Quantum batteries: Strange technology that could provide instant power

 

Quantum batteries: Strange technology that could provide instant power

By leveraging a bizarre property of quantum mechanics called entanglement, quantum batteries could theoretically recharge in a flash. Now, progress is being made towards making them a reality.

THE battery, as US comedian Demetri Martin pointed out, is one technology that we personify. “Other things stop working or they break,” he said. “But batteries – they die.” The observation is keener than it may at first appear. So beholden are some of us to smartphones, tablets and other digital technology, that our lives pretty much go on hold when they run out of juice. Even if it is just 30 minutes, we are apt to mourn the time lost to recharging.


If that seems like a laughable reaction, there is a serious side to this when it comes to the batteries that power electric vehicles. The fact that it usually takes hours to charge them is a major stumbling block to decarbonising transport, which is among the biggest global emitters of greenhouse gases. For humanity’s sake, charging times need to be slashed. Yet, with the fundamentals of battery science the same as they were half a century ago, the prospect of a drastic improvement looks slim.


Slim, but not impossible. Now, quantum physics could ride to our rescue. By leveraging the strange behaviour of subatomic particles, a quantum battery could charge itself much faster than any conventional device. As a handy bonus, the bigger a quantum battery, the better it performs. Although the concept is in its infancy, a recent experimental demonstration and some theoretical advances suggest that a world of uninterrupted portable power isn’t so far-fetched. One day, dead batteries could spring back to life in an instant.


Technically speaking, a battery is anything that can store energy, from the wind-up spring inside a child’s toy to the lake created by a dam that stems the flow of a river. For most of us, though, the word is associated with electrochemical batteries, either the single-use ones in your TV remote or the rechargeable ones in smart devices and electric cars, bikes and scooters. In fully charged electrochemical batteries, electrons and ions are forcibly separated from the electrodes they are attracted to, storing energy. Turn on the device the battery is in – that is, connect the battery’s electrodes – and suddenly these charged particles are able to flow, releasing their pent-up energy as electricity.


The limits of lithium ion


The current supremacy of the lithium-ion rechargeable battery, which won its developers, John B. Goodenough, M. Stanley Whittingham and Akira Yoshino, the Nobel prize for chemistry in 2019, is down to its relatively high capacity and longevity. It has also proved faster to charge than the competition, but only by so much. Charge a lithium-ion cell too quickly and the lithium ions stick irreversibly to the positive electrode, rendering the battery useless, even potentially explosive, over time. As a result, electric cars that use today’s lithium-ion technology will probably always take significantly longer to charge than the time to refill a petrol or diesel vehicle.


Practical issues such as these weren’t uppermost when Robert Alicki at the University of Gdańsk in Poland and Mark Fannes at KU Leuven in Belgium first formally proposed the concept of the quantum battery a little under a decade ago. As theoretical physicists, they were interested in whether the basic concept of such a battery could shed any light on a bigger question that has troubled physicists for several generations: why is the behaviour of small numbers of isolated particles so different to the behaviour of visible everyday objects, which are, after all, just sizeable collections of those very same particles? As physicists see it, there is a puzzling transition between the “quantum” world of the very small and the “classical” world of the reasonably big.


The quantum world is slippery by nature. Out of sight – or, to put it properly, unobserved by any means – a single particle ceases to have definite properties. Rather, it is in a superposition, simultaneously both here and there, in this state or that state. It can also act in concert with other isolated particles, the state of one instantly influencing the states of the others, even over vast distances, a phenomenon known as entanglement.



Quantum thermodynamics


Scientists who develop batteries don’t usually concern themselves with such mysterious goings-on. The charge of a conventional battery involves the displacement of 10 billion billion electrons or more, a long way from the usual quantum physics description of a small number of particles. For battery scientists, a classical theory that deals with tangible, large-scale properties such as heat, energy and physical work will do fine. That theory, historically, has been thermodynamics.


