Bose-einstein condensation of exciton polaritons – A condensate made of interacting photons

Bose-Einstein condensation (BEC) is a phase of quantum matter where the ground state of a bosonic system is occupied macroscopically by single particles, creating long-range order and global coherence. It was predicted by Bose and Einstein at the beginning of the 20th century and has been realised experimentally for the first time almost 80 years later in a trapped gas of ultracold atoms. Today, BEC can be achieved in the solid state with gases made not of atoms but of bosonic quasiparticles called exciton polaritons. These quasiparticles arise from the strong coupling between excitons and photons in a semiconductor microcavity and inherit the properties of light and matter, essentially becoming interacting photons. Here, we present an overview of the recent advances in understanding the interactions between exciton polaritons and its consequences as observed in experiments. Additionally, exciton-polariton condensates can be tuned to behave either similarly to matter condensates, like cold atoms, or to condensates of photonic character, deviating from equilibrium theories that are not yet understood by any existing theory.