Tiangong team sets cold record with a seven-second atom cloud

NASA’s Cold Atom Laboratory created the first condensate in Earth orbit in 2018. Later experiments aboard the International Space Station reached picokelvin energy scales. In 2021, a German-led drop-tower team reached the previous 38-picokelvin record but observed the freely expanding atoms for only about two seconds.

The Tiangong result combines a lower energy scale with a much longer observation window than earlier ground-based tests.

How did the team cool the atoms in microgravity?

The experiment began with roughly two million rubidium-87 atoms inside the Cold Atom Physics Rack, installed in Tiangong’s Mengtian laboratory module.

Crossed laser beams held the atoms in a tiny trap. The researchers gradually weakened it, allowing the most energetic atoms to escape. The process resembled hot coffee cooling as energetic molecules evaporate from its surface: the remaining atoms had less energy.

Microgravity removed an advantage that gravity normally provides. On Earth, gravity pulls escaping atoms away and makes the cooling process more efficient; in orbit, that assistance disappeared. The team used a magnetic field gradient—a field that changes strength across the trap—to imitate gravity’s useful effect.

They then produced a condensate of about 23,000 atoms at a conventional temperature of 3.1 nanokelvins. Nearly 90 percent of the atoms occupied the condensate.

To push farther, the researchers moved the cloud into a second, wider laser trap and loosened its grip over 1.175 seconds. That reduced the energy the atoms would release as they spread apart.

“Only the interaction energy changes,” coauthor Xuzong Chen of Peking University explained to Science.

When the team released the cloud, expansion speeds reached only about 70 to 88 micrometers per second, depending on direction. The cloud remained observable for seven seconds.

In practical terms, the team used a magnetic field to recover a cooling trick that Earth usually supplies for free.

Why does seven seconds matter more than the record?

A longer observation window could improve atom interferometry, a measurement method in which scientists split and recombine matter waves to reveal tiny changes in motion and forces. The more time the atoms remain available, the greater the potential sensitivity.

Such instruments could test Einstein’s equivalence principle, which says that objects of different composition fall at the same rate in a gravitational field. A heavy bowling ball and a light feather fall at the same speed in a vacuum, Einstein realized, and a body cannot distinguish Earth’s pull from the push of an accelerating rocket.

Any convincing deviation from the equivalence principle would provide a major clue to physics beyond current theories.

The Tiangong team also hopes to create arrays of laser light that mimic the structure of crystals and help investigate superconductivity, the flow of electricity without resistance. The record matters because it gives physicists more time to measure minute effects predicted by theories of gravity and matter.