2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/173574Cavity opto-mechanics studies the coupling between a mechanical oscillator and a cavity field, with the aim to shed light on the border between classical and quantum physics. Here we report on a cavity opto-mechanical system in which a collective density excitation of a Bose-Einstein condensate is shown to serve as the mechanical oscillator coupled to the cavity field. We observe that a few photons inside the ultrahigh-finesse cavity trigger a strongly driven back-action dynamics, in quantitative agreement with a cavity opto-mechanical model. With this experiment we approach the strong coupling regime of cavity opto-mechanics, where a single excitation of the mechanical oscillator significantly influences the cavity field. The work opens up new directions to investigate mechanical oscillators in the quantum regime and quantum gases with non-local coupling.F.B. and S.R. contributed equally to this work. 6 pages, 4 figuresQuantum PhysicsOther Condensed Mattertext