A Cloud of Atoms on China's Space Station Reached 9.7 Picokelvin

A cloud of rubidium atoms on China's space station has been cooled to 9.7 picokelvin — about fourteen orders of magnitude below room temperature, and the coldest matter anyone has recorded. It stayed cold, and observable, for seven seconds.

A picokelvin is a trillionth of a kelvin. Absolute zero, the floor thermodynamics allows, cannot be reached; the game is getting close and staying there long enough to measure something.

The previous record, set by a German team in 2021, reached 38 picokelvin and held for about two seconds. The new result is roughly a quarter of that temperature, with a window more than three times longer — which matters, because the physics at these temperatures is faint and needs time to show itself.

The enabling condition is weightlessness. On Earth, gravity pulls the cloud downwards, so it has to be held in a stronger trap, and the trap itself adds energy — which puts a floor under how cold the atoms can get. Ground experiments keep a stable cloud for about 0.05 seconds. In orbit, the atoms can float almost freely, so a very weak optical trap is enough.

That trap is where the approach differs. The team used an all-optical trap built from focused laser beams, rather than the magnetic chip traps other groups favour. An optical trap is less sensitive to magnetic disturbance and extends more naturally into the optical-lattice experiments that usually follow.

Getting the apparatus into orbit was a project in itself. The ground version occupied about 20 cubic metres and weighed 10 tonnes; the flight version fits into 0.7 cubic metres at roughly 500 kilograms. More than twenty laser beams converge on a cloud a few tens of micrometres across, aligned to within 5 micrometres — about a tenth of the width of a human hair.

At these temperatures the rubidium cloud becomes a Bose-Einstein condensate, in which atoms that would normally mill about thermally begin to behave as a single quantum object. The condensate held about 20,000 atoms. The paper, published in Science on 6 November, describes it as the first quantum degenerate gas produced with an all-optical trap in microgravity.

The uses are mostly instruments. Ultracold atoms underpin atom interferometers and quantum sensors, and serve as frequency references in space — useful for mapping gravity, for inertial navigation and for testing physical constants. They are also a platform for questions that are hard to ask on the ground, such as how quantum gases behave out of equilibrium.