  More than 30 years ago, Thouless introduced the concept of a topological
charge pump that would enable the robust transport of charge through an
adiabatic cyclic evolution of the underlying Hamiltonian. In contrast to
classical transport, the transported charge was shown to be quantized and
purely determined by the topology of the pump cycle, making it robust to
perturbations. On a fundamental level, the quantized charge transport can be
connected to a topological invariant, the Chern number, first introduced in the
context of the integer quantum Hall effect. A Thouless quantum pump may
therefore be regarded as a 'dynamical' version of the integer quantum Hall
effect. Here, we report on the realization of such a topological charge pump
using ultracold bosonic atoms that form a Mott insulator in a dynamically
controlled optical superlattice potential. By taking in-situ images of the atom
cloud, we observe a quantized deflection per pump cycle. We reveal the genuine
quantum nature of the pump by showing that, in contrast to ground state
particles, a counterintuitive reversed deflection occurs when particles are
prepared in the first excited band. Furthermore, we were able to directly
demonstrate that the system undergoes a controlled topological phase transition
in higher bands when tuning the superlattice parameters.
