  Cavity 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.
