  We consider a scheme of multiplexed cold atomic ensembles that generate a
frequency-entangled biphoton state with controllable entropy of entanglement.
The biphoton state consists of a telecommunication photon (signal) immediately
followed by an infrared one (idler) via four-wave mixing with two classical
pump fields. Multiplexing the atomic ensembles with frequency and phase-shifted
signal and idler emissions, we can manipulate and control the spectral property
of the biphoton state. Mapping out the entropy of entanglement in the scheme
provides the optimal configuration for entanglement resources. This paves the
way for efficient long-distance quantum communication and for potentially
useful multimode structures in quantum information processing.
