Continuous Real-Time Detection of Quasiparticle Trapping Part II: Quasiparticles Behavior

 Non-equilibrium quasiparticles are ubiquitous in superconducting circuits and are known to be a source of error for qubits. This two-part talk focuses on the use of sub-gap Andreev levels to investigate how non-equilibrium quasiparticles interact and trap in qubit-like circuits. Quasiparticle behavior depends on many environment variables including temperature, radiation levels, gap quality, and signal line filtering. Part 1 introduced quasiparticle trapping in Aluminum nanobridges [Farmer et al. APL 2021] and the measurement techniques we use. We discuss the effect of readout power on mean trap occupation and transition rates, highlighting how this power scaling can help us understand the interaction of Andreev levels with each other and with localized sub-gap states, such as those induced by magnetic contaminants. We present the temperature dependence of trap occupation and extrapolate to determine our non-equilibrium quasiparticle density. We discuss the effects of introducing eccosorb filters in our apparatus and conclude by discussing spectroscopic measurements of occupied Andreev trap energies.

 

*Funding was provided by the AFOSR under FA9550-19-1-0060 and the NSF under DMR-1900135