  In many experiments isolated atoms and ions have been inserted into
high-finesse optical resonators for the study of fundamental quantum optics and
quantum information. Here, we introduce another application of such a system,
as the realization of cavity optomechanics where the collective motion of an
atomic ensemble serves the role of a moveable optical element in an optical
resonator. Compared with other optomechanical systems, such as those
incorporating nanofabricated cantilevers or the large cavity mirrors of
gravitational observatories, our cold-atom realization offers direct access to
the quantum regime. We describe experimental investigations of optomechanical
effects, such as the bistability of collective atomic motion and the first
quantification of measurement backaction for a macroscopic object, and discuss
future directions for this nascent field.
