  We study supersymmetric field theories in three space-time dimensions doped
by various configurations of electric charges or magnetic fluxes. These are
supersymmetric avatars of impurity models. In the presence of additional
sources such configurations are shown to preserve half of the supersymmetries.
Mirror symmetry relates the two sets of configurations. We discuss the
implications for impurity models in 3d N=4 QED with a single charged
hypermultiplet (and its mirror, the theory of a free hypermultiplet) as well as
3d N=2 QED with one flavor and its dual, a supersymmetric Wilson-Fisher fixed
point. Mirror symmetry allows us to find backreacted solutions for arbitrary
arrays of defects in the IR limit of N=4 QED. Our analysis, complemented with
appropriate string theory brane constructions, sheds light on various aspects
of mirror symmetry, the map between particles and vortices and the emergence of
ground state entropy in QED at finite density.
