Secrets of slow light
(a) The amount of probe light transmitted by a cloud of ultracold sodium atoms in the presence of a "coupling laser". The coupling-laser field makes the cloud transparent to probe light within a very narrow range of frequencies.
(b) The refractive index of the cloud as a function of the frequency of the probe laser. The group velocity of a probe pulse is inversely proportional to the slope of the index the steeper the slope, the lower the light speed. When the probe is tuned to the frequency of the transition between states 1 and 3, then the refractive index is 1.0 the same as in free space.