What Quantum Batteries Have in Store

Quantum battery technology is approaching an inflection point similar to the one quantum computing crossed a decade or so ago, escalating it from a theoretical curiosity to an engineering challenge worth solving.

Quantum batteries exploit the strange physical laws of the very small — the quantum world — to gain performance advantages over classical batteries. Recent research on charging speed advantages and loss-free storage suggests this technology is poised for growth over the next three to five years.

The quantum world is probabilistic rather than deterministic, and that applies to quantum batteries as well as quantum computers. If, for example, an energy-storage unit exhibits either a ground state or an excited state in the classical world, it acts instead as a smearing of both in the quantum world, which can be described with probability functions.

Similarly, the likelihood that the same unit transitions between states also can be defined with a probability function. This function is known as a transition amplitude, and it is key to explaining why quantum batteries offer the advantages they do.

An Australian-Italian-UK research group published a paper in the journal Science Advances this year about photosensitive dye molecules, known as Lumogen-F Orange, which can be used as storage units. The researchers confined these units, in groups of varying sizes, in an optical microcavity — a quantum battery prototype — and measured the rate at which photons were able to excite the various groups.

“I saw the potential in what (quantum batteries) could do if someone could realize this in a lab,” said James Quach, who conceived and managed the project. “I wanted to take it from the blackboard out into the laboratory.”

Acting as quantum units, each dye molecule had its own transition amplitude describing the probability it would transition from a ground state to an excited state.

Superextensive Charging
The magic of quantum batteries emerged when the dye molecules’ transition amplitudes were allowed to interfere with each other.

“The way that quantum batteries work is that these transition amplitudes, when you put them in a coherent state, interfere with each other very much in the way that waves interfere with each other, and produce crests when they constructively interfere and troughs when they’re destructive,” said Quach. “Through this constructive interference, the combined transition amplitude of the whole system was greater than the sum of the individual parts if they weren’t acting as one.”

In contrast, the fastest way to deliver energy into a battery in the classical world is through a parallel charging configuration, where every cell is charged simultaneously. The battery’s charging speed, in this setup, is limited by how fast a single cell can charge.

What Quach’s team found in their quantum demonstration was that the interference allowed the battery as a whole to charge faster than a classical parallel setup. Even better, they found charging speed to be “superextensive,” meaning it increased as more and more dye molecules — storage units — were added to the battery.

The microcavity setup physically demonstrated, for the first time, superextensive energy absorption — superabsorption — a phenomenon that Quach says can benefit everything from small-scale consumer electronics to electric vehicles and grid-scale storage systems.

Hurdles and bounds
Just where the limits of this superextensive speed lie has been the subject of interest of Juyeon Kim at the Institute for Basic Science (IBS) in Daejeon, Korea. Last year, Kim and fellow researchers Dominik Safranek and Dario Rosa published a paper in Physical Review Letters quantifying the bounds of the quantum charging advantage — the ratio of quantum charging speed versus classical charging speed.

“I wanted to make a very tight bound for the expected power for the general case,” said Kim. “In classical batteries, the power increases with the number of cells in parallel. But in quantum batteries, we can make the power increase with the square of the number of cells.”

In practice, however, Quach’s team found their battery’s charging speed could only scale with the square root of N, a difference that warrants a deeper look into the implementation options for quantum batteries.

The charging advantages of these devices arise from an effect known as collective charging, where a battery’s units genuinely share the battery’s power source — in a way, communicate with each other — instead of the every-cell-for-itself strategy of classical batteries.

“Collective charging is kind of a shortcut,” said Kim. “We can separate the cells (in a classical setup) and there’s no other effect. But in the quantum battery, we cannot separate the cells if we want the quantum advantage, if we want collective charging.”

Quantum batteries can leverage one of two quantum phenomena to implement collective charging — quantum entanglement or quantum coherence.

Quantum entanglement, which Albert Einstein dubbed “spooky action at a distance,” existentially links particles together, allowing them to behave as a single unit despite physical separation. Although Kim’s team focused on entanglement in their paper in quantifying quantum advantage, they also acknowledged its fragility.

“Entanglement is very easily broken down by the environment” and notoriously difficult to maintain, said Kim. Quantum computers, for example, tend to operate at temperatures near absolute zero in pursuit of entanglement longevity.

