Beyond single-photon localization at the edge of a photonic band gap

We study spontaneous emission in an atomic ladder system, with both transitions coupled near resonantly to the edge of a photonic band gap continuum. The problem is solved through a recently developed technique and leads to the formation of a two-photon $+$ atom bound state with fractional population trapping in both upper states. In the long-time limit, the atom can be found excited in a superposition of the upper states and a direct two-photon process coexists with the stepwise one. The sensitivity of the effect to the particular form of the density of states is also explored.