Physicists aboard China’s Tiangong space station chilled roughly 20,000 rubidium atoms to 9.7 trillionths of a degree above absolute zero, establishing a new record-low temperature in space.
The experiment utilized the Cold Atom Physics Rack located within the space station’s Mengtian module. The research was led by Xiaoji Zhou and Xuzong Chen of Peking University, alongside Weibiao Chen of the Shanghai Institute of Optics and Fine Mechanics and colleagues from Fudan University.
Over a period of about 20 seconds, the laser power was eased down to facilitate the formation of a Bose-Einstein condensate, a state where many atoms share a single quantum state and behave collectively as a single quantum wave. This state provides a versatile platform for quantum simulations.
The researchers noted that weightlessness presented specific challenges. On Earth, gravity tilts the trap, which helps the hottest atoms spill out to assist in cooling. In orbit, the lack of this tilt meant that evaporation was slower and less complete; only about 65% of the atoms joined the condensate in an identical setup on the ground.
To resolve this, the team implemented a stand-in for gravity by using a magnetic field gradient. This pulled on the atoms’ spins to speed their escape and push the remaining atoms into a single, magnetically neutral state. This adjustment increased the condensate fraction to 89%, totaling about 23,000 atoms.
For 30 years, physicists have attempted to push atom clouds toward lower temperatures, but Earth’s gravity sets a floor. A trap that is too loose on the ground simply drops its atoms, causing ground-based laboratories to stall at approximately 350 trillionths of a degree in three dimensions. While drop towers and sounding rockets can provide a few seconds of free fall, they have limitations.
The previous record for a cloud of this type was 38 trillionths of a degree, set at the Fallturm Bremen drop tower in Germany. NASA’s Cold Atom Laboratory on the International Space Station, which utilizes magnetic fields to hold atoms, reached 52 trillionths of a degree while observing clouds for about one second.
The Chinese team distinguished their method by using optical dipole traps (ODT)—crossed laser beams—which can hold atoms regardless of their magnetic state, enabling experiments that magnetic traps cannot perform. No one had previously created a quantum gas in orbit using this specific method.
9.7 picokelvin (pK)! This is definitely the coolest experiment ever and exactly why we need space stations. They enable research like this that is difficult or impossible on the ground. Cooling atoms near absolute zero stretches their de Broglie wavelength to observable scales —… https://t.co/1I2xJxFzu6 pic.twitter.com/ZgmbA0koXu
This temperature is vastly lower than the 2.7 Kelvin temperature of the cosmic microwave background, or the temperature of empty space, according to OSEL. The authors of the study noted that this result is the lowest they know of.
The full study was published in the journal Science.
In the ultracold-atom physics experiment cabinet aboard the China Space Station, researchers have cooled a rubidium atom cloud to an equivalent temperature of 9.7 pK using all-optical trap. A new record for the lowest temperature achieved in artificial matter. Microgravity is… https://t.co/qwowkbTAmN pic.twitter.com/4AIJ3f0bW9