Causation is key to our experience of reality: for example, breaking a glass causes it to shatter, so it cannot have shattered before it shattered. But in the quantum world, these rules don't necessarily apply, and scientists have now demonstrated how to exploit this weirdness to charge a quantum battery.
In a sense, quantum batteries are driven by a paradox. On paper, they work by storing energy in quantum states of atoms and molecules—and of course, as soon as the word "quantum" is mentioned, you know something weird is going to happen. In this case, a new study finds that quantum batteries can work by violating what we know about cause and effect.
Chen Yuanbo, author of the study, said: "Current batteries used in low-power devices such as smartphones or sensors typically use chemicals such as lithium to store charge, while quantum batteries use microscopic particles such as arrays of atoms. Chemical batteries are governed by the laws of classical physics, while microscopic particles are quantum in nature, so we have the opportunity to explore ways to use them to bend or even break our intuitive concepts of what happens at small scales. I am particularly interested in how quantum particles violate one of our most fundamental experiences: time."
In classical physics, the kind of physics we experience in the large-scale world, cause and effect are clearly linear. Going back to the previous analogy, dropping a glass (event A) causes the glass to shatter (event B), but you can't reverse the relationship between the two events. The glass didn't fall because it was smashed. But in the ghostly realm of quantum physics, this limitation doesn't apply. Incorporating this paradox into quantum batteries could help improve their efficiency.
In the new study, scientists at the University of Tokyo conducted a laboratory experiment using lasers, lenses and mirrors as a large quantum battery. Charging these batteries usually requires multiple charging stages, working one after the other, but here, the research team took advantage of a quantum effect called indefinite causal order (ICO). Basically, once they brought the system into quantum superposition, the causal order could exist in both directions at once, allowing multiple charging steps to work simultaneously rather than sequentially.
"With ICO, we demonstrated that the way a battery composed of quantum particles is charged can greatly affect its performance," Chen said. "We found huge improvements in both the energy stored in the system and the thermal efficiency. And somewhat counter-intuitively, we found a surprising effect of an interaction that was the opposite of what was expected: a low-power charger could deliver more energy while being more efficient than a high-power charger using the same device."
It may be hard for most people to understand, but quantum batteries may one day become a reality. For now, they exist only as laboratory experiments, but scientists are slowly testing different aspects of them, with the ultimate goal of figuring out how to integrate the parts into a working whole.
The research was published in the journal Physical Review Letters.