Scientists have achieved an extraordinary breakthrough in ultra-cold physics after cooling a cloud of atoms to temperatures closer to absolute zero than ever previously recorded, setting a new benchmark in the study of quantum matter.
The experiment, conducted aboard China’s Tiangong space station, saw researchers cool approximately 20,000 rubidium atoms to an estimated temperature equivalent to between 7.0 and 12.9 picokelvin above absolute zero.
The achievement surpasses the previous record of 38 picokelvin established in 2021. The findings were published in the scientific journal Science.
Beyond setting a new temperature milestone, the experiment also allowed scientists to monitor the expanding atomic cloud for a considerably longer period than earlier studies, creating fresh opportunities to investigate unusual quantum phenomena.
Absolute zero, equivalent to -273.15 degrees Celsius, represents the lowest theoretically possible temperature. At this point, a system reaches its minimum possible thermal energy. Although scientists cannot reach this limit completely, researchers have spent decades developing techniques to bring matter increasingly close to it.
One of the most significant developments in this field came in June 1995, when researchers from the US National Institute of Standards and Technology and the University of Colorado Boulder successfully cooled rubidium atoms to approximately 20 billionths of a degree above absolute zero.
The experiment produced a new state of matter known as a Bose-Einstein condensate (BEC), in which atoms cooled to extremely low temperatures begin occupying the same quantum state and collectively behave like a single matter wave.
This unusual state enables scientists to investigate quantum mechanical effects on a much larger scale than would ordinarily be possible with individual particles.
Subsequent experiments continued pushing temperature boundaries. In 2003, researchers led by the Massachusetts Institute of Technology cooled sodium atoms to approximately 450 picokelvin using magnetic confinement. Their technique involved weakening the magnetic trap so that faster-moving atoms could escape, carrying away excess thermal energy in the process.
However, magnetic confinement comes with limitations because it can influence the spin properties of trapped atoms. Scientists have therefore continued searching for alternative techniques that provide greater control over atomic behaviour while achieving even lower temperatures.
A major advance followed in 2021, when researchers used a drop tower at the University of Bremen in Germany to create conditions of near-weightlessness. By counteracting gravity through free fall, they allowed a cloud of rubidium atoms to expand under the influence of a magnetic lens, achieving an effective temperature of 38 picokelvin.
The latest experiment builds on this approach by taking advantage of the prolonged microgravity conditions available in Earth’s orbit.
The research team used intersecting laser beams to confine the rubidium atoms inside an optical dipole trap. Unlike conventional magnetic traps, this arrangement allows atoms to be held without significantly affecting their magnetic properties.
The scientists initially slowed the atoms using laser cooling techniques before applying evaporative cooling to produce a Bose-Einstein condensate.
Although researchers had previously demonstrated the creation of this quantum state in space aboard the International Space Station in 2020, the latest experiment introduced a different approach to cooling the atomic cloud even further.
The team applied a carefully controlled magnetic-field gradient to remove the fastest-moving atoms. The remaining slower atoms were subsequently transferred into a larger optical trap, where the confinement was gradually weakened before the cloud was released.
On Earth, gravity would quickly pull the expanding atoms away, limiting how far researchers could reduce the confinement. However, the microgravity environment aboard Tiangong allowed the cloud to expand much more freely.
By capturing images of the expanding atoms from two different directions, scientists measured their movement along three spatial dimensions. The resulting data indicated an effective temperature ranging from 7.0 to 12.9 picokelvin above absolute zero.
Researchers emphasised that the figure represents an estimate derived from the energy associated with the cloud’s expansion, rather than a direct measurement obtained using a conventional thermometer.
Another important feature of the experiment was the time available to observe the expanding atomic cloud. The researchers managed to track its behaviour for approximately seven seconds, giving them a significantly longer observation period than previous comparable experiments.
This extended window could prove particularly valuable for future studies because scientists need sufficient time to examine quantum systems before their constituent atoms spread too far apart to remain useful.
The achievement could also advance the development of highly sensitive matter-wave interferometers. These instruments work by splitting and recombining matter waves to detect extremely small differences in their trajectories.
Such technology could help scientists investigate whether different types of atoms respond identically to gravity, providing new opportunities to test the equivalence principle, a fundamental concept underpinning Albert Einstein’s general theory of relativity.
Additionally, the ability to manipulate ultra-cold atoms with carefully designed laser fields could enable researchers to simulate unusual quantum systems and explore physical behaviours that are difficult to reproduce under ordinary laboratory conditions.
Although reaching absolute zero remains physically unattainable, the latest breakthrough demonstrates how microgravity experiments could push the boundaries of ultra-cold physics.
By combining exceptionally low temperatures with longer observation periods, scientists have opened another avenue for exploring quantum mechanics and developing technologies that could deepen our understanding of the universe.