Thermodynamics is unmatched in its ability to describe the workings of engines, heat pumps, boilers, batteries and other familiar sources of power. It has even been usefully applied to much more exotic objects, from black holes to the universe itself. The key to its success has been its insensitivity to what individual particles get up to. Most practical systems consist of large numbers of component particles, so it has been sufficient, in the scheme of thermodynamics, to deal with averages. In recent years, however, theorists have begun to pick at this assumption. Are thermodynamic laws still valid if they are applied to individual particles, with their quantum quirks? And might the answer to this shed light on the transition between the quantum and classical worlds? What better way to try to get at the answers than a device defined only by its ability to store energy temporarily, however big or small. The battery was, for Alicki and Fannes, a potential testing ground for such questions. The pair set out to devise a battery based on quantum rules.


The benefits of entanglement


Superficially, a quantum battery isn’t much different from an ordinary one. It consists of “stuff” that prefers to exist in a low-energy state, like water at the bottom of a dam, but which can be forcibly put in a high-energy state until it needs to power something. In a quantum battery, this stuff comprises quantum bits or “qubits”. This refers to anything that can exist in a superposition of different states at once, such as electrons, ions, molecules or pulses of light. Theoretically, it doesn’t matter what the qubits are made of, though practically they must be amenable to precise manipulation in order to generate quantum entanglement – in this case, an inextricable link between at least two qubits. In 2013, Alicki and Fannes calculated that, as more and more qubits in a quantum battery are entangled, the amount of energy that can be extracted from that battery approaches a thermodynamic limit. In other words, they found that entanglement makes a battery’s capacity as big as it can be in the classical view.


This lack of advantage over conventional batteries could be seen as an anticlimax. Despite the seemingly unreal nature of quantum physics, the capacity of a battery devised on its principles is still subject to the same limit as that of any other battery. “Thermodynamics is neither quantum nor classical,” says Alicki. “It provides a kind of ultimate censorship for physical theories.” Indeed, quantum physics may not even provide the leg-up to the ultimate thermodynamic limit that Alicki and Fannes thought possible. Later in 2013, Karen Hovhannisyan at the ICFO-Institute of Photonic Sciences in Barcelona, Spain, and others showed that the same amount of energy can still be extracted from a quantum battery without entanglement, so long as many attempts at extraction can be made.


Still, Alicki and Fannes’s work did fuel a suspicion that, whatever the thermodynamic constraints, a quantum battery could hold an advantage over a classical one. In 2015, theorists including Kavan Modi at Monash University in Victoria, Australia, realised that, while it might not improve the total extractable energy of a battery, quantum entanglement could improve something of perhaps greater practical utility: how fast a battery can be charged.


Why this is so takes us deep into the mysteries of quantum behaviour. When a classical battery charges, it effectively has to go on a journey from a low-energy state to a high-energy state. Think of it like a cyclist pedalling along a straight path from A to B. The bigger the energy gap, the longer the journey and the longer the charging time. In a quantum battery with only a single qubit, the same would be true. But entangle a second qubit with the first and a shortcut emerges. The original journey from A to B is a straight line only when seen from above; seen now from the side, myriad hills and valleys that were secretly slowing the cyclist’s journey become apparent. Working in tandem, the two qubits are able to ride a flatter, faster route to the high-energy, or charged, state.


It doesn’t end there. Entangle a third qubit and another unforeseen dimension to reality suddenly manifests, one that betrays a still-faster route to a full charge. A fourth, a fifth, a sixth qubit: with every addition, the indirectness of the original route is peeled away, and a faster line becomes possible. In fact, according to Modi and his colleagues, the charging time of a quantum battery is inversely proportional to the number of entangled qubits. The bigger the battery, the faster it charges.