Quach, for this reason, saw more practicality down the coherence route, even if it offered less of a quantum advantage. While quantum coherence also is susceptible to collapse, it maintains stability better than entanglement, even at room temperatures.

Furthermore, in addition to superabsorption, the optical cavity prototype demonstrated that decoherence, if applied judiciously to a quantum battery, can help control its storage and discharge phases. Or put in perspective, a little bit of a bad thing actually might be good.

“If I charge the battery very quickly, because quantum mechanics is time-symmetric, it should discharge very quickly,” explained Quach. “But decoherence makes this asymmetric, which means you can charge it quickly, but then it will discharge very slowly with decoherence.

Loss-free storage
Scientists from the University of Alberta, in partnership with the University of Toronto, published research in 2019 that detailed such symmetry-breaking perturbations and how quantum batteries might use them to enter dark states and achieve loss-free energy storage.

“The use of symmetry-protected dark states effectively decouples the battery from its environments, making it possible to perfectly store the excitation energy,” stated the Journal of Physical Chemistry C paper. “In contrast to conventional electrochemical batteries, the charged excitonic quantum battery does not ‘discharge’ over time in the presence of environments, a remarkable feature stemming from the quantum nature of the system.”

The research studied, as its quantum battery prototype, a para-benzene-like structure that accumulated excitons, and subjected the structure to numerical simulations that demonstrated immunity to environmentally-induced losses.

A possible inflection
The loss-free paper, one of the first to explore quantum batteries in a dissipative environment, and the real-world optical cavity demonstration, may herald a shift in how researchers approach the technology.

“Traditionally, because this was always the simplest way out, most of the works that dealt with quantum batteries initially dealt with isolated quantum systems, meaning subsystems that did not interact with the environment,” said Juzar Thingna, quantum thermodynamics researcher at the University of Massachusetts. “The goal was rather simplistic.”

Dissipative environments, however, represent “real situations much more than those idealistic situations where the quantum system were fully isolated,” Thingna said. The change in focus toward how these devices will interact with their parent systems suggests the field is moving closer to reality.

Another cause for shift, said Quach, is recognizing that the problems facing quantum batteries differ from those facing quantum computers, and how this recognition may help fast track quantum batteries to commercial applications.

“Any sort of decoherence in quantum computing [ruins it],” he said. “It just doesn’t work. And that’s the challenge (for quantum computing). But decoherence is a good thing for quantum batteries and, because of this, the big hurdle for quantum computing doesn’t apply for quantum batteries. In a sense, it is much easier than quantum computing, but it has started a lot later.”

Applications
“So why go quantum?” asked Thingna. “If a classical battery is just working fine, why do I need to go quantum? It’s storing energy. It’s doing the job that it’s meant to do. Why do I need to invest $1 billion or $2 billion of funding into something that will do as good?”

Certainly, batteries with charging speeds that thrive on scale and offer loss-free storage will find their place in the world.

The IBS paper on quantum charging advantage attracted a bit of media attention earlier this year, partly because it provided a layman’s gateway into the promise of quantum batteries — they can be used to charge electric cars much more quickly. But while the prospect of 10 hours of charging time compressed into seconds grabbed the public’s attention, practical considerations, such as charger power and coherence protection, are still in the research phase for electric vehicles.

One of the first applications for quantum batteries, believes Quach, will be light harvesting, which neatly side steps the charging power constriction by way of the sun providing a pseudo-ubiquitous power source. He intends to extend his existing work with photosensitive quantum batteries by scaling them up.

“The idea of superabsorption is that it should absorb better than classical absorption, and therefore we hope that it will take solar cell technology to a new level,” he said. But he notes that consumer electronics and electric vehicles far behind. Given sufficient funding, consumer applications could arrive within three to five years, he said.

Thingna envisions public transport — massive trains that require range and quick charging turnarounds — as a prime candidate for quantum battery use. But even without the lure of rapid charging and loss-free storage, engineers must soon contend with quantum batteries. “We are going to be miniaturizing things very soon,” he said. “We are already on that path. The problem with miniaturization is if you go too small, all your devices will hit a bottleneck and your classical laws of physics will no longer work. You cannot avoid quantum physics.”