Double-Λ microscopic model for entangled light generation by four-wave mixing

Motivated by recent experiments, we study four-wave-mixing in an atomic double-Lambda system driven by a far-detuned pump. Using the Heisenberg-Langevin formalism, and based on the microscopic properties of the medium, we calculate the classical and quantum properties of seed and conjugate beams beyond the linear amplifier approximation. A continuous-variable approach gives us access to relative-intensity noise spectra that can be directly compared with experiments. Restricting ourselves to the cold-atom regime, we predict the generation of quantum-correlated beams with a relative-intensity noise spectrum well below the standard quantum limit (down to -6 dB). Moreover, entanglement between seed and conjugate beams measured by an inseparability down to 0.25 is expected. This work opens the way to the generation of entangled beams by four-wave mixing in a cold-atom sample.