  Subradiance, i.e. the cooperative inhibition of spontaneous emission by
destructive interatomic interference, can be realized in a cold atomic sample
confined in a ring cavity and lightened by a two-frequency laser. The atoms,
scattering the photons of the two laser fields into the cavity-mode, recoil and
change their momentum. Under proper conditions the atomic initial momentum
state and the first two momentum recoil states form a three-level degenerate
cascade. A stationary subradiant state is obtained after that the scattered
photons have left the cavity, leaving the atoms in a coherent superposition of
the three collective momentum states. After a semiclassical description of the
process, we calculate the quantum subradiant state and its Wigner function.
Anti-bunching and quantum correlations between the three atomic modes of the
subradiant state are demonstrated.
