The goal of this internship is to theoretically investigate the behavior of a quantum fluid of light as it undergoes a spontaneous, abrupt jump in light intensity at the onset of a bistable regime, within a microcavity hosting exciton-polaritons. This system, which is currently the focus of several experimental studies, constitutes a paradigmatic example of a driven-dissipative many-body system, where the interplay between interactions, external driving, and losses gives rise to rich nonequilibrium physics. We will explore the evolution of quantum fluctuations in this regime, with the aim of revealing the possible generation of quantum-entangled photons. The strong enhancement of quantum fluctuations accompanying the abrupt change in mean-field intensity bears analogies with quantum quenches in ultracold atomic systems, as well as with more exotic quantum phenomena such as the dynamical Casimir effect.