Light normally spreads out and escapes from optical devices, so it is not obvious how photons can ever behave like the atoms in a Bose–Einstein condensate (BEC). Yet experiments have shown that, under the right conditions, photons trapped in microscopic semiconductor cavities can collect into a single quantum state, even at room temperature. What has been missing is a clear explanation of how this happens in real semiconductor materials.

New research provides that explanation. The authors develop a detailed theory that tracks how photons interact with the electrons and holes inside a semiconductor while the system is continuously pumped with energy. Unlike earlier models that treated the semiconductor as a simple thermal background, this theory follows how all parts of the system evolve together, including particle collisions, energy losses and heat exchange with the surrounding material.

The key finding is that collisions between charge carriers (Coulomb scattering) allow the photons to share energy and effectively cool down. At high particle densities, this process is strong enough to make photons behave as if they were in thermal equilibrium, enabling them to form a condensate. This mechanism is very different from that in dye‑based photon condensates, where vibrations of molecules do most of the thermalising.

The theory also predicts several distinct regimes: ordinary thermal light, single‑mode and multimode photon condensation, and standard laser behaviour. Importantly, experiments can move between these regimes by adjusting parameters such as the cavity design and the pumping strength.

By explaining how quantum states of light can emerge in compact, room‑temperature semiconductor devices, this work could enable new quantum photonic technologies, including advanced light sources for communications, sensing and information processing.