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

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.

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Cited by in corpus (12)

• High-coherence superconducting qubits made using industry-standard, advanced semiconductor manufacturing
• Mitigation of frequency collisions in superconducting quantum processors
• Enhancing the Coherence of Superconducting Quantum Bits with Electric Fields
• Identification of different types of high-frequency defects in superconducting qubits
• Alternating Bias Assisted Annealing of Amorphous Oxide Tunnel Junctions
• Mitigating Losses of Superconducting Qubits Strongly Coupled to Defect Modes
• Low-Energy Backgrounds in Solid-State Phonon and Charge Detectors
• The influence of pinholes and weak-points in aluminium-oxide Josephson junctions
• Strain-spectroscopy of strongly interacting defects in superconducting qubits
• In-situ scanning gate imaging of individual two-level material defects in live superconducting quantum circuits
• Robust gates with spin-locked superconducting qubits
• Why Superconducting Ta Qubits Have Fewer Tunneling Two-Level Systems at the Air-Oxide Interface Than Nb Qubits