Design and Verification of the Cold Atom Physics Rack Installed in the Chinese Space Station

Objective In microgravity, atoms can be cooled to very low temperatures, manipulated by a trap with a novel topology structure, and observed over long timescales. This phenomenon has garnered considerable attention, leading to exploration of ultracold atomic physics and its applications in microgravity. Over the past two decades, various state-of-the-art ground-based microgravity facilities and highly reliable ultracold atomic physics experimental systems have been developed to explore the lower temperature limit and applications of cold atoms in microgravity. However, space-based platforms, such as sounding rockets and space stations, have evolved into ideal environments because of their long free-fall time and stable microgravity environment. With the development of the Chinese Space Station (CSS), a Cold Atom Physics Rack (CAPR) that uses an all-optical approach has been deployed to investigate low-temperature and novel physical phenomena in microgravity based on the ultracold quantum degenerate gas of Rb-87 Bose-Einstein condensate (BEC). In addition, the CAPR serves as an open experimental platform for studying ultracold atomic physics and performing precision measurements in microgravity, with the major aim of cooling atoms at the pico-Kelvin scale through two-stage crossed beam cooling (TSCBC). Methods The CAPR needs to satisfy the restrictions on its size, weight, and power consumption. In addition, it needs to withstand the vibrations and impact during its launch as well as operate well after the launch. A highly reliable and integrated CAPR that integrated all the hardware for preparing, manipulating, and probing the Rb-87 BEC was designed. The designed CAPR included a physical system, a cooling laser system, an optical trap and lattice laser system, an electronic control unit, and a rack supporting system with dimensions of 1820 mmx1050 mmx815 mm. The dimensions and mass of the assembled physical system were approximately 590 mmx930 mmx510 mm and 170 kg, respectively. This system could provide a high-vacuum, optical, and magnetic environment for ultracold atoms. The cooling laser system consisted of a repumping laser, cooling laser, and probing laser, which provided three high-power outputs for cyclic cooling of Rb-87 atoms to temperatures of tens of microkelvins as well as for detecting the atoms. The optical trap and lattice laser system provided eight high-power outputs for evaporative cooling to attain the BEC, deep cooling via TSCBC, and manipulation of the ultracold atoms in the optical lattice. The electronic control unit controlled the experimental sequences as well as stored the experimental results and engineering parameters. The sizes and weights of the laser cooling system, optical trap and lattice laser system, and electronic control unit were similar (550 mmx470 mmx270 mm and less than 50 kg, respectively). To achieve the mission target, BEC and TSCBC tests were conducted on the ground before the launch.The realization of the Rb-87 BEC and the TSCBC were crucial and confirmed that the output of all the subsystems fulfilled the experimental requirements for the preparation, regulation, and detection of ultracold atoms. Results and Discussions The vacuum apparatus is the main part of the physical system and includes a two-dimensional magneto-optical trap (2D-MOT) chamber and science chamber for atomic cooling, manipulation, and probing. In addition, all the magnetic coils and optical modules, which provide the required magnetic and optical fields for the ultracold atoms, are fixed on the vacuum chambers. In the laser cooling system, the powers of the repumping, cooling, and probing lasers are 200,600,800 mW,respectively. The repumping laser is locked to the Rb-87 D2 |5(2)S(1/2), F=1 >->|5(2)S(3/2), F'=0,1 > crossover transition via modulation transfer spectroscopy (MTS), which is 193 MHz red-detuned from the repumping transition. The frequencies of the cooling and probing lasers are red-detuned by a few natural linewidths (Gamma=2 pi x6.065(9) MHz, which is the natural linewidth of the Rb-87 D2 line)from the Rb-87 D2 |5(2)S(1/2), F=2 >->|5(2)S(3/2), F'=3 > transition. The MOT loading process takes 10 s and more than 1.5x10(9 )atoms can be trapped with a temperature below 500 mu K. Furthermore, the atoms can be cooled to a temperature below 30 mu K using optical molasses, demonstrating the performance of the 780 nm cooling laser system. As to the optical trap and lattice laser system, the capability of the tight-confining laser is confirmed by loading more than 1.2x10(6) atoms and successfully cooling more than 1x10(5 )atoms via evaporative cooling to the BEC at a temperature below 30 nK. The performance of the loose-confining laser is verified by deeply cooling the ultracold atoms to 2.4 nK via TSCBC. Additionally, the CAPR performs well in space environmental qualification certification tests. Conclusions The CAPR flight model (FM) was installed in the Mengtian laboratory module, which was launched into the CSS on October 31,2022. The CAPR investigates low-temperature and novel physical phenomena in microgravity based on the quantum degenerate gas of Rb-87 BEC. Here, we report the design of the integrated CAPR, which includes a physical system, a cooling laser system, an optical trap and lattice laser system, an electronic control unit, and a rack supporting system. Ground based experiments have been conducted to confirm the ability of the CAPR to realize the Rb-87 BEC and lower its temperature from 30 nK to 2.4 nK with the TSCBC.