Drive-induced nonlinear behavior in Radio-frequency Quantum Upconverters

Radio-frequency quantum upconverters (RQUs) are continuous-variable quantum sensors that upconvert information from kHz-MHz signals to microwave frequencies (4-8 GHz), where quantum metrology techniques can be applied. The RQU consists of a microwave resonator loaded by an array of Josephson junctions. Low-frequency signal flux changes the effective inductance of the junctions, implementing a parametric interaction that upconverts the input signal to sidebands about the microwave drive tone. However, the microwave drive tone also introduces an amplitude-dependent shift of the effective inductance, giving rise to soft-spring Duffing oscillator dynamics. For large junction participation and high drive powers, the lineshape of the device resonance becomes non-Lorenztian and above a critical drive power exhibits bistability. In this talk, I will demonstrate modeling and measurements of this effect, describe the scaling of drive-induced nonlinearity with device parameters, and discuss future device designs that mitigate bifurcation under strong microwave drives through the use of additional Josephson junctions and loops.