  We report on a two-particle matter wave interferometer realized with pairs of
trapped 87Rb atoms. Each pair of atoms is confined at a single site of an
optical lattice potential. The interferometer is realized by first creating a
coherent spin-mixture of the two atoms and then tuning the inter-state
scattering length via a Feshbach resonance. The selective change of the
inter-state scattering length leads to an entanglement dynamics of the
two-particle state that can be detected in a Ramsey interference experiment.
This entanglement dynamics is employed for a precision measurement of atomic
interaction parameters. Furthermore, the interferometer allows to separate
lattice sites with one or two atoms in a non-destructive way.
