  We demonstrate theoretically that a dissipationless spin current can flow a
long distance through a diffusive normal metal by using superconductors
interfaced with magnetic insulators. The magnitude of this spin supercurrent is
controlled via the magnetization orientation of the magnetic insulators. The
spin supercurrent obtained in this way is conserved in the normal metal just
like the charge-current and interestingly has a term which is independent of
the superconducting phase difference. The quantum state of the system can be
switched between 0 and $\pi$ by reversing the insulators from a parallel to
antiparallel configuration with an external field. We show that the
spin-current is carried through the normal metal by superconducting triplet
(odd-frequency) correlations and that the superconducting phase difference can
be used to enhance these, leaving clear spectroscopic fingerprints in the
density of states.
