Probing defect densities at the edges and inside Josephson junctions of superconducting qubits

TL;DR

This study investigates the distribution of tunneling defects in Josephson junctions of superconducting qubits, finding that defects are evenly spread within the tunnel barrier rather than concentrated at edges, informing fabrication improvements.

Contribution

It demonstrates that defect densities are proportional to junction area and evenly distributed inside the tunnel barrier, regardless of fabrication process variations.

Findings

Defect density scales with junction area.

Defects are uniformly distributed inside the tunnel barrier.

Similar defect densities observed in different fabrication methods.

Abstract

Tunneling defects in disordered materials form spurious two-level systems which are a major source of decoherence for micro-fabricated quantum devices. For superconducting qubits, defects in tunnel barriers of submicrometer-sized Josephson junctions couple strongest to the qubit, which necessitates optimization of the junction fabrication to mitigate defect formation. Here, we investigate whether defects appear predominantly at the edges or deep within the amorphous tunnel barrier of a junction. For this, we compare defect densities in differently shaped Al/AlO$_x$/Al Josephson junctions that are part of a Transmon qubit. We observe that the number of detectable junction-defects is proportional to the junction area, and does not significantly scale with the junction's circumference, which proposes that defects are evenly distributed inside the tunnel barrier. Moreover, we find very similar defect densities in thermally grown tunnel barriers that were formed either directly after the base electrode was deposited, or in a separate deposition step after removal of native oxide by Argon ion milling.