Researchers working aboard China's Tiangong space station have cooled a cloud of atoms to about 10 picokelvin, ten trillionths of a degree above absolute zero, the lowest effective temperature yet reported for such a system. Absolute zero is minus 273.15 degrees Celsius.
The cloud was a Bose-Einstein condensate, a quantum state of matter, made from 20,000 rubidium atoms. The work was published in the journal Science on 5 November, with W. Xiong as lead author, and was reported by Science News.
The team held and cooled the atoms using laser-based traps, reducing their motion until the cloud was barely moving at all.
The temperature is not read from an instrument placed in the gas. It is an effective temperature, inferred from how slowly the atoms spread apart when the trap is released and the cloud is allowed to expand.
Microgravity is what makes the figure possible. On Earth, gravity makes a cloud of atoms sag, which limits how effectively it can be cooled. In orbit the apparatus is in free fall and the cloud holds its shape.
The previous marks were already far below anything an ordinary laboratory can reach. Condensates made on the International Space Station reached effective temperatures as low as 52 picokelvin, reported by Gaaloul and colleagues in Nature Communications in 2022. An experiment that dropped atoms down a tower on Earth to create brief weightlessness reached 38 picokelvin, reported by Deppner and colleagues in Physical Review Letters in 2021.
Drop towers are limited by how long the fall lasts. A space station is not.
It is worth being exact about what has been measured. The record is not a temperature in the everyday sense of a quantity a thermometer could confirm. It is a statement about how reluctant 20,000 atoms were to go anywhere once nothing was holding them in place.
On that definition coldness is slowness, and the coldest place ever recorded is the place where the least was happening. This is a considerable advance on the previous arrangement, under which coldness had to be demonstrated by being cold.
The scale extends in the obvious direction. Any cloud that disperses more slowly will take the record, and a cloud that declined to disperse at all would sit at absolute zero, which thermodynamics forbids and the measurement procedure does not.
Our analysts note that the remaining obstacle is gravity, which makes clouds sag. Since a space station is in permanent free fall, the record now belongs to whichever apparatus is falling for longest, and the coldest object in the universe is the one that has been dropped the furthest without landing.
The Department of Unnecessary Research notes that the measurement is taken only after the trap is switched off and the atoms are free to leave. The coldest thing ever recorded is therefore the one that was given every opportunity to move and did almost nothing, which the newsroom believes to be the first physical quantity ever defined by reluctance.