Detecting stray microwaves and nonequilibrium quasiparticles in thin films by single-electron tunneling

Superconducting thin films and tunnel junctions are the building blocks of many state-of-the-art technologies related to quantum information processing, microwave detection, and electronic amplification. These devices operate at millikelvin temperatures, and -- in a naive picture -- their fidelity metrics are expected to improve as the temperature is lowered. However, very often one finds in the experiment that the device performance levels off around 100--150 mK. In my presentation, I will address three common physical mechanisms that can cause such saturation: stray microwaves, nonequilibrium quasiparticles, and sub-gap quasiparticle states. The new experimental data I will present is based on a series of studies on quasiparticle transport in Coulomb-blockaded normal-insulator-superconductor tunnel junction devices. We have used a capacitively coupled SET electrometer to detect individual quasiparticle tunneling events in real time. We demonstrate the following record-low values for thin film aluminum: quasiparticle density $n_{\mathrm{qp}} < 0.033 / \mu\mathrm{m}^3$, normalized density of sub-gap quasiparticle states (Dynes parameter) $\gamma < 1.6 \times 10^{-7}$. I will also discuss some sample stage and chip designs that improve microwave shielding.