  Ken Wilson developed the Numerical Renormalization Group technique which
greatly enhanced our understanding of the Kondo effect and other quantum
impurity problems. Wilson's NRG also inspired Philippe Nozieres to propose the
idea of a large "Kondo screening cloud". While much theoretical evidence has
accumulated for this idea it has remained somewhat controversial and has not
yet been confirmed experimentally. Recently a new possibility for observing an
analogous crossover length scale has emerged, involving a Majorana fermion
localized at the interface between a topological superconductor quantum wire
and a normal wire. We give an overview of this topic both with and without
interactions included in the normal wire.
