Was a tower in Germany really the coldest place in the universe?

In August 2018, Bremen was having an ordinary summer: 17 to 28 °C (63 to 82 °F). Inside a capsule falling through a tower on the edge of the university campus, a cloud of rubidium atoms reached an effective temperature of 38 picokelvin.

The university later put it boldly: for two seconds, Bremen was "the coldest place in the universe".

38 trillionths of a kelvin above absolute zero.

Absolute zero, 0 kelvin or −273.15 °C (−459.67 °F), is the lowest possible temperature. It can be approached but never reached. 38 picokelvin is 0.000000000038 kelvin above it.

But that number isn't a thermometer reading.

The atoms formed a Bose–Einstein condensate: about 100,000 atoms acting together as a single wave of matter. Its temperature was worked out from how slowly the atoms moved. One of the physicists, Merle Cornelius, has said it isn't strictly correct to call it the coldest place in the universe, because no temperature can really be assigned to such a condensate.

So they dropped the experiment.

In the lab, the team could observe the cloud for only 22 milliseconds. In the Bremen Drop Tower, the capsule falls freely for 4.74 seconds, and the atoms, the cameras and the capsule all fall together. That bought them time.

After letting the cloud spread for a moment, they switched on a magnetic field that acted like a lens. Instead of focusing the atoms to a point, it slowed their spreading, like turning the widening beam of a torch into a parallel one. Setting the cloud gently changing shape before release, and timing the release, let the lens work in all three directions.

The cloud's effective temperature fell from 2 nanokelvin to 38 picokelvin, and the team could follow it for up to 2 seconds.

The cold record was a by-product. What the team wanted was a cloud of atoms that spreads as slowly as possible.

Slow, long-lived atom clouds are the raw material for atom interferometers: instruments that use waves of matter to measure gravity and motion with extreme precision. The researchers see them as a step towards tests of fundamental physics lasting many seconds, including in space.

Strictly, Bremen wasn't the coldest place in the universe. For two seconds in 2018, it held the slowest-spreading cloud of atoms in the world, which is what the scientists wanted all along.