  Atomic-ensemble spin waves carrying single-photon Fock states exhibit
nonclassical many-body correlations in-between atoms. The same correlations are
inherently associated with single-photon superradiance, forming the basis of a
plethora of quantum light-matter interfaces. We devise a scheme allowing the
preparation of spatially-structured superradiant states in the atomic
two-photon cascade using spin-wave light storage. We thus show that long-lived
atomic ground-state spin waves can be converted to photon pairs opening the way
towards nonlinear optics of spin waves via multi-wave mixing processes.
