Thermal cycling: Evidence for a generalized tunneling model and a tool to distinguish noise sources in quantum circuits

Structural two-level systems (TLSs) ubiquitous in amorphous solids are dramatically sensitive to thermal cycling to about 20K and then back to low temperature, a process upon which the excitation energy of most TLSs is significantly changed. Using Monte Carlo simulations we demonstrate that this phenomenon is not contained within the standard tunneling model, but is well explained by a model that includes an additional set of TLSs that are pseudogapped at low energies, yet possess a strong strain interaction through which they generate significant dynamical disorder upon thermal cycling. Our results provide additional support for the broad applicability of the Two-TLS model to amorphous solids at low temperatures, bringing us closer to a comprehensive understanding of the universal behavior of phonon attenuation in these materials. With regard to quantum superconducting circuits, our results suggest thermal cycling as a unique protocol to distinguish TLS noise from other noise sources. The possible relation of the Two-TLS model to ionizing radiation effects on long-time fluctuations in qubit relaxation times and on TLS scrambling is discussed.