Intermodulation spectroscopy of the nonlinear response due to two-level systems in superconducting resonators

Two-level system (TLS) loss is typically limiting the coherence of high-quality superconducting quantum circuits. The loss induced by TLS defects is nonlinear, resulting in quality factors with a strong dependence on the circulating microwave power. We observe frequency mixing due to this nonlinearity by applying a two-tone drive to a coplanar waveguide resonator fabricated from aluminum on a silicon substrate. The amplitude and phase of the resulting intermodulation products are measured using a multifrequency lock-in technique without analog up- or downconversion of microwave signals, potentially providing an efficient route to characterizing the dielectric loss due to TLS. We present intermodulation spectroscopy results and discuss progress towards reconstructing the TLS-induced nonlinear response using different reconstruction algorithms.