  The transport of a particle in the presence of a potential that changes
periodically in space and in time can be characterized by the amount of work
needed to shift a particle by a single spatial period of the potential. In
general, this amount of work, when averaged over a single temporal period of
the potential, can take any value in a continuous fashion. Here we present a
topological effect inducing the quantization of the average work. We find that
this work is equal to the first Chern number calculated in a unit cell of a
space-time lattice. Hence, this quantization of the average work is
topologically protected. We illustrate this phenomenon with the example of an
atom whose center of mass motion is coupled to its internal degrees of freedom
by electromagnetic waves.
