Quantum gas in free fall

In 2021 a team in Germany reported cooling a cloud of rubidium atoms to an effective temperature of 38 picokelvin, about 38 trillionths of a degree above absolute zero. They used a lensing technique on the gas while it floated in free fall in the Bremen drop tower, where nothing pulls the atoms down.

Record claims depend on how temperature is defined for a tiny quantum gas. Other labs quote very low figures by different methods, and true absolute zero cannot be reached.

Context

The experiment was carried out at the Bremen drop tower, a tall facility where an experiment capsule falls freely for several seconds, creating near-weightless conditions. In such conditions atoms can expand slowly without gravity pulling them, which allows an extremely low effective temperature.

Temperature for such a gas describes the spread of the atoms' velocities, not a reading taken by a thermometer. A magnetic lens was used to slow the expansion of the cloud, and the resulting figure of 38 picokelvin comes from analyzing how fast the cloud spreads.

The work is aimed at precision tests with atom interferometers, which measure gravity and other tiny effects. Absolute zero cannot be reached, according to the third law of thermodynamics, and for comparison, the cosmic microwave background in space is about 2.7 kelvin.