  We show how canonical transformations can map problems with impurities
coupled to non-interacting rings onto a similar problem with open boundary
conditions. The consequent reduction of entanglement, and the fact the density
matrix renormalization group (DMRG) is optimally suited for open boundary
conditions, increases the efficiency of the method exponentially, making it an
unprecedented tool to study persistent currents. We demonstrate its application
to the case of the one-channel and two-channel Kondo problems, finding
interesting connections between the two.
