Four-wave Mixing in an Electromagnetically Induced Transparency Medium 

Four-wave mixing (FWM) is a nonlinear optical process that can generate quantum-correlated twin beams of light with intensity noise below the shot-noise limit. Such nonclassical light sources are valuable for quantum-enhanced sensing, as demonstrated, for example, in experiments with the LIGO interferometer. Our goal is to produce squeezed light at 589 nm to enhance density measurements of a sodium Bose–Einstein condensate. We generate twin beams in a hot sodium vapor using a double-Lambda atomic configuration driven by a FWM process. Compared to alkali metals such as rubidium and cesium, sodium has lower vapor pressure, requiring higher operating temperatures (approximately 165 °C) to achieve sufficient atomic density. At these temperatures, Doppler broadening becomes significant, leading to an optical absorption linewidth on the order of several gigahertz. To enable efficient frequency conversion and high parametric gain for both the probe and conjugate fields, we employ electromagnetically induced transparency (EIT) to suppress resonant absorption of probe and conjugate beams. The transparency conditions are controlled by tuning experimental parameters including atomic density, pump intensity, relative angle between the pump and probe beams, and laser detunings. We report our experimental progress and characterize the dependence of the FWM gain on these parameters.