  Collective decays of multiply-excited atoms become subradiant and bound in
space when they are strongly coupled to the guided modes in an atom-waveguide
interface. In this interface, we analyze their average density-density and
modified third-order correlations via Kubo cumulant expansions, which can arise
and sustain for long time. The shape-preserving dimers and trimers of atomic
excitations emerge in the most subradiant coupling regime of light-induced
dipole-dipole interactions. This leads to a potential application of quantum
information processing and quantum storage in the encoded nonreciprocal spin
diffusion, where its diffusion speed depends on the initial coherence between
the excited atoms and is robust to their relative phase fluctuations. The
state-dependent photon routing can be viable as well in this interface.
