Ice cubes melt and hot coffee cools. This is the familiar way of the world, enshrined in the laws of thermodynamics. Left to its own devices, things tend to reach the same temperature, called thermodynamic equilibrium.
But things are not so straightforward when we consider individual atoms. University of California, Los Angeles (UCLA) researchers have studied the process of buffer gas cooling, a technique used to cool ions by immersing them in clouds of cold atoms. In the traditional picture, the cold atoms should steal some energy from the ions and you should end up with cooler ions.
This, however, is not always the case. In a paper published in Nature Communications, the team discovered that the final temperature of the ions depended on the initial temperature, but also on the number of ions.
"This apparent departure from the familiar laws of thermodynamics is akin to our warm apple pie either cooling as expected or spontaneously bursting into flames, depending on the pie's exact temperature when it is placed in the window," said co-author Professor Eric Hudson in a statement.
In this particular study, the team tried to cool up to 10 barium ions by immersing them in a cloud of 3 million laser-cooled calcium atoms. Once the calcium cloud was removed, the ions should have cooled down, but sometimes they didn’t.
The technique is in many respects a miniaturized fridge. There is a coolant (the cold atoms) taking away the heat from something else. Unfortunately, at that level, there are non-equilibrium activities at play. Buffer gas cooling must be more nuanced than previously thought; the complexity of the whole system needs to be taken into account.
"Our results demonstrate that you can't just throw any buffer gas into your device – no matter how cold it is – and expect it to work as an effective coolant," added lead author Steven Schowalter, who worked in Hudson's laboratory.
This discovery is obviously not a violation of the laws of thermodynamics, but it shows that even well-established techniques can go a bit funky when we move into the microworld. As buffer gas cooling is used in many different applications, from forensics to antimatter production, it is important to understand the full picture.