Illustration: Alan Stonebraker

Figure 1: (a) The three-level interaction scheme necessary for electromagnetically induced transparency (EIT) and the slowing of light. Without EIT both the control and signal optical fields drive an atom into the excited state that is followed by the spontaneous decay to the two ground states. Since the control field is much stronger, most atoms end up at the state $|c〉$, causing strong absorption for the signal field. Under EIT conditions (when $ωs-ωc$ is equal to the energy difference between states $|b〉$ and $|c〉$), however, the atoms are pumped into a quantum superposition of the states $|b〉$ and $|c〉$ (the dark state). Even though most of the atomic population is still in the state $|c〉$, the atoms do not interact with the signal field, resulting in a transparency window. (b) Schematic of diffractionless signal field propagation. By choosing the proper frequency difference between the control and signal frequencies, the diverging signal field couples preferentially to the atoms moving towards the propagation axis, which “drags” the signal light back.