Rice scientists find that matter mediates ultrastrong coupling between photons

24 Apr 2025

Discovery described as “foundation for advances in quantum computing and technologies”.

Researchers at Rice University, Houston, Texas, have developed a new way to control light interactions using a specially engineered structure called a 3D photonic-crystal cavity. Their work, published in Nature Communications, promises technologies that could enable transformative advancements in quantum-based technologies.

“Imagine standing in a room surrounded by mirrors,” said Fuyang Tay, an alumnus of Rice’s Applied Physics Graduate Program and first author of the study. “If you shine a flashlight inside, the light will bounce back and forth, reflecting endlessly. This is similar to how an optical cavity works — a tailored structure that traps light between reflective surfaces, allowing it to bounce around in specific patterns.”

These patterns with discrete frequencies are called cavity modes, and they can be used to enhance light-matter interactions, making them potentially useful in quantum information processing, developing high-precision lasers and sensors and building better photonic circuits and fiber-optic networks. Optical cavities can be difficult to build, so the most widely used ones have simpler, unidimensional structures.

Tay, together with Rice doctoral alumnus Ali Mojibpour and other team members, built a complex 3D optical cavity to study how multiple cavity modes interact with a thin layer of free-moving electrons exposed to a static magnetic field. The key question guiding their investigation was what happens when multiple cavity modes interact with the electrons simultaneously.

“It is well known that electrons strongly interact with each other, but photons do not,” said Junichiro Kono, the Karl F. Hasselmann Professor in Engineering, at Rice, and the study’s corresponding author. “This cavity confines light, which strongly enhances the electromagnetic fields and leads to strong coupling between light and matter, creating quantum superposition states, known as polaritons.”

Polaritons offer a way to control and manipulate light at very small scales, which could enable faster and more energy-efficient quantum computing and communication technologies. Polaritons can also behave collectively, giving rise to states of quantum entanglement that could be used for new types of quantum circuits and sensors.

Ultrastrong coupling

If the interaction binding photons and electrons into polaritons is extremely intense, to the point where the exchange of energy between light and matter happens so fast it resists dissipation, a new regime comes into effect known as ultrastrong coupling.

“Ultrastrong coupling describes an unusual mode of interaction between light and matter where the two become deeply hybridized,” said Tay, who is a postdoctoral researcher at Columbia University.

The researchers used terahertz radiation to observe how the cavity modes and electrons couple inside the 3D optical cavity, navigating experimental challenges such as the need for ultracold temperatures and strong magnetic fields.

They found not only that different cavity modes interact with moving electrons in an ultrastrong coupling regime but also that this multimodal light-matter coupling is dependent on the polarization of the incoming light, which triggers one of two forms of interaction. “This suggests we can engineer materials where different cavity modes ‘talk’ to each other through the electrons in a magnetic field, creating new correlated states,” said Tay.

If initially the researchers were mainly focused on how the 3D photonic crystal cavity served to increase light-matter coupling, the realization that the setup could be used to induce matter-mediated photon-photon coupling came as an “aha moment” in the research, said Andrey Baydin, an assistant research professor of electrical and computer engineering at Rice and study co-author. “Matter-mediated photon-photon coupling can lead to new protocols and algorithms in quantum computation and quantum communications,” said Kono.

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