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 with a collection of atoms by forcing them to behave first like bosons, then fermions, then bosons again.
The difference between these 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 used 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.
The researchers started with a collection of boson-like paired-up atoms. First they compressed them, then converted them into a collection of fermions, which increased their collective energy.
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Next, Widera and his colleagues let the atoms expand. They also tweaked the magnetic fields to re-form the atomic pairs and switch them back to boson state – which lowered their overall energy. 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 fluid.
Currently, the ultracold atom engine has an efficiency of about 25 per cent, but Widera says scientists could tweak it to be 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, setting the new engine apart from existing devices. This difference makes it 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 devices, such as quantum batteries, says Gabriele De Chiara at Queen’s University Belfast in the UK. They could also be used “in reverse”, he says, to cool down devices like quantum computers which are known to make more errors when warm.