Electric field spectroscopy of material defects in transmon qubits

TL;DR

This paper introduces a novel electric field spectroscopy technique to analyze individual defects in superconducting transmon qubits, helping identify and mitigate sources of decoherence for improved quantum device performance.

Contribution

The study presents a new method for defect analysis in superconducting qubits by tuning defects with electric fields, revealing defect distributions and their impact on qubit decoherence.

Findings

Defects at circuit interfaces cause about 60% of dielectric loss.

Approximately 40% of defects are in tunnel barriers of parasitic Josephson junctions.

Only about 3% of defects are strongly coupled and likely in small-area qubit junctions.

Abstract

Superconducting integrated circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the downside of the solid-state approach is that superconducting qubits suffer strongly from energy dissipation and environmental fluctuations caused by atomic-scale defects in device materials. Further progress towards upscaled quantum processors will require improvements in device fabrication techniques which need to be guided by novel analysis methods to understand and prevent mechanisms of defect formation. Here, we present a new technique to analyse individual defects in superconducting qubits by tuning them with applied electric fields. This provides a new spectroscopy method to extract the defects' energy distribution, electric dipole moments, and coherence times. Moreover, it enables one to distinguish defects residing in Josephson junction tunnel barriers from those at circuit interfaces. We find that defects at circuit interfaces are responsible for about 60% of the dielectric loss in the investigated transmon qubit sample. About 40% of all detected defects are contained in the tunnel barriers of the large-area parasitic Josephson junctions that occur collaterally in shadow evaporation, and only about 3% are identified as strongly coupled defects which presumably reside in the small-area qubit tunnel junctions. The demonstrated technique provides a valuable tool to assess the decoherence sources related to circuit interfaces and to tunnel junctions that is readily applicable to standard qubit samples.