Guillaume Cassabois: Matter in Light

Guillaume Cassabois works on the scale of the infinitely small, where quantum laws reign. For those who might worry they won’t understand his research, the physicist at the Charles Coulomb Laboratory and professor at the University of Montpellier offers reassurance:“We all live in a quantum world. ” Lasers, MRI imaging, and the transistors that make up computers are technologies born of a century of research in quantum mechanics.

From arsenide to graphene

He studies semiconductor nanostructures using optical spectroscopy. In doing so, he is contributing to the second quantum revolution—the one that began in the 2010s and seeks to apply quantum properties at the atomic scale to new technologies. Among these technologies, the best-known—and perhaps the least advanced—project is the quantum computer, which would use matter’s ability to exist in two states at once to compute much faster.

After defending his dissertation in 1999 at the École Normale Supérieure, Guillaume Cassabois devoted the early part of his career to the qubit—that quantum bit which, unlike our current computers where transistors manipulate 0s or 1s, can be both 0 and 1 at the same time. While serving as a lecturer at Jussieu, he attempted to fabricate this qubit by creating a quantum dot—an object just a few nanometers in size made of indium arsenide. Ultimately, superconductors prevailed over quantum dots in the race to create the qubit. And the researcher shifted his focus to nanotubes, a nanostructure made from graphene—a material consisting of a single layer of carbon atoms arranged in a honeycomb pattern—which can emit single photons. Among quantum applications, the single photon is a highly sought-after commodity. As in quantum cryptography, where the impossibility of creating a perfect copy of a photon allows messages to be encoded in an unbreakable way.

The drawbacks of silicon

When he joined the University of Montpellier in 2010, Guillaume Cassabois set out to explore yet another new avenue: optical defects in silicon.“Such a defect exists in diamond and allows for the emission of single photons on demand at visible wavelengths. My intuition was to find the equivalent in silicon, a material widely used in microelectronics that could emit in the infrared, at the wavelengths of telecom optical fiber.”

Bingo. Her team was the first in 2019 to observe the defects they were looking for. Guillaume Cassabois emphasizes the role of the team in this achievement:“Anaïs Dréau [a researcher at L2C and the 2025 winner of the Gustave Ribaud Prize fromthe Academy of Sciences] was in the process of setting up a highly efficient experimental setup to study silicon carbide, an effort that Vincent Jacques (L2C) was able to support financially using leftover funds from his ERC grant. ” He further argues that collaboration andthe “small savings”in laboratories are crucial for staying competitive against the U.S. and Chinese heavyweights. Today, the Montpellier team is contributing to pioneering international advances in the technological application of this discovery, which enables the integration of quantum emitters into silicon chips.

Boron nitride on board

Taking advantage of another strength of his laboratory—ultraviolet optical spectroscopy, a field in which Bernard Gil is a local expert—Guillaume Cassabois is investigating the properties of another semiconductor material: boron nitride (BN). “At first, working on bulk crystals a few millimeters in size, I felt more like I was doing physics from the 1960s. I had no idea about the incredible physical properties of this material, which has a structure similar to graphene.”  Together with his colleague Pierre Valvin, they achieved an experimental breakthrough“using two-photon spectroscopy in the deep ultraviolet” and revealed the exceptional optical properties of this material, which could enable the manufacture of deep-ultraviolet LEDs for water sterilization and surface disinfection.

“This groundbreaking work marked the birth of a new field that is now booming: boron nitride-based photonics,” noted an article by the French Physical Society on the occasion ofthe 2025 Jean Ricard Grand Prize—an award that recognizes a physicist for outstanding and original work. As just one example of the international impact of his work, Guillaume Cassabois launched the first international workshop on boron nitride in Montpellier in 2023, a conference that has since been held in Australia, the United States, and will soon take place in Korea…