The laws of physics assert that an organised system will grow increasingly disordered over time until it dissolves into featureless mush – but a new experiment shows that some extremely cold atoms could avoid such entropy.
Any system beginning with low disorder, or low entropy, is bound to eventually become more of a mess. Picture flowers arranged in a bouquet: their entropy will gradually keep increasing until the brightly-coloured bundle breaks down into brown dust.
For more than a century, physicists believed the process behind this, called thermalisation, was unavoidable. In the 1950s, however, it became clear that quantum effects can cause exceptions.
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at Duke University in North Carolina and his colleagues discovered that such exceptions could be selectively created. In their experiment, some atoms thermalised, while others defied entropy and remained close to their original state.
“This has been postulated and conjectured in the past, but never observed in an experiment,” he says.
Zhao and his colleagues focused on atoms of the element rubidium, which they cooled to only 19 millionths of a degree kelvin above absolute zero by hitting them with lasers and electromagnetic fields. They used the same tools – lasers and electromagnetic fields – to arrange up to 19 such atoms into a chain.
These atoms were also supersized in diameter, meaning their electrons orbited their nuclei at a large distance. As a result, the atoms were extremely sensitive to light – which could then easily be used to control them.
Using laser light, the researchers could make the atoms interact with each other in a very specific way. Light also allowed the team to precisely set the atoms’ quantum properties, such as the energies of their electrons, at the beginning of the experiment. After establishing the initial conditions, the researchers gave the atoms time to naturally change states – an opportunity to thermalise – before measuring those quantum properties and determining the atoms’ eventual state.
Strikingly, with the right combination of initial properties and interactions, some atoms in the chain resisted thermalisation. Instead of joining their neighbouring atoms in forming one state that would experience lots of entropy, they ended up with properties very similar to those they had at the start of the experiment.
at Iowa State University says it is unusual for part of a system to somehow fail to reach the same high-entropy state as the rest of it. “Typically, whatever initial state you started in shouldn’t matter,” he says.
Now that the researchers have demonstrated that this type of behaviour can be engineered and controlled, it may have practical applications. The ability to selectively avoid thermalisation could be useful in experiments where ultracold atoms are used for simulating materials or where changes in their quantum states are used to process information. Iadecola says that making sure some atoms always behave differently from their neighbours could be an extra control method in such experiments.
One especially promising use could be enabling quantum computers built from ultracold atoms to catch and correct their own errors, says Zhao. In this case, the researchers would try to ensure that any malfunction stayed confined to only a few atoms instead of spreading through the whole computer.
Journal reference:
Physical Review X,
