Atoms on China's Space Station Hit 9.7 Picokelvin, a Fourfold Record Chill

Physicists working on China's Tiangong space station have cooled a cloud of atoms to the coldest equivalent temperature ever reported, 9.7 picokelvin, or just under ten trillionths of a degree above absolute zero. The result, published in the journal Science under the title "Quantum gases below 10 picokelvin in space," roughly quadruples the previous record of 38 picokelvin, which was set in a drop-tower experiment on Earth. The uncertainty range is 7.0 to 12.9 picokelvin. Wei Xiong is the first author, and the work is credited to a team from Peking University.

The sample was a Bose-Einstein condensate of about 20,000 rubidium atoms. In that state, the atoms lose their individual identities and behave as a single quantum wave. The team held the atoms in an optical dipole trap, a configuration of focused laser beams that grips neutral atoms without touching them. They first used laser cooling and evaporative cooling, with a magnetic-field gradient to improve the early stages, then moved the atoms into a wider optical trap with weaker confinement. Releasing atoms from a looser trap reduces the energy they carry as they expand, which is what allows an extremely low effective temperature.

Microgravity is the key ingredient. On Earth, gravity pulls cold atoms out of a trap within a fraction of a second, so scientists measure how fast the cloud expands after release and convert that into a temperature. The longer the cloud can float freely, the more precisely a very low temperature can be measured and reached. The Tiangong team tracked the expansion for 7 seconds. A drop tower gives about 2 seconds, and the Cold Atom Laboratory on the International Space Station gives about 1 second in the experiment that had reached 52 picokelvin earlier.

These numbers are an equivalent temperature that describes how little the atoms' motion spreads, not a thermometer reading of a solid object. Still, the milestone matters because cold, slowly expanding atom clouds are the raw material for the most sensitive quantum instruments. The researchers list two goals for the technique. One is quantum simulation, using clouds of ultracold atoms to mimic exotic phases of matter that are impossible to study directly. The other is an atom Mach-Zehnder interferometer in space, a device that splits and recombines matter waves to test physical principles with extreme precision, including the equivalence principle, which says gravity accelerates all masses equally.

The result also adds to the growing competition in orbital quantum physics. NASA's Cold Atom Laboratory on the space station has produced Bose-Einstein condensates in orbit for years, and the new Chinese platform pushes the observation time and the temperature well past what that facility reported. Whether those gains translate into new tests of gravity will depend on follow-up experiments, and the paper presents the 9.7 picokelvin figure as a technical demonstration rather than a physics discovery in itself. Even so, a quantum gas colder than anything made before, floating undisturbed for seven seconds, shows how much the quiet of space can improve a measurement that gravity ruins on the ground.