  The interplay between the Kondo screening of quantum impurities (by the
electronic channels to which they couple) and the interimpurity RKKY
interactions (mediated by the same channels) has been extensively studied.
However, the effect of unidirectional channels (e.g., chiral or helical edge
modes of 2D topological materials) which greatly restrict the mediated
interimpurity interactions, has only more recently come under scrutiny, and it
can drastically alter the physics. Here we take Wilson's numerical
renormalization group (NRG), the most established numerical method for treating
quantum impurity models, and extend it to systems consisting of two impurities
coupled at different locations to unidirectional channel(s). This is
challenging due to the incompatibility of unidirectionality with one of the
main ingredients in NRG -- the mapping of the channel(s) to a Wilson chain -- a
tight-binding chain with the impurity at one end and hopping amplitudes which
decay exponentially with the distance. We bridge this gap by introducing a
"Wilson ladder" consisting of two coupled Wilson chains, and demonstrate that
this construction successfully captures the unidirectionality of the
channel(s), as well as the distance between the two impurities. We use this
mapping in order to study two Kondo impurities coupled to a single chiral
channel, showing that all local properties and thermodynamic quantities are
indifferent to the interimpurity distance, and correspond to two separate
single-impurity models. Extensions to more impurities and/or helical channels
are possible.
