Measurement of steep dispersion associated with magnetically assisted gain in Zeeman degenerate two-level system of cold atoms

We report on the measurements of magnetically assisted dispersion and gain in a Zeeman degenerate two-level system associated with the closed D 2 hyperfine transition <?CDATA Fg = 3 → Fₑ = 2?> F g = 3 → F e = 2 cold cesium atoms. We use a Mach–Zehnder interferometer to measure the dispersive properties of this system in the presence of a magnetic field, transversal to the quantization direction, when the atomic system is excited by a coupling and probe beams with orthogonal polarizations. We have measured the spectrum of the refraction index for different magnitudes of the external magnetic field and different coupling beam intensities. A previously published theoretical model, using a simpler atomic transition <?CDATA Fg = 1 → Fₑ = 0?> F g = 1 → F e = 0 , accounts reasonably for the experimentally observed gain and dispersive lineshapes.