Understanding and mitigating losses in superconducting radio frequency (SRF) cavities using two-tone spectroscopy*

It was shown that two-level systems (TLSs) are among the major detrimental factors causing energy loss and decoherence in superconducting quantum devices. Understanding and mitigating this loss mechanism is of high importance for practical applications of quantum computing systems. Microscopically, majority of TLSs are defects at dielectric surfaces and interfaces of superconducting quantum hardware. For instance, the major source of TLS in Nb SRF cavities is the layer of natural oxide formed on their inner surface.

Typical linewidth of TLSs is 10-20 MHz which provides a great opportunity to probe these systems using TM₀₁₀ pass-band modes of multicell SRF cavities of elliptical shape. In our experiments we used 1.3 GHz π pass-band mode (3.9 GHz in another cavity measurements) to pump the TLS bath with a persistent power tone and probed the quality factors of the other pass-band modes with frequencies within 30 MHz of the π-mode. The Q-factors of the probed modes were measured by performing the time decay of the output signal after the probing signal was turned off. We observe the increase of the Q-factors of the probed modes when the π-mode is continuously pumped comparing to the case when there is no π-mode pumping. Our work shows a new way to characterize losses in various dielectric materials and provides a strategy to mitigate TLS losses in superconducting quantum circuits.