  We calculate the Casimir-Polder frequency shift and decay rate for an atom in
front of a nonreciprocal medium by using macroscopic quantum electrodynamics.
The results are a generalization of the respective quantities for matter with
broken time-reversal symmetry which does not fulfill the Lorentz reciprocity
principle. As examples, we contrast the decay rates, the resonant and
nonresonant frequency shifts of a perfectly conducting (reciprocal) mirror to
those of a perfectly reflecting nonreciprocal mirror. We find different power
laws for the distance dependence of all quantities in the retarded and
nonretarded limits. As an example of a more realistic nonreciprocal medium, we
investigate a topological insulator subject to a time-symmetry breaking
perturbation.
