Study of repeater brings quantum communication closer to reality
To realize the long-distance quantum communication beyond 100 km, a concept of BACZ quantum repeaters to combine technologies such as quantum memory, entanglement exchange and purification was put forward at the end of the 1990s.
Recently the theory was successfully demonstrated in an experiment by a research team headed by Prof. PAN Jianwei from the University of Science and Technology of China, CAS. Experts say that the feat, which was reported by 28 August issue of Nature, overcomes another obstacle toward the long-distance quantum communication.
Although scientists have made progress in setting up a quantum communication network for efficient and secure exchange of information, attempts have failed to establish such a network over the distance of 100 km, mainly due to inevitable photon loss in the transmission channel.
To address this problem, Austrian scientists (Briegel, Dur, Cirac and Zoller) introduced the concept of BDCZ quantum repeaters in 1998. Although entanglement swapping has been experimentally demonstrated, the implementation of BDCZ quantum repeaters has proved challenging owing to the difficulty of integrating a quantum memory.
Now following a scheme that incorporates the strategy of BDCZ with atomic quantum memories, Pan and his colleagues realized entanglement swapping with storage and retrieval of light, a building block of the BDCZ quantum repeater.
In their experiments, two atomic ensembles, each originally entangled with a single emitted photon, were projected into an entangled state by performing a joint Bell state measurement on the two single photons after they have passed through a 300-m fibre-based communication channel. The results showed that the entanglement is stored in the atomic ensembles and later verified by converting the atomic excitations into photons. The method is intrinsically phase insensitive and establishes the essential element needed to realize quantum repeaters with stationary atomic qubits as quantum memories and flying photonic qubits as quantum messengers.
