  Ultracold atoms interacting with the optical modes of a high-Q optical ring
cavity can synchronize their motion. The collective behavior makes the system
interesting for quantum computing applications. This paper is devoted to the
study of the collective coupling. We report on the first observation of a
collective dynamics and on the realization of a laser, the gain mechanism of
which is based on collective atomic recoil. We show that, if the atoms are
subject to a friction force, starting from an unordered distribution they
spontaneously form a moving density grating. Furthermore, we demonstrate that a
1D atomic density grating can be probed via Bragg scattering. By heterodyning
the Bragg-reflected light with a reference beam, we obtain detailed information
on phase shifts induced by the Bragg scattering process.
