Fig. 1: Principle of high-order photon processes.
From: Spectral signature of high-order photon processes enhanced by Cooper-pair pairing
a Electrical circuit depicting a superconducting LC oscillator (black) shunted by a generalized Josephson junction (grey) that only permits Cooper-pair tunneling in pairs. The circuit is threaded by an external flux denoted ϕext. b Potential and energy levels (solid lines) of this nonlinear oscillator [Eq. (1) with parameters \(\Omega /2\pi=2.86\,\,{\mbox{GHz}}\,,{{{{\mathcal{E}}}}}_{J}/h=0.795\,\,{\mbox{GHz}}\,,\eta=3.4\)] and its linear equivalent (dashed lines) as a function of the superconducting phase difference at ϕext = π. Crucially, the adjacent transition frequency shift δn from the bare frequency ω0 alternates in sign when ascending the ladder. c Photon process diagrams for the first four interaction orders and corresponding interaction energies Jn.
