Scientists aboard China’s Tiangong space station have cooled a cloud of atoms to an equivalent temperature of just 9.7 picokelvin, setting a new record for the coldest temperature achieved in this type of experiment. Using laser-based optical traps in the microgravity environment of space, the team reached temperatures about four times lower than the previous record while tracking the expanding atomic cloud for seven seconds. The achievement could open new possibilities for studying unusual quantum states of matter and developing highly sensitive instruments in space.
At temperatures approaching absolute zero, matter can behave in ways that are very different from what we experience in everyday life. One particularly interesting example is a Bose–Einstein condensate, or BEC.
A BEC forms when a group of atoms is cooled so much that they begin behaving collectively, acting like a single shared quantum wave rather than a collection of separate particles. This unusual state of matter gives scientists a way to investigate quantum physics on a larger, more accessible scale. The study authors describe BECs as “versatile platforms for quantum simulation.”
Lower temperatures are especially useful because they reduce the motion of atoms. With less thermal motion to interfere with measurements, scientists can investigate delicate quantum behavior with greater precision.
However, getting atoms this cold is not easy. Gravity presents a particular challenge because it pulls atoms downward, making it harder to hold them in a stable trap and study their behavior.
One way to overcome this problem is to use magnetic levitation, which counteracts gravity. But this approach comes with a limitation: it forces the atoms into the same internal spin state, restricting how scientists can manipulate them.
Optical traps offer another option. These systems use focused laser beams to hold neutral atoms in place, allowing researchers greater flexibility in controlling their properties. Before the new experiment, however, optical traps had not directly produced a Bose–Einstein condensate under microgravity conditions.
Space offers an important advantage. Without the usual effects of gravity, researchers can study freely expanding atomic clouds for longer periods, rather than being limited by the short free-fall windows available in ground-based experiments.
How the experiment broke the temperature record
The new experiment took advantage of the microgravity environment aboard China’s Tiangong space station.
The researchers began with approximately 20,000 rubidium atoms. They cooled the atoms using two established techniques: laser cooling, which slows atoms with laser light, and evaporative cooling, which removes higher-energy atoms from the group.
A magnetic-field gradient provided additional help during the initial cooling process. According to the researchers, this produced a condensate that was less sensitive to magnetic disturbances.
Once the condensate had formed inside an optical dipole trap, the team transferred the atoms into a wider optical trap. They then weakened the trap’s confinement, reducing the energy associated with the cloud’s expansion.
Finally, they measured how the atomic cloud expanded in three dimensions.
The result was an equivalent temperature of 9.7 picokelvin. A picokelvin is one trillionth of a kelvin, making this an extraordinarily small measure of temperature.
The reported uncertainty ranged from 7.0 to 12.9 picokelvin, meaning the researchers’ measurement was not an exact single value.
There is also an important distinction between this figure and an ordinary temperature measurement. At such extreme conditions, conventional thermodynamic temperature is not practical for describing the experiment. Instead, scientists use an equivalent temperature based on the atomic cloud’s expansion energy and velocity.
This provides a way to compare how cold different ultracold-atom experiments have become.
The new result surpasses two notable previous achievements. A drop-tower experiment had reached an equivalent temperature of 38 picokelvin by placing atoms in a brief free-fall environment. Meanwhile, an experiment aboard the International Space Station’s Cold Atom Laboratory reached 52 picokelvin using magnetic trapping and matter-wave lensing.
The Tiangong experiment therefore pushed the measured equivalent temperature substantially lower than either result.
Seven seconds to study an expanding quantum cloud
The temperature record is only part of the achievement. The time available to observe the atoms is another important advance.
In experiments involving ultracold atoms, researchers often allow a trapped cloud to expand freely before measuring its properties. Tracking this expansion helps them understand the cloud’s energy and motion.
But gravity makes long observation periods difficult in many experiments. A drop tower, for example, provides only a brief period of free fall before the experiment ends.
The new space-based system allowed the researchers to follow the atomic cloud’s free expansion for seven seconds.
That is considerably longer than the two seconds achieved in the previous drop-tower experiment and the one second reported for the International Space Station experiment.
Having more time to observe the cloud could make it easier to investigate subtle changes in its behavior and carry out measurements that require longer observation periods.
The combination of extremely low equivalent temperatures and extended observation time makes the experiment particularly interesting for future quantum physics research.
What could scientists do with atoms this cold?
The new system could provide a platform for investigating quantum states that are difficult to study under ordinary conditions.
One possibility is to use carefully shaped laser fields to create unusual environments for atoms and examine how quantum matter behaves within them.
The researchers are particularly interested in exotic quantum phases, including Bose-glass and fractal Mott phases. These are unusual states of matter associated with complex quantum behavior in systems where disorder and interactions between particles play important roles.
Another proposed direction involves studying Tomonaga-Luttinger liquids, a type of one-dimensional quantum system in which particles behave collectively in ways that differ from those in ordinary three-dimensional materials.
The team also sees potential in developing space-based atom interferometers.
An atom interferometer uses the wave-like nature of atoms to make extremely precise measurements. One possible design is a space-based atom Mach-Zehnder interferometer, which could be used to test fundamental principles of physics, including the equivalence principle.
The equivalence principle concerns the relationship between gravity and acceleration. Testing it with highly sensitive instruments could help scientists examine one of the foundations of modern physics.
These applications remain potential research directions rather than demonstrated capabilities of the new experiment.
The researchers also suggest that adjustable-volume optical traps could make it possible to work with larger numbers of atoms, expanding the range of experiments the system could support.
For now, the central achievement is a new benchmark in ultracold-atom research: an atomic cloud with an equivalent temperature of 9.7 picokelvin, observed during seven seconds of free expansion in space.
The study was published in Science.
