We describe the probabilistic but heralded generatio n of a single spin-wave quantum (magnon) in an atomic ensemble inside an optical resonator. The magnon can be converted at a later time on demand into a photon with a conversion efficiency near 90%. If the resonator contains a second ensemble, the magnon can be transferred adiabatically and phase-coherently between the two ensembles via a dark state of the resonator. Partial transfer results in an entangled state, where a single spin-wave quantum is shared between the two ensembles.
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