  We demonstrate a source for correlated pairs of atoms characterized by two
opposite momenta and two spatial modes forming a Bell state only involving
external degrees of freedom. We characterize the state of the emitted atom
beams by observing strong number squeezing up to -10 dB in the correlated
two-particle modes of emission. We furthermore demonstrate genuine two-particle
interference in the normalized second-order correlation function $g^{(2)}$
relative to the emitted atoms.
