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The ladder configuration of atomic levels provides a source for telecomphotons (signal) from the upper atomic transition. \ For rubidium and cesiumatoms, the signal field has the range around 1.3-1.5 $\mu$m that can be coupledto an optical fiber and transmitted to a remote location. \ Cascade emissionmay result in pairs of photons, the signal entangled with the subsequentlyemitted infrared photon (idler) from the lower atomic transition.\ Thiscorrelated two-photon source is potentially useful in the(Duan-Lukin-Cirac-Zoller) DLCZ protocol for the quantum repeater.\ We implementthe cascade emission to construct a modified DLCZ quantum repeater andinvestigate the role of time-frequency entanglement in the protocol.\ Thedependence of protocol on photon-number resolving and non-resolving detectorsis also studied.\ We find that frequency entanglement deteriorates theperformance but the harmful effect can be diminished by using shorter pumppulses to generate the cascade emission.\ An optimal cascade-emission-basedDLCZ scheme is realized by applying a pure two-photon source in addition tousing detectors of perfect quantum efficiency.

Spectral analysis for cascade-emission-based quantum communication in atomic ensembles