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A device connected to a quantum battery can gain more charge than the battery loses, if they are specially correlated. Understanding how this works could help us learn how to more efficiently power devices like quantum sensors and quantum computers.

鈥淚f [a battery and a device] have some information about each other, if they are correlated, then sometimes your device will be able to get more than the battery gives,鈥� says at the University of Potsdam in Germany. Here, the device and the battery don鈥檛 become correlated simply by one being plugged into the other. Instead, they have to start off in a special shared quantum state, like being entangled, which guarantees that the way the quantum state of each changes during charging always depends on information that they 鈥渒now鈥� about each other.

The amount of useful work that a device can gain from a quantum energy-storage device like a quantum battery is called ergotropy, and some of this is typically lost when the two connect. While traditional batteries can give off almost all of their charge, quantum batteries do not always manage to pass on all of their ergotropy. But use a quantum process to make the behaviour of the battery and the device inseparable, and it鈥檚 as if you can get a AAA battery to produce the power of a larger AA battery.

Hovhannisyan and his colleagues discovered this through mathematical analysis and computer simulations of the quantum charging process. They modelled the battery and the device as a pair of quantum bits, like those used to build quantum computers, then compared how much ergotropy was exchanged if they chose their respective quantum states from two different mathematical sets 鈥� one where all pairs of device-and-battery states were inseparable, or correlated, and one where the two could always be treated as independent.

Team member at the University of Exeter in the UK says it was challenging to rigorously prove that, without correlations, some of the ergotropy is always lost during the transfer, but they ultimately determined that correlations are an invaluable resource for quantum batteries and let them give off more charge than they hold.

This type of analysis is crucial for understanding when quantum batteries may be useful in practice, says at the Vienna University of Technology in Austria. For every operation that such a battery would power, it is useful to know what kind of correlations could make the process more efficient, as well as how much energy it would take to put those correlations in place, he says.