Chinese scientists cool atoms to record-low temperature on Tiangong space station

Scientists aboard China’s Tiangong space station have cooled a gas of rubidium atoms to 9.7 picokelvin, the lowest temperature ever artificially achieved, according to results published in the journal Science on November 5. One picokelvin is one trillionth of a kelvin. The measured temperature is an “equivalent temperature” obtained by measuring the expansion speed of the atomic cloud, translating it into kinetic energy, and expressing it as a temperature. The uncertainty is +3.2/−2.7 picokelvin.

The experiment

A joint research team including Peking University in Beijing and the Shanghai Institute of Optics and Fine Mechanics at the Chinese Academy of Sciences carried out the work using the Cold Atom Physics Research facility (CAPR), installed in the Mengtian experiment module of Tiangong. Tiangong’s core module, Tianhe, was launched in April 2021, and the Mengtian experiment module in October 2022.

The team first cooled about 23,000 rubidium-87 atoms in an optical trap where two laser beams intersect, using evaporative cooling to bring them down to 3.1 nanokelvin. About 89% of all atoms condensed into the same quantum state. The atoms were then transferred to a second optical trap that confines them in a larger region, and a two-step cooling process was carried out by gradually lowering the trap strength over about 1.175 seconds to allow the atomic cloud to slowly expand. Analysis of the freely expanding atoms showed the equivalent temperature of 9.7 picokelvin. The ultracold state and long free-expansion time are expected to improve the precision of atom interferometers, which measure tiny gravitational fields and accelerations using matter-wave interference of atoms.

Comparison with previous records

This record is about four times lower than the previous minimum equivalent temperature of 38 picokelvin achieved in the microgravity environment of a drop tower on Earth, and about five times lower than the 52 picokelvin recorded by the Cold Atom Laboratory (CAL) on the International Space Station (ISS). In drop-tower experiments, the free expansion of atomic clouds was observed for about 2 seconds, whereas in the new study the freely expanding atomic cloud was observed for 7 seconds.

Scientific context

When atoms are cooled to ultracold temperatures, multiple atoms share a single quantum state and behave collectively, a phenomenon known as Bose–Einstein condensation (BEC). This enables quantum simulation, precision measurement, and experiments that test the relationship between gravity and quantum mechanics. On Earth, gravity drags ultracold atoms down, forcing researchers to grip them tightly and cutting short how long they can be watched. In orbit, the atoms can float freely, making space the ideal lab for such experiments.

The research team believes the results can be used in future to test fundamental physical laws, such as the equivalence principle, with greater precision in space. The equivalence principle is a key aspect of Einstein’s general theory of relativity, stating that all objects fall the same way under gravity regardless of mass or composition. Using an atom interferometer based on ultracold atoms, it is possible to precisely measure minute differences in the falling speeds of different kinds of atoms. In addition, the researchers expect their achievement to be used to explore new physical phenomena or to study exotic quantum states that emerge at ultralow temperatures.

Why it matters

• The 9.7 picokelvin measurement is the lowest temperature ever artificially achieved, according to DongA Science.
• The experiment used lasers alone to trap the atoms, unlike previous space experiments that relied on magnets.
• The freely expanding atomic cloud was observed for 7 seconds, longer than the 2 seconds in drop-tower experiments and about one second aboard the ISS.
• Researchers say the results could improve atom interferometers for measuring gravitational fields and testing the equivalence principle in space.

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Sources

• sciencenews.org: Record-low temperature created on China’s space station
• scmp.com: Chinese scientists chill atoms to record low on Tiangong space station | South China Morning Post
• dongascience.com: China’s Tiangong Space Station Achieves Record 9.7 Picokelvin Atomic Gas - DongA Science

This article was drafted with AI assistance and checked against the sources above. Company claims are reported as claims. Cover image is AI-generated.