Experiments probing quasiparticles in semiconductor microcavities offer unprecedented insights into the dynamics of quantum fluids of light.

Superfluidity [1, 2], the ability of a fluid to flow without friction, isn’t restricted to systems described by hydrodynamics. Over a decade ago, optics researchers started to take an interest in superfluids and other quantum fluids [3], driven by the realization that light propagating in a nonlinear medium can exhibit quantum hydrodynamics features [4]. Two platforms emerged for the study of these “fluids of light”: semiconductor microcavities in which photons are confined [5] and propagating geometries in which photons travel in a bulk medium [6–8]. Both configurations allow photons to acquire an effective mass and experience an effective mutual interaction—properties that can lead them to collectively behave as a quantum fluid.