  A gas of electrons in a one-dimensional periodic potential can be transported
even in the absence of a voltage bias if the potential is modulated slowly and
periodically in time. Remarkably, the transferred charge per cycle is only
sensitive to the topology of the path in parameter space. Although this
so-called Thouless charge pump has first been proposed more than thirty years
ago, it has not yet been realized. Here we report the first demonstration of
topological Thouless pumping using ultracold atoms in a dynamically controlled
optical superlattice. We observe a shift of the atomic cloud as a result of
pumping and extract the topological invariance of the pumping process from this
shift. We demonstrate the topological nature of the Thouless pump by varying
the topology of the pumping path and verify that the topological pump indeed
works in the quantum region by varying speed and temperature.
