This setup eliminated the need for an additional vacuum setup to lock the laser frequency. Reducing the length of the Zeeman slower is an effective method to diminish the volume of the physical package.