Absence of Charge Offset Drift in a Transmon Qubit

TL;DR

This study demonstrates a tantalum-based transmon qubit with remarkably stable charge offset over months, attributed to a superconducting layer formed during fabrication, which could lead to more stable superconducting quantum circuits.

Contribution

The paper introduces a transmon qubit with unprecedented charge offset stability and identifies a fabrication-related superconducting layer as the cause, suggesting a new approach to improve qubit stability.

Findings

Charge offset remains at zero for nearly three months.

Stability is linked to a superconducting layer formed during fabrication.

Deliberate engineering of this layer can eliminate charge-offset drift.

Abstract

Superconducting quantum circuits are sensitive to their electrostatic environment: uncontrolled charges accumulating on the electrodes of a Josephson junction shift the energy levels of a qubit, perturbing its operation and restricting their design. This effect is captured by a single parameter - the charge offset - whose slow, unpredictable drift has proven difficult to eliminate in practice. Here, we report a tantalum-based transmon qubit in which the charge offset remains pinned at zero over nearly three months of measurements, including two thermal cycles, with no observable compromise to the qubit lifetime. This exceptional stability disappears in later cooldowns, indicating a fragile mechanism at play. We attribute it to the inductance of a thin superconducting layer inadvertently formed in parallel with the Josephson junction during fabrication. X-ray surface spectroscopy suggests this layer arises from an incomplete wet-etch of tantalum on sapphire. Deliberately engineering such a layer offers a route to eliminating charge-offset drift in superconducting circuits more broadly.