Experimental realization of a quantum ratchet through phase modulation

We report on an experimental realization of unidirectional transporting island structures in an otherwise chaotic phase space of the δ-kicked rotor system. Using a Bose-Einstein condensate as a source of ultracold atoms, we employ asymmetric phase modulation in the kicks, with the narrow momentum distribution of the atoms allowing us to address individual island structures. We observe quantum ratchet behavior in this system, with clear directed momentum current in the absence of a directional force, which we characterize and connect to ε-classical theory.