Superradiant parametric conversion of spin waves

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.