  The study of topological effects in physics is a hot area, and only recently
researchers were able to address the important issues of topological properties
of interacting quantum systems. But it is still a great challenge to describe
multi-particle and interaction effects. Here, we introduce multi-particle
Wannier states for interacting systems with co-translational symmetry. We
reveal how the shift of multi-particle Wannier state relates to the
multi-particle Chern number, and study the two-boson Thouless pumping in an
interacting Rice-Mele model. In addition to the bound-state Thouless pumping in
which two bosons move unidirectionally as a whole, we find topologically
resonant tunneling in which two bosons move unidirectionally, one by the other,
provided the neighboring-well potential bias matches the interaction energy.
Our work creates a new paradigm for multi-particle topological effects and lays
a cornerstone for detecting interacting topological states.
