Observation of quantum accelerator modes

We have realized a quantum accelerator mode using a system consisting of ul tracold cesium atoms falling through a pulsed standing wave of off resonant light. We present a picture of this system based on diffraction and show t hat the effect arises from the application of blazed matter wave diffractio n gratings. The implications of our results for quantum chaos and the prosp ect of constructing a large angular separation matter wave beam splitter ar e discussed.