Continuous Real-Time Detection of Quasiparticle Trapping Part I: Detection and Analysis

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. We briefly introduce the Andreev picture of the Josephson effect and describe how Andreev levels may be used as quasiparticle traps. We discuss measurement techniques using Aluminum nanobridge SQUID resonators [Farmer et al. APL 2021] to detect non-equilibrium quasiparticle trapping in Andreev levels. An algorithm for assigning trap occupation from noisy I-Q data is presented and we discuss methods for distinguishing multiple trapping events from excitation of a single Andreev state. We conclude with methods for extracting relevant information from the trap occupation time series data. In part 2, we present measurement results and discuss implications.