Two-tone spectroscopy of decoherence-causing defect in superconducting devices*

Despite fast progress towards scalable superconducting quantum computing (QC) systems, one of the main detrimental factors hindering further advancement is the loss caused by so-called two-level system defects (TLS) which are believed to be hosted in amorphous native oxides on the surfaces of superconducting CQ devices.

Good phenomenological understanding of TLS is provided by standard tunneling model, and recent experiments with various superconducting materials and fabrication approaches led to improvements of QC devices. Yet, proper detailed characterization and understanding of microscopic nature of TLS is lacking.

We propose and demonstrate a new experimental technique to characterize spectroscopic features of TLS defects. To do that we use 3.9 GHz multi-cell 3D resonators and two-tone spectroscopy where we pump one of the pass-band modes of the 3D cavity with continuous wave signal, and probe internal quality factors of the other modes which are close in the frequency domain. Our measurements allow us to estimate the spectroscopic linewidth of TLS defects hosted in amorphous oxide layers of superconducting materials. This information allows to optimize fabrication procedures and choose optimal materials for high-coherence QC devices.