Direct measurement of light speed reduction in a rubidium vapour medium coherently prepared by electromagnetically induced transparency

We have experimentally observed the reduction of light speed in a rubidium vapour medium coherently prepared by electromagnetically induced transparency. The light speed reduction was deduced by directly measuring the time delay of a probe light when it passed through the medium. The time delay varies with the intensity of the coupling laser, and the typical time delay we recorded was 1.8 mus, corresponding to a light speed of 56000 m/s.