Atom Cloud on China's Space Station Hits Record Cold of 9.7 Picokelvins

Physicists working aboard China's Tiangong space station say they have produced the lowest equivalent temperature ever measured: 9.7 picokelvins, or 0.0000000000097 kelvins. ScienceAlert reported the result, which appears in a paper published in the journal Science.

The figure is about 280 billion times lower than the roughly 2.7 kelvins of background temperature left over from the Universe's early history. The advance matters mostly for what it could allow: longer observation of ultracold atoms, which could make some precision measurements more sensitive.

The key numbers behind the record

• 9.7 picokelvins: the new record, which the researchers describe as, to their knowledge, "the lowest equivalent temperature ever measured."
• 280 billion times: how much lower this is than the background temperature of space, which hovers around 2.7 kelvins, according to ScienceAlert.
• 38 picokelvins: the previous record, set during a free-fall experiment in Germany's Bremen Drop Tower and described in a 2021 paper.
• 100 nanokelvins: the temperature NASA reached in 2018 when it created the first Bose-Einstein condensate in orbit, aboard the International Space Station.
• 20,000 atoms at 3.1 nanokelvins: the size and temperature of the rubidium condensate in the Chinese experiment before the final cooling stage.
• Seven seconds: how long after release the team could still detect the expanding atom cloud.

How the team got there

The experiment was led by physicists Xiaoji Zhou and Xuzong Chen of Peking University and Weibiao Chen of the Chinese Academy of Sciences. They worked with rubidium-87 atoms, an isotope well suited to laser cooling and to making Bose-Einstein condensates. In this exotic state, atoms act collectively, almost like one large quantum object.

On Earth, gravity limits how cold such clouds can get. The team therefore ran the work in orbit, in an apparatus called the Cold Atom Physics Rack. They first cooled the atoms with lasers in an optical trap. They then used evaporative cooling, in which the most energetic atoms escape and leave the rest with less energy.

That step posed a problem in orbit, where everything is in continuous free fall and gravity does not help pull escaping atoms away. The researchers applied a magnetic field gradient that mimicked gravity's effect on the atoms, according to ScienceAlert.

Next they moved the condensate into a second, wider laser trap and gradually weakened it, letting the cloud spread out and lowering the energy it would release as it expanded. Finally they switched the trap off and watched the cloud disperse. The cloud expanded so slowly that the atoms were still detectable seven seconds later.

What "equivalent temperature" does and does not mean

The 9.7 picokelvin figure is not a conventional temperature reading. ScienceAlert explained that equivalent temperature describes the energy tied to the cloud's expansion. The team measured how quickly the cloud spread, used that to determine how much energy the atoms had, and expressed the result in kelvins.

Absolute zero, or zero kelvins, is the theoretical lower limit. ScienceAlert noted that it cannot be reached, because removing the last bits of thermal energy would require a cooling process that could never be completed.

Why it matters beyond the record

For most readers, the result will not change daily life soon. Its significance is in research tools. According to ScienceAlert, the more slowly atoms disperse, the longer scientists can observe and manipulate them, and the more sensitive their measurements can become.

One promising use is atom interferometry, which relies on the wave-like properties of atoms to detect extremely small changes in their surroundings. With more time to watch the atoms, researchers could build more sensitive instruments for testing fundamental physics. Bose-Einstein condensates also let scientists study quantum behavior on a larger scale than individual atoms allow.

The result comes from a single experiment, though it has passed peer review at Science. The record is also an equivalent temperature, not a direct thermometer reading, which is a distinction that matters when comparing it with earlier figures.

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Around Prague, based on reporting by ScienceAlert