Guillaume Cassabois: Matter in Light

Guillaume Cassabois works on an infinitely small scale, where quantum laws reign supreme. For those who fear they might not understand his research, the physicist from the Charles Coulomb Laboratory and professor at the University of Montpellier is reassuring: "We all live in a quantum world. " Lasers, MRI imaging, and the transistors that make up computers are technologies inherited from 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, which 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 least successful—project is the quantum computer, which would use the ability of matter to be in two states at once to calculate much faster.

After defending his thesis in 1999 at the École Normale Supérieure, Guillaume Cassabois devoted the early part of his career to the qubit, a quantum bit which, unlike our current computers where transistors manipulate 0s and 1s, can be both 0 and 1 at the same time. Then a lecturer at Jussieu, he attempted to manufacture this qubit by creating a quantum box, an object measuring a few nanometers made of indium arsenide. Ultimately, superconductors prevailed over the quantum box in the race for the qubit. The researcher then turned his attention to nanotubes, a nanostructure made from graphene, a material consisting of a layer of carbon atoms arranged in a honeycomb pattern, which can emit single photons. Among quantum applications, single photons are a sought-after commodity. As in quantum cryptography, where the impossibility of creating a perfect copy of a photon makes it possible to encode messages in an unbreakable way.

Silicon defects

When he joined the University of Montpellier in 2010, Guillaume Cassabois explored yet another new avenue: optical defects in silicon. "Such a defect exists in diamonds and allows single photons to be emitted 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 wavelengths used in telecom fiber optics."

Bingo. His 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 [researcher at L2C and winner of the 2025 Gustave Ribaud Prize fromthe French Academy of Sciences] was setting up a high-performance experimental device to study silicon carbide, an operation that Vincent Jacques (L2C) was able to support financially with leftover funds from his ERC grant. " He argues that collaboration and "nest eggs" in laboratories are crucial to staying in the race against the heavyweights from the United States and China. Today, the Montpellier team is contributing to pioneering international advances in the technological application of this discovery, which makes it possible to integrate quantum emitters into silicon chips.

Boron nitride on board

Taking advantage of another asset in his laboratory, ultraviolet optical spectroscopy, which Bernard Gil has mastered locally, Guillaume Cassabois is tackling the properties of another semiconductor material, boron nitride (BN). "At first, working on massive crystals measuring a few millimeters, I felt 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 feat "using two-photon spectroscopy in the deep ultraviolet" and revealed the exceptional optical properties of this material, which could be used to manufacture 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 Physics Society on the occasion ofthe 2025 Jean Ricard Grand Prize, which rewards a physicist for remarkable and original work. One example among many of the international influence of his work is that 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 soon in Korea.