Observation of chaos-assisted tunneling between islands of stability

We report the direct observation of quantum dynamical tunneling of atoms be tween separated momentum regions in phase space. We study how the tunneling oscillations are affected as a quantum symmetry is broken and as the initi al atomic state is changed. We also provide evidence that the tunneling rat e is greatly enhanced by the presence of chaos in the classical dynamics. T his tunneling phenomenon represents a dramatic manifestation of underlying classical chaos in a quantum system.