Quantum communication is a method that offers efficient and secure ways forthe exchange of information in a network. Large-scale quantum communication (ofthe order of 100 km) has been achieved; however, serious problems occur beyondthis distance scale, mainly due to inevitable photon loss in the transmissionchannel. Quantum communication eventually fails when the probability of a darkcount in the photon detectors becomes comparable to the probability that aphoton is correctly detected. To overcome this problem, Briegel, D\"{u}r, Ciracand Zoller (BDCZ) introduced the concept of quantum repeaters, combiningentanglement swapping and quantum memory to efficiently extend the achievabledistances. Although entanglement swapping has been experimentally demonstrated,the implementation of BDCZ quantum repeaters has proved challenging owing tothe difficulty of integrating a quantum memory. Here we realize entanglementswapping with storage and retrieval of light, a building block of the BDCZquantum repeater. We follow a scheme that incorporates the strategy of BDCZwith atomic quantum memories. Two atomic ensembles, each originally entangledwith a single emitted photon, are projected into an entangled state byperforming a joint Bell state measurement on the two single photons after theyhave passed through a 300-m fibre-based communication channel. The entanglementis stored in the atomic ensembles and later verified by converting the atomicexcitations into photons. Our method is intrinsically phase insensitive andestablishes the essential element needed to realize quantum repeaters withstationary atomic qubits as quantum memories and flying photonic qubits asquantum messengers.
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