Identification of different types of high-frequency defects in superconducting qubits

TL;DR

This paper introduces an experimental spectroscopy method to identify and distinguish different high-frequency TLS defects in superconducting qubits, aiding in understanding and mitigating decoherence sources.

Contribution

The authors develop a novel spectroscopy technique using strong qubit drive to classify types of TLS defects and their coupling mechanisms in superconducting qubits.

Findings

Detected nonlinear TLS defects caused by critical-current fluctuations.

Identified conventional TLS defects with linear charge fluctuation coupling.

Proposed method can be used for defect inspection and fabrication quality control.

Abstract

Parasitic two-level-system (TLS) defects are one of the major factors limiting the coherence times of superconducting qubits. Although there has been significant progress in characterizing basic parameters of TLS defects, exact mechanisms of interactions between a qubit and various types of TLS defects remained largely unexplored due to the lack of experimental techniques able to probe the form of qubit-defect couplings. Here we present an experimental method of TLS defect spectroscopy using a strong qubit drive that allowed us to distinguish between various types of qubit-defect interactions. By applying this method to a capacitively shunted flux qubit, we detected a rare type of TLS defect with a nonlinear qubit-defect coupling due to critical-current fluctuations, as well as conventional TLS defects with a linear coupling to the qubit caused by charge fluctuations. The presented approach could become the routine method for high-frequency defect inspection and quality control in superconducting qubit fabrication, providing essential feedback for fabrication process optimization. The reported method is a powerful tool to uniquely identify the type of noise fluctuations caused by TLS defects, enabling the development of realistic noise models relevant to noisy intermediate-scale quantum (NISQ) computing and fault-tolerant quantum control.