Identifying and Engineering Two-Level Systems in Superconducting Qubits – Part II

Two-level systems (TLSs) are parasitic defects that exist in the amorphous oxide junction of superconducting qubits. While normally seen as a source of decoherence, we instead consider new avenues for identifying and harnessing TLSs for controlling superconducting and interfacial properties. Building on a first-principles study of amorphous aluminium oxide [1], we suggest localised soft modes (or "vibrons") as a likely candidate for dipole-activated TLSs. Such soft modes subsequently result in a new scattering channel for normal electrons, resulting in "hot spots" of the Josephson critical current at the superconductor/oxide interface [2]. Similarly, the vibron adds weight to the bulk electron-phonon spectral function, resulting in either an enhancement or suppression of the superconducting critical temperature and zero temperature gap [3]. Our work as a whole highlights new opportunities for tailoring material properties through TLS engineering.