Low-temperature defects in quantum-regime superconducting devices
Amorphous dielectrics have long been known to have low-temperature properties dictated by two-level system (TLS) defects which can be inferred from a tunneling-system model. These defects lead to absorption and dispersion of electromagnetic and acoustic waves. Modern superconducting resonators used for single-photon detection in astronomy and superconducting qubits and resonators in quantum computing are at milli-Kelvin temperatures and operate at multi-GHz photon frequencies, such that they can operate in the quantum regime. These devices exhibit deleterious phenomena which are very often attributed to TLS defects, and here I will present two new experiments which study relevant defects. In the first experiment, the defects of amorphous silicon nitride are studied and are found to differ from those described by a standard TLS model. In particular, we see evidence for TLS-TLS interactions which lower the loss at below 70 mK due to interactions. By turning on a dc electric field of up to 300 mV /nm, we see temporal changes in these interactions. In the second experiment, TLS defects within the aluminum oxide barrier of a Josephson junction are observed with a new frequency-tunable resonator circuit. This device is a convenient strong-coupling probe of TLS defects in Josephson junctions and will allow for rapid testing of new Josephson junction barrier materials for qubits in the near future.