  Ordered ensembles of atoms, such as atomic arrays, exhibit distinctive
features from their disordered counterpart. In particular, while collective
modes in disordered ensembles show a linear optical response, collective
subradiant excitations of subwavelength arrays are endowed with an intrinsic
non-linearity. Such non-linearity has both a coherent and a dissipative
component: two excitations propagating in the array scatter off each other
leading to formation of correlations and to emission into free space modes. We
show how to take advantage of such non-linearity to coherently prepare a single
excitation in a subradiant (dark) collective state of a one dimensional array
as well as to perform an entangling operation on dark states of parallel
arrays. We discuss the main source of errors represented by disorder introduced
by atomic center-of-mass fluctuations, and we propose a practical way to
mitigate its effects.
