Measurement of Dispersive Properties of Electromagnetically Induced Transparency in Rubidium Atoms

The dispersive properties of the atomic transition in the rubidium $D_2$ line ($5S_{\frac{1}{2}}-5\mathrm{}{P}_{\frac{3}{2}}$) at 780.0 nm are measured with a Mach-Zehnder interferometer when an additional coupling field at 775.8 nm is applied to an upper transition ($5P_{\frac{3}{2}}-5\mathrm{}{D}_{\frac{5}{2}}$). This ladder-type system is observed to exhibit electromagnetically induced transparency together with a rapidly varying refractive index. A reduction in group velocity for the probe beam ($v_g=\frac{c}{13.2}$) is inferred from the measured dispersion curve with 52.5% suppressed absorption on resonance.