Single-photon bus connecting spin-wave quantum memories

. Here, we report the phase-coherent transfer of a single magnon from one atomic ensembleto another via an optical resonator serving as a quantum busthat in the ideal case is only virtually populated. Partial transferdeterministically creates an entangled state with one excitationjointlystoredinthetwoensembles.Theentanglementisverifiedby mapping the magnons onto photons, whose correlations canbe directly measured. These results should enable deterministicmultipartiteentanglementbetweenatomicensembles.