Power-dependent internal loss in Josephson bifurcation amplifiers

TL;DR

This study investigates the nonlinear behavior and internal power-dependent loss in Josephson junction-based superconducting resonators, revealing discrepancies between observed hysteresis and theoretical predictions, likely due to quasiparticle conductance effects.

Contribution

It provides experimental characterization of power-dependent internal loss in Josephson bifurcation amplifiers and discusses quasiparticle conductance as a potential cause.

Findings

Hysteresis observed in resonators at low temperatures.

Internal loss increases with drive power.

Discrepancies explained by quasiparticle conductance effects.

Abstract

We have studied nonlinear superconducting resonators: lambda/2 coplanar-waveguide (CPW) resonators with Josephson junctions (JJs) placed in the middle and lambda/4 CPW resonators terminated by JJs, which can be used for the qubit readout as "bifurcation amplifiers." The nonlinearity of the resonators arises from the Josephson junctions, and because of the nonlinearity, the resonators with appropriate parameters are expected to show a hysteretic response to the frequency sweep, or "bifurcation," when they are driven with a sufficiently large power. We designed and fabricated resonators whose resonant frequencies were around 10 GHz. We characterized the resonators at low temperatures, T<0.05 K, and confirmed that they indeed exhibited hysteresis. The sizes of the hysteresis, however, are sometimes considerably smaller than the predictions based on the loaded quality factor in the weak drive regime. When the discrepancy appears, it is mostly explained by taking into account the internal loss, which often increases in our resonators with increasing drive power in the relevant power range. As a possible origin of the power-dependent loss, the quasiparticle channel of conductance of the JJs is discussed.