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Quantum batteries: rethinking energy storage is possible

James Quach
Chief Scientist at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia
Key takeaways
  • Quantum batteries have the potential to accelerate charging time and even harvest energy from light.
  • Unlike electrochemical batteries that store ions and electrons, a quantum battery stores the energy from photons.
  • Quantum batteries charge faster as their size increases thanks to quantum effects such as entanglement and superabsorption.
  • They will not be able to power electric vehicles but could improve the efficiency of solar cells and be used for small electronic devices.
  • Ultimately, the challenge is to evolve these batteries, as these devices could serve as true small off-grid power sources.

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.

Isabelle Dumé 

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