A battery is a device that stores energy: a quantum battery is no exception. In theory, it is a quantum mechanical system that stores the energy of photons rather than electrons and ions, as is the case with conventional electrochemical batteries. Unlike normal batteries, quantum batteries charge faster as their size increases thanks to quantum effects such as entanglement and superabsorption – a property that could prove useful in making more efficient light – harvesting devices, such as solar cells.
Several research teams around the world are working on the quantum battery concept, which was first formally proposed just 10 years ago by Robert Alicki of the University of Gdańsk in Poland and Mark Fannes of KU Leuven in Belgium. These devices take advantage of quantum particles which, unlike classical particles that have defined properties, can simultaneously be in a superposition of several states. Quantum particles can also influence other isolated particles, with the state of one instantly influencing the state of the others – regardless of the distance between them. This phenomenon, known as entanglement, allows a quantum battery to recharge more quickly, as the greater the number of entangled particles, the faster they collectively move from a low-energy state to a high-energy state1.
Quantum batteries could be exploited to improve the efficiency of solar cells.
Last year, James Quach and colleagues at the University of Adelaide in Australia demonstrated that this concept works even if all the quantum particles in the systems could not be fully entangled. Based on a simplified version of a model created by a team at the Italian Institute of Technology in Genoa2, their battery comprises molecules of a semiconducting organic dye, known as Lumogen F Orange, that are all identical and have a low-energy and a high-energy state. When exposed to light of a certain wavelength, a molecule in the low-energy state can absorb a photon and switch to the excited state.
A distributed Bragg reflector
James Quach and his colleagues placed the molecules between two highly reflective, micron-sized mirrors in a device known as a distributed Bragg reflector, which consists of several alternating layers of dielectric material. They then loaded these molecules with laser light. To ensure that the molecules absorbed the photons efficiently, they suspended them in an inert polymer matrix.
The researchers observed that the rate at which the mirror cavity absorbed light – that is, the rate at which the system charged – far exceeded what would be possible if each molecule absorbed light individually without any entanglement3. This effect is known as superabsorption and occurs because all the molecules act collectively through quantum superposition. They also found that the charging time decreased as they increased the size of the microcavity, and therefore the number of molecules.
With a billion extra molecules, a quantum battery would provide enough energy to light up a light-emitting diode.
Like any other quantum system, the battery will need to be isolated from its environment before it can be scaled up. This is due to a phenomenon called decoherence, which is the transition at which a quantum system starts to behave like a classical system. In the short to medium term, therefore, it is unlikely that quantum batteries will be able to power large objects such as electric vehicles. “However, they could be exploited to improve the efficiency of solar cells by improving the capture of low-light energy in photovoltaic materials,” explains James Quach. In this context, a small amount of decoherence may actually be beneficial for charge storage, as it would prevent quantum effects that rapidly discharge the battery.
“However, we still have a lot of work to do before we can reliably exploit superabsorption outside the lab,” he admits. “For example, current solar cells and cameras can store energy from a wide range of wavelengths, whereas our quantum battery can only absorb light at a specific wavelength. However, we are confident that we can scale the system and produce devices that can be easily integrated into existing technologies.”
Many challenges remain
While, in principle, quantum batteries could contribute to the energy transition, many challenges remain. One of these is finding a way to maintain the right level of energy that they can store and release it in a simple and reliable way.
Last but not least, the molecular cavity developed by James Quach and his colleagues only stores photons of light. To convert this light into usable electricity, they need to incorporate a conductive layer into which electrons from charged molecules can be transferred. Many more molecules will also have to be added to the system. With a billion more molecules, for example, a quantum battery might be able to provide enough energy to light up a light-emitting diode. These devices could also be used in small electronic devices such as watches, phones, tablets, or laptops – in fact, any product that needs the stored energy.
In the long term, the researchers obviously want to develop their batteries further. The stakes are high, because these devices could serve as small off-grid energy sources and power Internet of Things devices. They would be similar to current solar panels and batteries, but because the charging and storage functions are housed in a single system, they would be easier to integrate and use.
“The aim is to produce such devices within three to five years,” says James Quach.
