Quantum engine could power devices with an ultracold atom cloud

It’s possible to build a quantum engine that is powered by a constant shifting in the fundamental quantum nature of the particles it contains. Such devices could one day be used to power other quantum technologies.

All known particles can be categorised as either fermions or bosons, and which category they fall into determines how they behave in large groups. Artur Widera at the University of Kaiserslautern-Landau in Germany and his colleagues worked out how to make an engine out of a collection of atoms by forcing them to behave first like bosons, then fermions, then bosons again.

The difference between the two types of particles is most prominent in the quantum realm, particularly at extremely low temperatures, so the researchers built their engine from a few hundred thousand lithium atoms cooled to a temperature within a fraction of a degree of absolute zero.

Under these conditions, they could use carefully tuned magnetic fields to make the lithium atoms behave either like a group of fermions or, after they forced them to form molecule-like pairs, a group of bosons.

Starting with a collection of boson-like paired-up atoms, the team first compressed them, then converted them into a collection of fermions. This increased their collective energy.

Next, Widera and his colleagues let the atoms expand. They also tweaked the magnetic fields to reform the atomic pairs and switch them back to being a collection of bosons – which lowered their overall energy. The energy difference between these steps was effectively the work produced by the ultracold atom engine. In this way, the researchers used the atoms like the “working fluid” in conventional engines, where work is extracted through a repeated cycle of compressing and expanding this fluid.

Currently, the engine has an efficiency of about 25 per cent, but Widera says that it could be tweaked to be made more efficient. “It’s not that we want to drive the next Mercedes Benz with a quantum engine, but we have shown for the first time that you can drive an engine with a purely quantum form of energy,” he says.

Sebastian Deffner at the University of Maryland, Baltimore County points out that the difference between fermions and bosons is a quantum property that is not possible to exploit in conventional machines, which sets the new engine apart from existing devices. This difference is a genuinely quantum resource and it is important to explore whether it can be used in a technologically advantageous way, he says.

If they can be made more practical, quantum engines could eventually be used to charge other quantum devices, like quantum batteries, says Gabriele De Chiara at Queen’s University Belfast in the UK. They could also be used “in reverse”, says De Chiara, to cool down devices like quantum computers which are known to make more errors when warm.