The harsh reality of quantum batteries

In theory, quantum batteries recharge extremely rapidly but translating this to the real world is a different matter.

There’s been plenty written about quantum batteries over the past 10 years but it’s unlikely they will replace conventional batteries for everyday appliances anytime in the near future.

When quantum batteries were envisaged about a decade ago, scientists did not foresee any technological application of their model. Quantum batteries were invented to study how energy moves in quantum systems, not to be sold in shops.

A quantum battery is only a theoretical concept for now (albeit buttressed by recent proof-of-principle experiments).

Yet, somehow the idea of quantum batteries took off with the media and scientists alike. In the past 10 years, nearly 300 papers on this topic have been written. These works examine different platforms for quantum batteries, different charging and discharging mechanisms, stability conditions, energy density and everything else that is required to build batteries.

Still, some researchers do not see strong potential for quantum batteries to be a commercial product. It is hard to see how quantum batteries could ever surpass today’s batteries.

This is because the former would require careful quantum control possible only in certain environments, like a laboratory. The batteries we use daily do not require such careful control and can be used almost anywhere. Still, it is a scholarly topic that is worth pursuing.

Quantum batteries offer a fertile playing ground for exploring and testing new ideas of optimal control of quantum systems – a highly relevant topic as quantum engineers are constructing ever more complex implementations of quantum technology with an increasing number of control parameters and system components.

The story begins with researchers re-examining the laws of thermodynamics in the 2010s with advances in the theory of quantum entanglement.

The first quantum revolution brought us into the information age and exotic materials like graphene. The second revolution promises new ways of computing, communicating and sensing.

A quantity central to the second quantum revolution is entanglement. It seems to play a crucial role for quantum computing and other nascent quantum technologies, including batteries. While the term dates back to the 1930s, it was not fully understood until the late 1980s and 1990s.

Because of its importance, a great deal of effort was dedicated to the theory of entanglement.

These quests always have two sides. Scientists wanted to understand how entanglement could be used for practical purposes. And there were many foundational tasks (posing philosophical questions) that gave surprising answers in the presence of entanglement.

Pioneering experiments that probed these foundational implications underlie the most recent Nobel Prize in Physics last year.

In the early 2010s, quantum information scientists began to apply the newly discovered toolkit of quantum entanglement to re-examine the theory of thermodynamics.

The hope was to discover thermodynamic phenomena that are truly quantum. This is mostly motivated by foundational questions, as the accepted facts of thermodynamics are at odds with those of quantum (and classical) mechanics.

Identifying thermodynamic phenomena with genuine quantum features turned out to be surprisingly difficult as energetic phenomena seem to be indifferent as to whether the underlying system is quantum or classical.

However, some researchers noted that time behaved somewhat differently in the quantum realm than in classical, which in turn affects power (work done in a unit of time).

They found that quantum mechanics allows for energy to be deposited (extracted) significantly faster when multiple systems are addressed collectively rather than individually. This theoretical quantum system was dubbed a quantum battery.

Scientists then demonstrated what is the maximum charging speed allowed within the rules of quantum physics, as well as what is realistic when taking into account the structure of real physical interactions. The latter showed that we should anticipate only modest gains in power.

To formally prove these relationships, they made use of a quantum phenomenon known as the quantum speed limit. Originally discovered by two Soviet physicists in 1945, the quantum speed limit can be understood in analogy to Werner Heisenberg’s uncertainty principle which states a trade-off relationship between precision in momentum and position, albeit with respect to time and energy.

Using the quantum speed limit along with the effects of quantum entanglement, researchers first showed that one can move more energy in less time in quantum systems.

Later it turned out that entanglement is not strictly needed. It appears that the capacity to generate it in principle suffices for a speedup, even if no entanglement is actually created.

There is a fundamental connection between energy and information, which mandates processing information, for example by a computer, must lead to wasted energy (or heat). This leads to a trade-off in the speed of a computer and its heat output, which accounts for over a tenth of global energy consumption.

Quantum control through quantum batteries research could reduce the heat production by computers, data centres and networks and thus contribute to faster decarbonisation.

But there also lie unanswered questions: is there a fundamental connection between energy waste, energy-density, time, and information? Could we write down a single equation (principle) that proves this? In other words, there remains a lot of interesting research to be done on whether or not quantum batteries will ever become a replacement for conventional batteries.

With further research, the science that underlies them, however, may prove rather useful in other applications such as quantum computing and sensing. In these technologies precise quantum control of small systems is of the essence for their very function and not an additional hurdle.

Professor Felix Binder is with the School of Physics, Trinity College Dublin and Professor Kavan Modi is with the School of Physics and Astronomy, Monash University, Melbourne. They declare no conflict of interest.

Originally published under Creative Commons by 360info™.