Fig. 6: Spectral interlacing.
From: Spectral signature of high-order photon processes enhanced by Cooper-pair pairing
a Adjacent transition frequencies obtained from two-tone measurements (open circles) and numerical diagonalization of the one-mode Hamiltonian of Eq. (4) (solid curves) along θext = π. b Transition frequency shifts δn and (c) interaction strengths Jn extracted from measurements (open circles) and analytic expressions depending on the fitted circuit parameters (full circles) at the starred external flux value. The transition frequency shifts alternate in sign while remaining much smaller than the transition frequency itself, directly corresponding to similarly-sized Hamiltonian coefficients for two-, three-, and four-photon interaction strengths.
