Premise: I work on quantum computer science mainly at a circuit level abstraction. During my studies I spent some time to get an understanding of how abstract gates relate to physical settings. My background is computer science, so it gets hard for me when it comes to QED. I could cover some basics during my free time (e.g. faynman diagrams, B-E condensate, photon cavities, ions, superconductivity).
I am trying to understand how "distant" qubits and, specifically to the case of this question, superconducting-based qubits can interact by means of flying particles.
With some browsing on google.scholar, some papers got my attention as possible starting point. I will not list them all. Rather, there are a few sentences and pictures that I think are fundamental to understand the basic ingredients to achieve such interaction.
1. Superconducting-based can be coupled by means of a (1D) waveguide. I'd like to receive a qualitative explanation of what happens in the diagram below, taken from here
2. In this big survey authors refer to three states systems as a possible configuration to perform photon emission and/or routing. See Figure 4d, also reported below.
I have the feeling that these two pictures are related somehow, in the sense that the coupling between the stationary qubits happens by means of state transitions of the type in figure 4d.
My main doubt come from the fact that in both the papers they refer to photon-photon correlation. I am missing how such an interaction can couple the (distant) qubits.