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A single atom inside of a reflective cavity could be enough to drive a piston in a tiny, quantum version of an engine.

The essential feature of any engine is that it converts heat into work, which can then set mechanical parts into motion. For internal combustion engines, burning gas makes it expand and push on pistons, which eventually results in car wheels or turbine blades moving. at the University of Granada in Spain and his colleagues designed a similar engine for the quantum realm.

鈥淵ou can imagine keeping a classical piston but replacing the classical gas with the simplest quantum system, which is one atom. That was our starting point,鈥� he says.

The atom and the piston in the quantum engine would sit inside a small cavity made from a reflective material, like a tiny box made of mirrors. The piston would be a solid microscopic object made of a similar reflective material. In a conventional engine, the piston moves because it gets repeatedly hit by many gas atoms, but here its motion would come from quantum radiation.

This could happen a few different ways. If the atom warms up or gains energy from the engine鈥檚 environment in some other way, such as by being illuminated, it would release it as radiation that would then get stuck bouncing within the reflective cavity and repeatedly interact with the atom. This would make radiation build up and its pressure would move the piston. Alternatively, the cavity could warm up and start to radiate, and that radiation would again bounce off the walls and interact with the atom.

Quantum engines have been modelled and even built in experiments before, says at the University of Granada, who was part of the research team, but rigorous proof that they could produce useful mechanical work has been lacking. The researchers calculated that not only would their engine work, but it could be used to implement the two most prototypical sequences of piston motions for a heat engine, called the Otto cycle and the Carnot cycle.

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, another team member at the University of Granada, says that some of these results were surprising as the quantum world of single atoms and small objects is so different from the much larger machines that conventionally count as engines that even seemingly simple ideas like defining work or temperature are not a given.

It is challenging to get a clear mathematical understanding of the notions of heat or work at the quantum level, as the researchers have done here, says at the Singapore University of Technology and Design. This could be useful for building nanosized devices or controlling how they heat up 鈥� which is crucial for many emerging quantum technologies such as computing and communications.

The fact that this engine design relies on solid materials rather than the less practical gases of very cold atoms used in some past studies could also inspire many new experiments, says Poletti.