  We demonstrate atomic four-wave mixing of two-component matter waves in a
collinear geometry. Starting from a single-species Bose-Einstein condensate,
seed and pump modes are prepared through microwave state transfer and
state-selective Kapitza-Dirac diffraction. Four-wave mixing then populates the
initially empty output modes. Simulations based on a coupled-mode expansion of
the Gross-Pitaevskii equation are in very good agreement with the experimental
data. We show that four-wave mixing can play an important role in studies of
bosonic mixtures in optical lattices. Moreover our system should be of interest
in the context of quantum atom optics.
