© Lene V. Hau.
After the cooling process, all the atoms are in the lowest internal energy state which is state |1> in the figure. To slow light, we first illuminate the atom cloud with the coupling laser beam and then send the probe laser pulse into the cloud. The frequencies of the two laser fields match two different transitions in the atoms, both transitions connecting one of the lower states to the upper state |3>. If an atom actually ends up in state |3>, it will fluoresce and send out radiation in a random direction and decay to a lower state. However, with the coupling laser illuminating the atom, and as we turn on the probe laser field, the two laser beams together nudge the atom partly into state |2>—the more the probe laser field is turned on, the more the atom will be in state |2>. This process where the atom is partly transferred from state |1> to state |2> and ends up in a superposition of the two states—it is in both states at the same time—happens via the atom's absorption of a probe laser photon and the stimulated (rather than fluorescent) emission of a photon into the coupling laser field. (Unit: 7)