"Quantum materials could have an impact way beyond quantum computing."Mackillo Kira, University of Michigan

A new tool that uses light to map out the electronic structures of crystals could reveal the capabilities of emerging quantum materials and pave the way for advanced energy technologies and quantum computers, according to researchers at the University of Michigan, and the universities of Regensburg and Marburg in Germany. The researchers report their work in a paper in Science.

"Quantum materials could have an impact way beyond quantum computing," said Mackillo Kira, professor of electrical engineering and computer science at the University of Michigan, who led the theory side of the new study. "If you optimize quantum properties right, you can get 100% efficiency for light absorption."

Silicon-based solar cells are already becoming the cheapest form of electricity, although their sunlight-to-electricity conversion efficiency is rather low, at around 30%. Emerging 2D semiconductors, which consist of a single layer of crystal, could do much better – potentially converting up to 100% of sunlight. They could also elevate quantum computing to room temperature, compared with the near-absolute-zero temperatures required for the quantum devices demonstrated so far.

"New quantum materials are now being discovered at a faster pace than ever," said Rupert Huber, professor of physics at the University of Regensburg, who led the experimental work. "By simply stacking such layers one on top of the other under variable twist angles, and with a wide selection of materials, scientists can now create artificial solids with truly unprecedented properties."

The ability to map these properties down to the atomic level could help streamline the process of designing materials with the right quantum structures. But ultrathin 2D materials are much smaller and messier than earlier crystals, and the old analysis methods don't work. Now, these 2D materials can be measured with the new laser-based method at room temperature and pressure.

The tool measures processes that are key to solar cells, lasers and optically driven quantum computing. Essentially, electrons pop between a 'ground state', in which they cannot travel, and states in the semiconductor's 'conduction band', in which they are free to move through space. They do this by absorbing and emitting light.

The new quantum mapping tool uses a 100 femtosecond (100 quadrillionths of a second) pulse of red laser light to pop electrons out of the ground state and into the conduction band. Next, the electrons are hit with a second pulse of infrared light, which pushes them so that they oscillate up and down an energy 'valley' in the conduction band, a little like skateboarders in a halfpipe.

The researchers use the dual wave/particle nature of electrons to create a standing wave pattern that looks like a comb. They discovered that when the peak of this electron comb overlaps with the material's band structure – its quantum structure – electrons emit light intensely. That powerful light emission, along with the narrow width of the comb lines, helps create a picture so sharp that researchers call it super-resolution.

By combining that precise location information with the frequency of the light, the researchers were able to map out the band structure of the 2D semiconductor tungsten diselenide. They could also get a read on each electron's orbital angular momentum, through the way the front of the light wave twisted in space. Manipulating an electron's orbital angular momentum, known also as a pseudospin, is a promising avenue for storing and processing quantum information.

In tungsten diselenide, the orbital angular momentum identifies which of two different 'valleys' an electron occupies. The messages that the electrons send out can reveal not only which valley an electron is in but also what the landscape of that valley looks like and how far apart the valleys are, which are the key elements needed to design new semiconductor-based quantum devices.

For instance, when the team used the laser to push electrons up the side of one valley until they fell into another, the electrons emitted light at that drop point, too. This light gives clues about the depths of the valleys and the height of the ridge between them. With this kind of information, researchers can figure out how the material would fare for a variety of purposes.

This story is adapted from material from the University of Michigan, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.