Apparatus and methods for high fidelity quantum communication over long
communication channels even in the presence of significant loss in the
channels are disclosed. The invention employs laser manipulation of
quantum correlated atomic ensembles using linear optic components,
optical sources of low intensity pulses, beam splitters, and
single-photon detectors requiring only moderate efficiencies. The
invention provides fault-tolerant entanglement generation and connection,
using a sequence of steps that each provide built-in entanglement
purification and that are each resilient to the realistic noise. The
invention relies upon collective excitation in atomic ensembles rather
than single particle excitations in atomic ensembles so that
communication efficiency scales polynomially with the total length of a
communication channel.