Lattice Renormalization of Configurational TLS in Superconducting Films

Two-level systems (TLS) are well-known to be present in the substrates, interfaces, superconducting films, and amorphous surface oxide layers of superconducting qubits. Configurational TLS are a subclass of the total TLS population and occur due to the presence of degenerate or nearly-degenerate minima in the nuclear Hamiltonian separated by a small energy barrier. We show that the structure and tunnel splitting of configurational TLS is mediated by an intrinsic broad-spectrum coupling to lattice phonon modes. This coupling is separate to the well-understood coupling to low frequency phonon modes which modulates the well-to-well energy asymmetry of configurational TLS. Our work demonstrates the underlying renormalization of the tunnel splitting which occurs due to the coupling of the TLS to lattice phonon modes and which is not accounted for by considering the tunneling atom as a light particle. We additionally show that accounting for the renormalization of the TLS tunnel splitting is necessary to accurately assess the impact of a particular TLS relative to the operational frequency of a qubit.