One of the central paradigms for classical and quantum chaos in conservative systems is the twodimensional
billiard in which particles are confined to a closed region in the plane, undergoing elastic
collisions with the walls and free motion in between. We report the first realization of billiards using
ultracold atoms bouncing off beams of light. These beams create the desired spatial pattern, forming an
‘optical billiard.’? We find excellent agreement between theory and our experimental demonstration of
chaotic and stable motion in optical billiards, establishing a new testing ground for classical and quantum
chaos.