Broadband Two-Level System Defect Spectroscopy

Two-level system (TLS) defects are widely believed to be the main cause of decoherence in superconducting circuits. However, their atomistic origins, frequency distribution, and dipole moments are still not well understood because current probes based on qubits or resonators require complex fabrication and can only measure defects within a narrow frequency range and limited mode volume. These constraints make it challenging to obtain statistically significant measurements of TLS defects and their properties, ultimately hindering the development of low-loss superconducting circuits. In this talk, I will describe how we have addressed these limitations by developing a new spectroscopy tool, Broadband Cryogenic Transient Dielectric Spectroscopy (BCTDS), which allows for direct probing of TLS defects over several GHz in a wide range of devices and materials. Importantly, our approach does not require device packaging, which enables longitudinal device characterizations that establish causal links between specific fabrication steps and TLS defect characteristics, ultimately informing the design of the next generation of ultra-low-loss superconducting circuits.