Tiangong team sets estimated rubidium temperature record in orbit

By Priya Raman

Inside China’s Tiangong space station, an expansion of about 20,000 rubidium atoms corresponded to an estimated 9.7 trillionths of a degree above absolute zero, roughly one-quarter of the 38-picokelvin record that researchers set in 2021.

The result does not come from a conventional thermometer. Images taken from two directions revealed how quickly the cloud spread along three axes, and those measurements suggested a temperature between 7.0 and 12.9 picokelvin. The 9.7-picokelvin figure was an estimate based on the energy associated with expansion.

Hundreds of miles above Earth, researchers used intersecting laser beams to hold the atoms in an optical dipole trap. The cage confined rubidium in a magnetically insensitive state, unlike magnetic traps, while a combination of laser braking and evaporative cooling produced a Bose-Einstein condensate.

How did the team reach the record in orbit?

The cooling sequence comprised several distinct interventions:

• Intersecting laser beams formed the optical dipole trap and confined the rubidium atoms.
• Lasers slowed fast-moving atoms, and evaporative cooling brought the population into a Bose-Einstein condensate.
• A gentle gradient in an applied magnetic field expelled the faster-moving atoms.
• The team transferred the remaining “slow” atoms into a second, wider optical trap, reduced the trap’s strength and released the cloud.
• Orbital free fall allowed researchers to relax the confinement much further than gravity would permit on Earth’s surface.

The condensate itself was not the first of its kind in orbit. Researchers aboard the International Space Station achieved that milestone in 2020. The Tiangong advance combined a lower inferred temperature with an expansion window longer than those in previous experiments.

The rationale echoed a 2021 University of Bremen drop-tower experiment. There, free fall counteracted gravity long enough for a magnetic lens to force rubidium to expand and cool, yielding an effective temperature of 38 picokelvin. In orbit, the team could relax the trap further and track the expanding cloud for seven seconds.

The technique rests on a half-century of controlled cooling. In June 1995, a collaboration between the US National Institute of Standards and Technology and the University of Colorado at Boulder cooled rubidium to 20 billionths of a degree above absolute zero, or 20 picokelvin. The work produced a Bose-Einstein condensate.

At sufficiently low temperatures, multiple atoms occupy the same quantum state and behave collectively as one wave of matter. Phenomena normally accessible only to individual particles can then emerge at a larger scale, and Bose-Einstein condensates have become a standard tool in physics.

By 2003, an MIT-led team had cooled sodium to 450 picokelvin in a magnetic trap. The researchers weakened the confinement and allowed the most energetic atoms to escape, carrying heat with them. Magnetic traps can also affect atomic spin, so researchers sought ways to reduce that influence while reaching lower temperatures. The Tiangong optical trap addressed that constraint by confining the atoms in a magnetically insensitive state.

What does the seven-second observation window enable?

A gradual expansion keeps the cloud usable longer, giving physicists more time to interrogate its quantum behavior before diffusion makes it too diffuse for measurements. The seven-second tracking period therefore adds practical value beyond the record temperature itself.

Proposed interferometers could split and recombine matter waves to reveal tiny differences in their paths. Such instruments could test whether different atoms fall identically, providing a new way to examine general relativity. Carefully patterned laser fields could also turn the platform into a simulator for unusual quantum states.

These remain proposed applications rather than measured outcomes of the Tiangong experiment. The demonstrated result covers condensate formation, controlled release, expansion-based temperature estimation and seven seconds of observation.

What remains uncertain?

The temperature estimate depends on expansion energy rather than a direct thermometer reading. The account reports the 7.0–12.9-picokelvin range but does not describe an independent thermometer cross-check. It also does not report the number of repeated clouds or experimental runs, the funder, the lead researchers or their institutional affiliations.

The figure of about 20,000 atoms describes the size of one cloud, not the number of experimental replications. For portfolio decisions, teams should therefore treat the result as evidence for a longer orbital measurement window, while requesting a complete error budget, replication record and cost breakdown before using it as a procurement benchmark.

Absolute zero remains mathematically out of reach, but the practical research question is whether repeated orbital runs can turn the seven-second demonstration into a calibrated platform for tests of gravity and unusual quantum states.