Bogoliubov excitations driven by thermal lattice phonons in a quantum fluid of light

Abstract

Elementary excitations in weakly interacting quantum fluids have a correlated particle-hole nature that leads to spectacular macroscopic quantum phenomena such as superfluidity. This many-body character was established in the context of cold-atom condensates at thermal equilibrium in the framework of Bogoliubov's celebrated theory of the weakly interacting Bose gas. Bogoliubov excitations were also found to be highly relevant to driven-dissipative quantum fluid of light, with certain resulting phenomena strikingly analogue to their equilibrium counterparts, but also genuine out-of-equilibrium aspects. In this work, we investigate both theoretically and experimentally a regime in which the elementary excitations in a quantum fluid of light result dominantly from their interaction with thermal lattice phonons, namely the elementary vibrations of the crystal. By an accurate comparison with the theoretically predicted spectral function of the driven-dissipative quantum fluid we achieve a quantitative understanding of the particle-hole nature of the elementary excitations, and unveil a remarkable decoupling from thermal excitations which is expected to be relevant in equilibrium quantum fluids as well. Finally, we exploit this quantitative understanding to identify a crossover temperature around $1\,$K, below which the lattice phonons are sufficiently quieted down for the quantum fluctuations to take over in the generation of Bogoliubov excitations. This regime is highly desired as it is characterized by strong quantum correlations between Bogoliubov excitations.Comment: Main text: 22 pages 6 figures, Supplementary Information : 4 pages, 3 figure