Traditionally, the resonator losses are only attributed to TLS defects using a power dependent model for the quality factor. In his dissertation, Dr. Alexander developed a two- temperature, power, and temperature dependent model to evaluate resonator losses. The model combines a standard TLS model with a new modified two-temperature quasiparticle model where the driven quasiparticle density is defined by an effective temperature that may be different than the bath temperature. This model also explores the power and temperature dependence of the internal quality factor. The model was applied to interpreting microwave loss in high quality factor resonators fabricated from molecular beam epitaxy-grown superconducting aluminum (Al) and titanium nitride (TiN) thin films. The unique power and temperature dependence of TLS and QP loses are used to more fully understand the relative contributions of these two loss mechanisms.

Dr. Alexander also explored a small bandgap superconductor in contact with a larger bandgap superconductor as a quasiparticle trap. The quasiparticle loss in the larger bandgap superconductor can be reduced by confining quasiparticles into a smaller bandgap superconductor. These devices were then analyzed at different microwave powers and temperatures. The parametric study of resonator and trap geometries identified devices where the quasiparticle traps clearly show performance improvement over bare resonators.

In the near term, Dr. Alexander will continue his research at LPS as a post-doctoral research scientist with a focus on correlating process variation with internal loss mechanisms of superconducting resonator waveguides.