Atoms Aboard China's Space Station Reach a Record-Low 9.7 Picokelvin

Researchers from Peking University, the Shanghai Institute of Optics and Fine Mechanics and the Chinese Academy of Sciences used a laser trap aboard China’s Tiangong space station to cool about 20,000 rubidium atoms to an equivalent temperature of 9.7 picokelvin. That is the lowest such temperature reported for a gas of atoms. The work was published Nov. 5 in the journal Science.¹⁴

The previous record was 38 picokelvin, set during a weightless fall down a tall drop tower. Atom experiments on the International Space Station had reached about 52 picokelvin.³ The new result comes in about four times lower than the old record.⁴

Why chase a number that small? Ultracold atoms make extremely sensitive measuring tools, and colder, slower atoms can be watched for longer. The paper’s authors, led by Wei Xiong and including Peking University’s Xuzong Chen, see this as the start of a new line of orbital experiments.¹

“This record-low temperature could stimulate a new generation of ODT-based experiments in quantum simulation and precision measurements in space,” the authors wrote, as quoted by Live Science.¹

ODT stands for optical-dipole trap, a setup that uses focused laser light, sometimes called optical tweezers, to hold atoms in place.¹

How you take the temperature of almost nothing

Nobody stuck a thermometer into this cloud. The 9.7 picokelvin figure is an “equivalent temperature,” worked out from how fast the cloud of atoms spreads after the trap lets it go. The estimate carries an uncertainty range of 7.0 to 12.9 picokelvin.⁴

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The atoms were barely moving, at speeds below 100 micrometers per second.² That is a tenth of a millimeter each second. At that pace, an atom would need about three hours to cross a single meter.

Before cooling all the way, the atoms formed a Bose-Einstein condensate, a quantum state of matter in which a cloud of extremely cold atoms behaves less like a crowd and more like one shared quantum object.¹

The team could follow the expanding cloud for seven seconds after releasing it. Holger Mueller, an atomic physicist at the University of California, Berkeley, thinks that is the headline that matters most.²

“The 7 seconds is an even more important figure of merit than the 10 picokelvin,” Mueller told Science.²

For people who study atoms, seven seconds of watching a slowly drifting cloud is a long, luxurious look.

Why it took a space station

On Earth, gravity tugs on everything, including atoms that physicists would very much like to sit still. In the weightlessness of orbit, the atoms float, which lets them cool more efficiently. Combined with the laser trap restricting their motion, that let the team reach temperatures lower than are possible on the ground.¹

It is also a sign that Tiangong is working as a research lab, not just a place to live in orbit.

The record still leaves a gap between the atoms and absolute zero itself, which is 0 kelvin, or minus 273.15 degrees Celsius. That gap is not closing all the way. Absolute zero is considered unreachable, and one version of the third law of thermodynamics says getting there would take an infinite number of steps.¹ Physicists have taken that as a challenge to see how many of those steps they can manage anyway.