Ultra-cold atomic physics laboratory cabinet.

A few days ago, one of the main scientific loads of the Mengtian experimental module of the Chinese space station, the ultra-cold atomic physics experiment cabinet (hereinafter referred to as the ultra-refrigerated cabinet, CAPR), successfully completed its first self-inspection and will carry out related platform tasks.

This is China's first space microgravity ultra-cold atomic physics experiment platform, and it is also the world's second space station ultra-cold atomic cabinet after the United States. What are the ingenious ideas and exquisite designs of this ultra-freezer? What temperature limit can be reached? What scientific experiments can be done in space?

To this end, "China Science Daily" invited Li Tang, a researcher at the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, the deputy chief designer of the scientific experiment system of the ultra-cold atomic physics experiment cabinet (hereinafter referred to as the scientific experiment system), and an associate researcher at the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences. 4 scientists, including Wang Bin, deputy director designer of the scientific experiment system, Chen Xuzong, professor of the School of Electronics of Peking University and chief scientist of the scientific experiment system, and Xiong Wei, assistant researcher of the School of Electronics of Peking University and deputy director designer of the scientific experiment system, deciphered "the coldest universe in the universe". place".

"The ultra-freezer is actually a laboratory for ultra-cold atomic physics experiments on the space station. It uses laser cooling technology to cool atoms to the pK (10-12, only one ten-billionth of a degree from absolute zero) level, and On this basis, carry out major physical research such as superconducting mechanism, topological phase transition, and basic standard model testing." Li Tang introduced.

"The requirements for establishing an ultra-cold atomic physics laboratory in the space station are very demanding." Wang Bin added, "In addition to high relevant indicators, resource constraints such as volume, weight, and power consumption must also be solved, and the requirements can be met. Special requirements such as safety, reliability, and on-orbit maintenance for manned spaceflight.”

Building an ultra-cold atomic physics laboratory on the earth usually requires more than 30 cubic meters of space, and the various equipment weighs several tons, and its power consumption is very high (several kilowatts), plus the need to carry out scientific experiments Materials, the laboratory occupies a huge amount of resources. In the limited space of the space station, it is absolutely impossible to "move" the ground laboratory directly.

"This involves a series of technological updates. For example, the original laser system in the traditional laboratory can no longer be used. We have tackled key problems in the laser system, and have overcome a series of technologies such as all-fiber laser technology, large-frequency laser phase-locking technology, and high-precision laser power stabilization technology. A series of difficulties, using an all-fiber laser link to achieve Bose-Einstein condensation on an ultracold atomic system, and reaching the predetermined target." Wang Bin said.

In response to the special requirements of manned spaceflight, the research team has also overcome the ultra-high vacuum long-term power-off maintenance technology, and developed CPLD/FPGA circuit control technology and high-precision vibration isolation technology for the characteristics of ultra-cold atomic physics experiments. In the end, a traditional laboratory is "compressed" into an ultra-freezer of less than 1 cubic meter (volume less than 1 cubic meter, weighing about 300 kilograms, and power consumption about 700 watts).

According to Chen Xuzong, the characteristic of the ultra-freezer is to obtain the "lowest temperature in the universe", obtain the fifth state of matter predicted by Einstein in 1925, and use this coldest substance in the universe to carry out scientific experiments.

"In terms of cooling schemes and scientific experiments, my country's experimental cabinets are different from the CAL (Cold Atom Laboratory) of the US space station." Chen Xuzong explained that the US CAL uses the traditional method of magnetic field plus radio frequency field for evaporative cooling. The "optical trap scheme" has two-stage cooling, which is lower than the American scheme in terms of initial temperature, and can more conveniently carry out various experiments such as quantum simulation.

"The US CAL was launched in May 2018, and the current reported result is 52pK. The index proposed by the Chinese program is relatively conservative (better than 100pK)." Xiong Wei said, "Actually, scientists have obtained the temperature on the ground at hundreds of pK. The cold atomic gas of pK level is expected to be one or two orders of magnitude lower (about 10pK) in the microgravity environment of the space station.”

After the ultra-freezer is working, the atomic temperature must be lowered to the order of nK to hundreds of pK, which is very close to absolute zero (0K=-273.15 degrees Celsius).

"This is the human pursuit of the temperature limit." Li Tang said that a series of scientific experiments can be carried out at pK temperature. From January 2023 to December 2032, the experimental plan is four categories: quantum simulation, quantum new state of matter, basic physical theory test, dark matter, and Higgs model test. Through these experiments, we can discover new physical phenomena, test the basic laws of physics, and promote the progress of basic physics research.

The goal of the ultra-cooler is to build an ultra-low temperature, large-scale, high-quality, and suitable for long-term precision measurement Bose and Fermi quantum degenerate gas open experimental platform in the Chinese space station, providing a long-term on-orbit stable operation for ultra-cold atomic physics research experimental system. Since it is an open ultra-cold atomic physics experiment platform, it needs to meet the needs of different ultra-cold atomic science experiments.

At the beginning of the system design, the research team conducted extensive project collection at home and abroad, classified the collected projects, and sorted out, merged and summarized the current hot and difficult experimental projects in ultracold atomic physics. Under the constraints of limited external resources during the construction period of the space station, they provided common conditions for ultracold atomic physics experiments such as optical traps, three-dimensional optical lattices, and hexagonal optical lattices to meet the needs of early on-orbit ultracold atomic physics experiments. At the same time, a modular design was carried out for the ultra-cold atomic physics experiment system, and a powerful and flexible software system was developed. For different ultra-cold atomic science experiments, the ultra-freezer scientific experiment system calls different modules and corresponding experimental timings according to ground plans and instructions, so as to meet the normal operation of different ultra-cold atomic physics experiments on the space station.

"During the operation period of the space station, the ultra-freezer project team will continue to pay attention to the development of this discipline and the progress of related technologies, combined with the existing resources of the ultra-freezer, extensively collect scientific experimental projects related to ultra-cold atomic physics at home and abroad, and conduct research on the relevant modules of the ultra-freezer. Continuous and small upgrades, and if necessary, re-upgrade and restructure the ultra-cold atomic physics experiment system control and software systems to meet the new needs of follow-up scientific experiment projects." Wang Bin said.

(Originally titled "Deciphering "The Coldest Place in the Universe")