A fast and robust approach to long-distance quantum communication with atomic ensembles

Quantum communication holds promise for cryptography, teleportation of arbitrary quantum states, and violation of Bell's inequalities over long distances. While commercial implementation of simple quantum communication protocols are well established, attenuation in optical fibers makes longer distances exponentially challenging. Quantum repeaters overcome the time overhead associated with fiber attenuation by using a quantum memory and local quantum operations. A simple implementation uses atomic ensembles and linear optics (Duan, Lukin, Cirac, Zoller, Nature 414, 413 (2001)). Motivated by experimental progress, here we develop an efficient scheme compatible with the active purification of arbitrary errors. Using similar resources as the DLCZ approach, our approach intrinsically purifies leakage out of the logical subspace and all errors within the logical subspace, leading to greatly improved performance in the presence of experimental inefficiencies. We find that about one, 1280 km-distant entangled pair per minute could be realized with a fidelity sufficient to violate Bell's inequality