The peculiar nature of light-matter fluids
Recent experiments with many-body systems, strongly coupled to light, challenge our common notions of quantum matter. The emergence of many-body correlations is augmented with the need to pump these systems with lasers, defining a new class of matter far from thermal equilibrium. In this talk I will focus on the fundamental questions raised by experiments purporting to show evidence for Bose condensation of exciton-polaritons in two dimensional quantum-well optical cavities. What is the nature of the phase transition at which these correlations are established? Can a two dimensional superfluid exist under the non-equilibrium conditions set by the drive?
I will present surprising answers to these question, arguing in particular that the non-equilibrium fluctuations are fundamentally different from the effect of temperature. In isotropic systems these fluctuations hinder the formation algebraic correlations that would have been established in an isotropic two dimensional thermal Bose fluid. The condensate seen in current experiments is thus interpreted as an intermediate scale crossover phenomena, while a correlated “fluid of light” with no equilibrium counterpart is established at longer scales. We further predict that equilibrium-like condensation can occur only in sufficiently anisotropic systems, but it does so in a peculiar reentrant manner. We obtain these results through a mapping of the long-wavelength condensate dynamics onto the Kardar-Parisi-Zhang equation.