  A theoretical proposal for realizing and detecting spin supercurrent in an
isotropic antiferromagnetic insulator is reported. Superfluid spin transport is
achieved by inserting the antiferromagnet between two metallic reservoirs and
establishing a spin accumulation in one reservoir such that a spin bias is
applied across the magnet. We consider a class of bipartite antiferromagnets
with Neel ground states, and temperatures well below the ordering temperature,
where spin transport is mediated essentially by the condensate. Landau-Lifshitz
and magneto-circuit theories are used to directly relate spin current in
different parts of the heterostructure to the spin-mixing conductances
characterizing the antiferromagnet|metal interfaces and the antiferromagnet
bulk damping parameters, quantities all obtainable from experiments. We study
the efficiency of spin angular-momentum transfer at an antiferromagnet|metal
interface by developing a microscopic scattering theory for the interface and
extracting the spin-mixing conductance for a simple model. Within the model, a
quantitative comparison between the spin-mixing conductances obtained for the
antiferromagnet|metal and ferromagnet|metal interfaces is made.
