Long Distance Quantum Communication Using Cascade Emission in Atomic Ensembles

The ladder configuration of atomic levels provides a source for telecom photons (signal) from the upper atomic transition. For rubidium and cesium atoms, the signal field has the range around 1.3-1.5 $\mu$m that can be coupled to an optical fiber and transmitted to a remote location. Cascade emission may result in pairs of photons, the signal entangled with the subsequently emitted infrared photon (idler) from the lower atomic transition. This correlated two- photon source is potentially useful in the DLCZ protocol for the quantum repeater. We investigate the role of time- frequency entanglement in the protocol, and find that it deteriorates the performance but the harmful effect can be diminished by using shorter pump pulses to generate the cascade emission.