Bulk and surface loss in superconducting transmon qubits

TL;DR

This paper investigates the sources of decoherence in superconducting transmon qubits, highlighting the role of surface loss and revealing an additional unknown loss mechanism in certain sapphire substrates.

Contribution

It introduces a systematic study of surface loss contributions in transmon qubits with varying sensitivities and substrate types, uncovering a new loss mechanism.

Findings

Surface loss is a significant decoherence factor in transmon qubits.

An unknown additional loss mechanism was observed in one sapphire substrate.

Testing many devices is crucial for identifying different decoherence sources.

Abstract

Decoherence of superconducting transmon qubits is purported to be consistent with surface loss from two-level systems on the substrate surface. Here, we present a study of surface loss in transmon devices, explicitly designed to have varying sensitivities to different surface loss contributors. Our experiments also encompass two particular different sapphire substrates, which reveal the onset of a yet unknown additional loss mechanism outside of surface loss for one of the substrates. Tests across different wafers and devices demonstrate substantial variation, and we emphasize the importance of testing large numbers of devices for disentangling different sources of decoherence.