  This Letter shows that chains of optical or microwave emitters coupled to a
1D waveguide support subradiant states with close pairs of excited emitters,
which have longer lifetimes than even the most subradiant states with only a
single excitation. Exact, analytical expressions for non-radiative excitation
dimer states are obtained in the limit of infinite chains. To understand the
mechanism underlying these states, we present a formal equivalence between
subradiant dimers and single localized excitations around a chain defect
(unoccupied site). Our analytical mapping permits extension to emitter chains
coupled to the 3D free space vacuum field.
