  We consider a quantum impurity model in which a bosonic impurity level is
coupled to a non-interacting bosonic bath, with the bosons at the impurity site
subject to a local Coulomb repulsion U. Numerical renormalization group
calculations for this bosonic single-impurity Anderson model reveal a
zero-temperature phase diagram where Mott phases with reduced charge
fluctuations are separated from a Bose-Einstein condensed phase by lines of
quantum critical points. We discuss possible realizations of this model, such
as atomic quantum dots in optical lattices. Furthermore, the bosonic
single-impurity Anderson model appears as an effective impurity model in a
dynamical mean-field theory of the Bose-Hubbard model.
