  We have observed parametric generation and amplification of ultracold atom
pairs. A 87Rb Bose-Einstein condensate was loaded into a one-dimensional
optical lattice with quasimomentum k0 and spontaneously scattered into two
final states with quasimomenta k1 and k2 . Furthermore, when a seed of atoms
was first created with quasimomentum k1 we observed parametric amplification of
scattered atoms pairs in states k1 and k2 when the phase-matching condition was
fulfilled. This process is analogous to optical parametric generation (OPG) and
amplification (OPA) of photons and could be used to efficiently create
entangled pairs of atoms. Furthermore, these results explain the dynamic
instability of condensates in moving lattices observed in recent experiments.
