Extremely cold atoms defy entropy and refuse to heat up

Repeatedly energising a collection of ultracold atoms should destroy their collective structure, but quantum effects seem to counteract the process.

The ultimate fate of any physical system ought to be “thermalisation,” a process by which everything heats up and becomes even and featureless, like an ice sculpture becoming a puddle of water. Intuitively, we would assume that repeatedly throwing rocks at the sculpture could only speed this process up, but Hanns-Christoph Nӓgerl at the University of Innsbruck in Germany and his colleagues ran an experiment that essentially did this to some of the coldest atoms on the planet and did not see them thermalise.

“We expected to see the opposite,” says Nӓgerl. The researchers used about 100,000 atoms of caesium that they cooled within billions of a degree of absolute zero by hitting them with lasers and electromagnetic forces – at this temperature, the behaviour of atoms is fully quantum. The team arranged the atoms into thousands of one-atom-thick tubes. Then, they started “kicking” them by shining an extra laser pulse on the atoms over and over again.

Because these kicks gave the atoms extra energy, they should have made them heat up and fly away with different velocities. Team member Yanliang Guo says that they never saw this happen, even as he and his colleagues tried applying kicks of different strengths and tweaked how strongly the atoms interacted with each other. The atoms kept moving with a very similar velocity, as if they were all “frozen” into only one quantum state.

The idea of quantum particles beating thermalisation is not new – it dates to the 1950s – and the question of when this can happen has long fuelled physicists’ debates. Team member Manuele Landini says that previous experiments that explored how kicking the atoms affects whether they heat up found that they eventually do but his team’s experiment explored a different range of parameters, so it may have captured truly new physics.

Mathematical theories of what is going on are also challenging and conflicted. Adam Rançon at the University of Lille in France says that calculating whether interacting atoms will heat up is so difficult that researchers can often only complete calculations for two or three atoms. There are ideas about how the quantum states of atoms that interact very strongly can overlap in just the right way to produce a state that does not absorb energy, but, in his view, the picture is incomplete.

Experiments like the new one can act as quantum simulators that can go further, but Rançon says that some kick and interaction strengths remain to be explored.

Robert Konik at Brookhaven National Laboratory in New York has worked on a mathematical model of a system like the one in the new experiment, which did predict the atoms’ odd behaviour. He says identifying systems that don’t keep absorbing energy from the kicks may also offer inspiration for novel quantum technologies. This is because the quantum state that the atoms get stuck in becomes long-lasting and could be reliably used for sensing or storing information. “Thermalisation is always the kiss of death to quantum effects,” he says.

The researchers are already working on follow-up experiments to arrange atoms into thicker tubes and let them to move between different tubes to see whether that may “unfreeze” their velocities.