© Reprinted by permission from Macmillan Publishers Ltd: Nature 397, 594-598 (18 February 1999).
The refractive index and transmission are shown as functions of the frequency of the probe laser. A "detuning" of 0 MHz means that the probe laser frequency matches the energy difference between levels 1 and 3. The very rapid variation of the refractive index around 0 MHz created in this system is responsible for ultra-slow light. (Unit: 7)