Switching 'spin' on and off in quantum materials

Researchers have found a way to control the interaction of light and quantum ‘spin’ in organic semiconductors that works at room temperature. Spin is known as the intrinsic angular momentum of electrons, which is referred to as up or down. Using the up/down spin states of electrons instead of the 0 and 1 in conventional computer logic could enhance how computers process information. And sensors based on quantum principles could improve the ability to measure and study the world around us.

Researchers from the University of Cambridge have used particles of light as a ‘switch’ that can connect and control the spin of electrons, making them behave like tiny magnets that could be used for quantum applications. The researchers designed modular molecular units connected by tiny ‘bridges’. Shining a light on these bridges allowed electrons on opposite ends of the structure to connect to each other by aligning their spin states. Even after the bridge was removed, the electrons stayed connected through their aligned spins.

This level of control over quantum properties can only be achieved at ultra-low temperatures. However, the researchers have been able to control the quantum behaviour of these materials at room temperature, which could enhance the potential of quantum applications by reliably coupling spins to photons. The research findings have been published in the journal Nature.

Almost all types of quantum technology — based on the behaviour of particles at the subatomic level — involve spin. As they move, electrons form stable pairs, with one electron spin up and one spin down. However, it is possible to make molecules with unpaired electrons, called radicals. Most radicals are reactive, but with careful design of the molecule, they can be made chemically stable. First author Sebastian Gorgon said these unpaired spins change the rules for what happens when a photon is absorbed and electrons are moved up to a higher energy level. “We’ve been working with systems where there is one net spin, which makes them good for light emission and making LEDs,” Gorgon said.

Organic semiconductors are currently used for lighting and commercial displays, and they could be a more sustainable alternative to silicon for solar cells. However, they have not yet been widely studied for quantum applications, such as quantum computing or quantum sensing. The researchers have now linked the optical and magnetic properties of radicals in an organic semiconductor. “These new materials hold great promise for completely new applications, since we’ve been able to remove the need for ultra-cold temperatures,” Gorgon said.

The researchers designed a new family of materials by determining how they wanted the electron spins to behave; then they were able to control the properties of the end material by using a building block method and changing the ‘bridges’ between different modules of the molecule. These bridges were made of anthracene, a type of hydrocarbon. For the ‘mix-and-match’ molecules, the researchers attached a light-emitting radical to an anthracene molecule. After a photon of light is absorbed by the radical, the excitation spreads out onto the neighbouring anthracene, causing three electrons to start spinning in the same way. When a further radical group is attached to the other side of the anthracene molecules, its electron is also coupled, bringing four electrons to spin in the same direction.

“In these materials we’ve designed, absorbing a photon is like turning a switch on. The fact that we can start to control these quantum objects by reliably coupling spins at room temperature could open up far more flexibility in the world of quantum technologies. There’s a huge potential here to go in lots of new directions,” Gorgon said.

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