No one knows how best to interpret this higher-dimensional character of the quantum realm – whether the extra dimensions have a physical reality or whether they represent the surface of a deeper reality we have yet to fully uncover. What physicists do know about calculations involving them is that, to put it simply, they work. Super-powerful quantum computers, super-secure quantum communications, super-sensitive quantum detectors – all these technologies involve similarly intangible higher-dimensional mathematics, and have been proven experimentally.


In January, James Quach at the University of Adelaide in Australia and others demonstrated the advantage of the quantum battery in practice, too. Based on a blueprint created by physicists at the Italian Institute of Technology in Genoa, their simplified quantum battery consisted of molecules of an organic dye – not qubits proper, as they couldn’t be fully entangled, but all identical and having a low-energy and a high-energy state. The experimenters put the molecules in a cavity between two small mirrors and shone a laser on them. The result was a speed of light absorption, in effect a charging of the overall battery, that far exceeded what would have been possible if every molecule were absorbing by itself, without entanglement of any sort.


The full impressiveness of this experiment depends on your point of view. There were about 10 billion molecules, a lot for a quantum system, but holding an electronic charge equivalent to just one-billionth that of an AA battery. Because the quantum charging advantage depends on the system’s isolation from its surroundings, theorists, including Alicki and Modi, are doubtful that it will prove possible to scale up for practical use. “For us, the theory was just an interesting playground to explore fundamental ideas of time and energy,” says Modi. “I don’t think there will be technological applications. Of course, I may be completely wrong.”



Drive-through charging


Others are more optimistic, stressing that problems of isolation affect all quantum technologies, not just batteries. In April, Ju-Yeon Gyhm, Dominik Šafránek and Dario Rosa at the Institute for Basic Science in South Korea carried out further theoretical work looking at the maximum possible charging speed-up for quantum batteries. They point out that, while relatively small in total, the energy stored in the experiment by Quach and his colleagues had a density roughly equivalent to that of a common lead-acid car battery. In principle, they say, future quantum batteries could charge so swiftly that the operation is practically unnoticeable. For electric cars, it is possible to imagine a drive-through charging station, one in which you don’t even need to stop. Such a future may be some way off, however. “Maybe I won’t live long enough to see it,” says Rosa, who is in his late 30s.


Help could actually be hiding in plain sight. Dario Ferraro, a theorist at the University of Genoa in Italy, believes progress can be made by capitalising on work on existing quantum computers, which are also built from qubits and which, more importantly, have huge investment behind them. In April, he and his colleagues in Genoa and at the CERN particle physics laboratory near Geneva, Switzerland, showed that an IBM quantum computer might actually perform better as a battery. This is because errors that affect its quantum computation, and which are hard to reduce as such machines scale up, have little effect on energy storage. “Quantum computers are already quite well designed to be quantum batteries,” says Ferraro, possibly even as self-powering devices.


Quach sees advances in unexpected areas as well. For example, the quantum boost behind the accelerated light absorption in his group’s experiment could also be exploited to improve the efficiency of solar cells, he says. But he isn’t losing sight of the quantum battery as a potentially revolutionary concept in itself. Although the isolation required for quantum effects poses issues for scaling, he points out that a certain amount of contact with the classical environment, technically known as decoherence, is actually good for charge storage, as it prevents quantum effects from rapidly discharging the battery again. “It needs some tuning,” he says. “Too much and it stops the charging behaviour.” For the quantum battery, purely from a practical point of view, controlling the tension between the quantum and classical worlds looks important.


On that practical note, Quach and his colleagues have work to do. Their molecular cavity stored only photons of light. To convert that light to electricity that is usable in classical, modern technology, they must somehow incorporate a conductive layer that electrons in the charged molecules can hop into. They need to rope in more molecules, too, perhaps billions more. After all that, he says, they might have a quantum battery powerful enough to illuminate a tiny light-emitting diode, like the one that shows your TV is on standby.


No immediate solution to the climate crisis, then. But coming from the quantum world, the dazzling workings of which are almost always hidden from view, that little light would be an exciting glimmer of progress.

CAM