Single-photon bus connecting spin-wave quantum memories

Generation of non-classical correlations (or entanglement) between atoms(1-7), photons(8) or combinations thereof (9-11) is at the heart of quantum information science. Of particular interest are material systems serving as quantum memories that can be interconnected optically(3,6,7,9-11). An ensemble of atoms can store a quantum state in the form of a magnon-which is a quantized collective spin excitation-that can be mapped onto a photon(12-18) with high efficiency(19). Here, we report the phase-coherent transfer of a single magnon from one atomic ensemble to another via an optical resonator serving as a quantum bus that in the ideal case is only virtually populated. Partial transfer deterministically creates an entangled state with one excitation jointly stored in the two ensembles. The entanglement is verified by mapping the magnons onto photons, whose correlations can be directly measured. These results should enable deterministic multipartite entanglement between atomic ensembles.