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By Rachel Hergett, MSU News Service

BOZEMAN — Researchers from Montana State University, working with colleagues at Columbia University, have made a breakthrough in the science of materials used for quantum information technologies with a study involving two-dimensional materials and individual photons of light.

Nicholas Borys, an assistant professor in the Department of Physics in the College of Letters and Science, and frequent collaborator James Schuck, an associate professor of mechanical engineering at Columbia University's Engineering School in New York, have been conducting research into 2D materials — that is, materials with atoms arrayed in a single layer — since a flurry of research findings in 2015 showed sources within the materials that emit light one photon at a time.

But findings then weren’t able to determine why this single-photon release was happening, which would be key to further development of the materials for use in quantum technologies that work on the single-photon level, Borys explained. Single-photon emitters in 2D materials would be more easily integrated into devices than those made of other, more complex materials.

“We are trying to identify the functional materials for technologies down the road,” Borys said. “There’s lots of promise for quantum computing, quantum communications and quantum sensing.”

For single-photon emission, a major problem is that it occurs on very small length scales.

“This makes it difficult to understand what specifically in the material results in the single-photon emission,” said Tom Darlington, the study’s lead author and a post-doctoral researcher with Schuck. "You need light to observe these states, but their sizes are so small that they can't be studied with standard microscopes.”

Instead, they used nano-optical microscopy, which can be 30 times more sensitive than traditional microscopy. That method allowed Borys, Darlington and Schuck to directly image potential single-photon emitters around nanobubbles in tungsten diselenide, which is a stable 2D semiconductor composed of one layer of tungsten atoms sandwiched between two layers of selenium atoms.

The scientists’ results were published Monday as “Imaging strain-localized excitons in nanoscale bubbles of monolayer WSe2 at room temperature” in the journal Nature Nanotechnology.

“Our results mean that fully tunable, room-temperature single-photon emitters are now within our grasp, paving the way for controllable — and practical — quantum photonic devices,” Schuck said.

The nanobubbles, typically ultrasmall pockets of fluid or gas that get trapped between layers of 2D materials, create strain in the 2D material and lead to a ring of wrinkles around the edge. Picture strain as the flex of a rubber balloon, Borys explained. Press on the balloon and it gives. It becomes more translucent. Similarly, strain in 2D materials is a stretching of the atomic bonds that changes the electronic and optical properties of the material.

Conventional models say light should be released at the center of the nanobubble, at the apex of its curve where the strain is at a maximum. But a 2019 study led by Frank Jahnke at Germany’s Institute for Theoretical Physics predicted wrinkles at the edges of the strained area could create space to emit single photons. Borys and Schuck’s research corroborated Jahnke’s theoretical models. It also shows that strain, not imperfections in the crystalline structure of the material, elicits this single-photon emitting behavior.

For the most recent study, the data from nano-optical microscopes was fed directly into Jahnke’s numerical calculations, showing a strong correlation between the sites of single-photon emitters and the edge wrinkles. Some agreement was expected, but Borys said he was amazed that the two worked so strongly together.

“We have basically a new understanding of how strain controls the light-emitting properties in 2D materials,” Borys said. “It also demonstrates the power of combining this sophisticated, microscopy technique with its complement in numerical calculations. The results emerge from a nearly one-of-a-kind synergy between cutting-edge theoretical calculations with cutting-edge experimental work.”

Borys and Schuck will build on this new understanding to develop controllable and tailorable single-photon emitters from 2D materials as well as even more sophisticated non-classical sources of light.

“We have new many new routes to explore for controllably generating quantum states of light using 2D material,” Borys said.

Borys and Schuck will build on this new understanding to develop controllable and tailorable single-photon emitters from 2D materials as well as even more sophisticated non-classical sources of light.

“We have new many new routes to explore for controllably generating quantum states of light using 2D materials,” Borys said.