Evidence for unexpectedly low quasiparticle generation rates across Josephson junctions of driven superconducting qubits

TL;DR

This study experimentally investigates quasiparticle generation rates in driven superconducting qubits, finding unexpectedly low energy loss rates that challenge existing theoretical models and have implications for quantum computing reliability.

Contribution

The paper provides the first quantitative experimental estimation of QPG rates in strongly driven SCQs, revealing discrepancies with standard models and highlighting the need for refined theories.

Findings

QPG energy loss rates are much lower than predicted by Floquet-Markov models.

High-frequency cutoffs in conductance explain experimental results.

Results suggest limitations in current QPG models and Markovian assumptions.

Abstract

Recent studies find that even drives far below the superconducting gap frequency may cause drive-induced quasiparticle generation (QPG) across Josephson junctions (JJs) of superconducting qubits (SCQs), posing a serious concern for fault-tolerant superconducting quantum computing (FTSQC). Nonetheless, quantitative experimental estimation on QPG rates has remained vague. Here, we investigate QPG using strongly driven SCQs, reaching qubit drive amplitudes up to $2\pi\times$300 GHz by applying intense drive fields through the readout resonators. The resonator nonlinear responses enable quantification of the energy loss at SCQs, including the contribution from QPG. Surprisingly, the estimated total energy loss rates are far lower than those expected by the Floquet-Markov formalism with QPG as the sole loss mechanism. Meanwhile, calculations that incorporate high-frequency cutoffs (HFCs) in the QPG conductance at approximately 17-20 GHz effectively explain the experimental observations. These results suggest limitations in either the QPG conductance model or the Markovian treatment of the QPG processes. Both possibilities possess crucial implications for handling QPG problems toward FTSQC and for a more deeper understanding of Josephson junctions.