Non-equilibrium Bose–Einstein condensation in photonic systems

The study of Bose–Einstein condensation effects in photonic systems has revealed a rich phenomenology related to spontaneous coherence generation in driven-dissipative spatially extended systems and is providing a new platform for the study of non-equilibrium phase transitions and critical behaviours. In this Review, we give an interdisciplinary overview of condensation phenomena in photonic systems. We cover a wide range of systems, from lasers to photon condensates in dye-filled cavities, to excitons in semiconductor heterostructures, to microcavity polaritons, as well as emerging systems, such as mode-locked lasers and classical light waves. Our aim is to highlight novel universal phenomena that stem from the driven-dissipative, non-equilibrium nature of these systems and affect the static, dynamic, superfluid and coherence properties of the condensate. Finally, we provide our perspectives on the future of fundamental science and technological applications in this field.