Figure 2
From: Photon-Assisted Perfect Conductivity Between Arrays of Two-Level Atoms
Ladder diagram. Diagrammatic expansion of \(\langle q|\Gamma ({P}_{0},P)|p\rangle \) which is the sum of the repeated and continuous scatterings between the upper and the lower fields (ladder diagrams). The external legs are only for the eyes, the black dot “●” is U in Eq. (5), and the internal double-line (single line) represents the propagator of the upper (lower) field.
