Astronauts on the Chinese Space Station have recorded the coldest temperature ever achieved. The experiment cooled a collection of rubidium atoms to 9.7 picokelvins, equivalent to 9.7 trillionths of a degree above absolute zero.

The achievement is the first cold temperature record to be set in space. Although such records are broken relatively frequently, the new result involved just over a quarter of the heat associated with the previous record.

The pursuit of ever colder temperatures is about more than securing a place in the record books. Supercooled gases can form Bose-Einstein Condensates, also known as BECs, in which thousands or millions of atoms display quantum behavior normally associated with subatomic particles.

By scaling quantum activity to much larger collections of atoms, BECs and other unusual states of matter give researchers new ways to study the quantum world. They allow scientists to examine behavior that would normally be inaccessible.

Extreme cooling has also been proposed as a potential tool in the search for dark energy. It could additionally contribute to more advanced gravitational wave experiments, according to the source paper.

The difficulty is that Earth’s gravity restricts how far conventional experiments can go. The paper’s authors explain that gravitational sag makes it difficult to retain atoms during evaporative cooling, requiring a finite confining trap frequency that ultimately limits the achievable temperature.

Temperatures of 3.5x10 -10 K have already been achieved on Earth. For researchers attempting to move even closer to absolute zero, however, that result is still not cold enough.

One existing method uses magnets to counteract gravity and suspend gas molecules. The paper’s authors note that this approach introduces residual curvature, which places another limit on how much further the gas can be cooled.

Researchers can also allow atoms to fall through a tall tower or float inside a zero gravity aircraft. Both approaches temporarily simulate microgravity, creating a short opportunity to reduce the effects of Earth’s gravitational pull.

The record achieved with those methods was 3.8x10 -11 K. However, the temperature could be sustained for only 2 seconds because the falling material eventually encountered the ground or the aircraft had to pull up.

Low Earth orbit offers a way to extend that limited window. A team from several Chinese institutions sent its research equipment to the Tiangong Space Station, where sustained microgravity made a longer cooling experiment possible.

The experiment involved 20,000 rubidium atoms. Researchers ultimately cooled those atoms to 9.7 picokelvins, also expressed as 9.7x10 -12 degrees above absolute zero.

The team used the optical dipole trap method, with lasers counteracting the atoms’ movement. As the atoms shed almost all their energy, their temperature initially dropped to just 6 nanokelvins.

One nanokelvin is one billionth of a Kelvin. Even that extraordinarily low temperature was only an intermediate stage in the Tiangong experiment.

The researchers then allowed the atoms to expand for 7 seconds. That expansion cooled the system further, producing the record temperature of 9.7 picokelvins.

A conventional thermometer cannot be inserted into material under these conditions. Instead, the researchers calculated the temperature by observing the expansion energy of the atoms.

Temperatures in the picodegree range have previously been achieved in space. In 2022, the Cold Atom Laboratory on the International Space Station reached 52 picokelvins for 1 second.

The Tiangong experiment pushed substantially below that result while sustaining its expansion stage for 7 seconds. It demonstrates how low Earth orbit can give researchers more time to investigate matter at temperatures extraordinarily close to absolute zero.