  Networks of globally coupled oscillators exhibit phase transitions from
incoherent to coherent states. Atoms interacting with the counterpropagating
modes of a unidirectionally pumped high-finesse ring cavity form such a
globally coupled network. The coupling mechanism is provided by collective
atomic recoil lasing (CARL), i.e. cooperative Bragg scattering of laser light
at an atomic density grating, which is self-induced by the laser light. Under
the rule of an additional friction force, the atomic ensemble is expected to
undergo a phase transition to a state of synchronized atomic motion. We present
the experimental investigation of this phase transition by studying the
threshold behavior of the CARL process.
