Quantum batteries recharge more efficiently, bypassing laws of causality

Energy

With information from New Scientist – 11/09/2023

Quantum batteries

Quantum batteries emerged as a curiosity in theoretical physics, but soon caught the attention of hands-on people.

One of the reasons for this interest is that theories indicate that, thanks to quantum effects, all the cells of these futuristic batteries will be charged simultaneously, which will give them an unbeatable recharging speed.

The first experimental quantum battery was only presented in 2020, but it is still far from becoming practical for powering cell phones or cars, looking more like the superconducting qubit system of a quantum computer. But the essential thing is that the concept has been demonstrated to work.

Trying to advance not just the concepts but also the experiments, Gaoyan Zhu and colleagues at the University of Tokyo in Japan decided to see whether a particularly counterintuitive quantum phenomenon could play a role in the charging and discharging of these batteries. And it worked, both theoretically and experimentally.

Indefinite causal order

The bizarre phenomenon explored by the team hurts our intuition because it seems to challenge something that is very dear to our traditional way of seeing the world, the idea of ​​causality, which we commonly call the law of cause and effect, that is, the much-loved notion that a one thing leads to another.

In the classical world we are used to, causality goes in only one direction: If event A causes event B, then of course B did not also cause A. But on the quantum scale, it may be impossible to say in which direction such causality goes. , something physicists call “indefinite causal order.”

This happens because both directions of causality can be placed in a quantum superposition, where both and neither are simultaneously true and only “collapse” into the real order when we make a measurement of it.

In other words, quantum causality questions the sequence of cause and effect, since quantum events are independent of space and time – or, at least, the variables seem to be outside of time, neither in the past nor in the future.

In the traditional scenario, with a defined causal order, a battery can only choose one of two possible paths to recharge. But this restriction can be removed using a quantum key, which generates an undefined causal structure.
[Imagem: Gaoyan Zhu et al. – 10.48550/arXiv.2105.12466]

In theory it works, in practice it almost works

Physicists have applied this idea of ​​indefinite causal order to two chargers working to put energy into a quantum battery.

They calculated how much power the battery would gain and how efficiently it would charge for three protocols: One in which the chargers were connected sequentially (in series), one in which the chargers simultaneously powered the battery (in parallel), and one in which it was impossible to know which charger was working because these two causalities were in superposition.

Calculations proved that this last approach provides the battery with more energy in the most efficient way, even when the connection between the two chargers and the battery was relatively weak. This means that the charging process can work well even for suboptimal chargers.

To test their calculations, the team carried out a proof-of-principle experiment using photons, proving that this undefined causality protocol can be implemented with a component called a quantum key. However, they have not yet directly tested charging a complete quantum battery.

Therefore, to be sure about how the absence of the cause and effect sequence could influence the performance of quantum batteries, we will have to wait for more complete experiments. But now it’s another question of how to set up the experiment in a way that leaves no doubts, and the team is already working on that.

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