Our paper Finite sizes and smooth cutoffs in superconducting circuits was recently published in Physical Review A. In this paper we discuss the strong interaction between an atom and a one dimensional electromagnetic field, a regime that we have demonstrated in recent experiments. We discuss the role of atomic shape and of cutoff function for coupling to fields. We find that the cutoff scale type is highly relevant in superconducting circuits and observable in experiments measuring qubit spontaneous excitation or decay by the field vacuum.