Strongly driven quantum Josephson circuits

Radio Frequency driven Josephson circuits provide a rich platform to engineer a variety of nonlinear Hamiltonians for superconducting quantum circuits. While Josephson junctions mediate strong interactions between microwave photons, some particular types of interaction Hamiltonians can only be obtained through the application of microwave drives (pumps) at well-chosen frequencies. For various applications, it is important to increase the pump strength without introducing undesired couplings and interferences that limit the fidelity of the operations. In this Letter, we analyze these limitations through the theoretical study of the steady state behavior of the driven-dissipative systems. Our general analysis, based on the Floquet-Markov theory, indicates that the ubiquitous circuit consisting of a transmon coupled to a harmonic oscillator suffers from strong limitations in this regard. In accordance with a parallel experimental study, we find that above a fairly low critical pump power the transmon state escapes the Josephson potential confinement and is sent to a statistical mixture of free-particle like states. Next, we illustrate that by diluting the non-linearity of the Josephson junction through a parallel inductive shunt, the picture changes significantly and one achieves very large dynamic ranges in the pump power. This theoretical study provides the ground for drastic modifications in Josephson circuit designs to be used in parametric Hamiltonian engineering experiments.