Macroscopic quantum memories get entangled from far away

We report an experimental implementation of a fundamental unit for long-distance quantum communication. By means of entanglement swapping, entanglement is generated between two remote atomic ensembles connected with either 6 m or 300 m fibre-based optical channel, where the flying qubits--two emitted photons from the atomic ensembles are sent to an intermediate station for a joint Bell-state measurement. Afterwards, the measurement induced entanglement between the atomic ensembles are verified by the violation of Bell's inequality or by an entanglement witness. The striking features, phase-error insensitiveness and scalable flexibility, promise the present setup as a fundamental unit for future quantum information processing.