Ultrathin Magnesium-based Coating as an Efficient Oxygen Barrier for Superconducting Circuit Materials

TL;DR

This paper introduces an ultrathin magnesium coating that effectively suppresses tantalum oxide formation, enhancing superconducting properties and coherence times in quantum circuits, thereby advancing scalable quantum computing technologies.

Contribution

It presents a novel magnesium capping technique to reduce tantalum oxide, improving superconducting performance and providing insights into surface oxide mechanisms for quantum materials.

Findings

Magnesium capping confines tantalum oxide to an ultrathin region.

Superconducting transition sharpness and temperature are improved with Mg capping.

The study offers a new materials design principle for quantum circuit fabrication.

Abstract

Scaling up superconducting quantum circuits based on transmon qubits necessitates substantial enhancements in qubit coherence time. Among the materials considered for transmon qubits, tantalum (Ta) has emerged as a promising candidate, surpassing conventional counterparts in terms of coherence time. However, the presence of an amorphous surface Ta oxide layer introduces dielectric loss, ultimately placing a limit on the coherence time. In this study, we present a novel approach for suppressing the formation of tantalum oxide using an ultrathin magnesium (Mg) capping layer deposited on top of tantalum. Synchrotron-based X-ray photoelectron spectroscopy (XPS) studies demonstrate that oxide is confined to an extremely thin region directly beneath the Mg/Ta interface. Additionally, we demonstrate that the superconducting properties of thin Ta films are improved following the Mg capping, exhibiting sharper and higher-temperature transitions to superconductive and magnetically ordered states. Based on the experimental data and computational modeling, we establish an atomic-scale mechanistic understanding of the role of the capping layer in protecting Ta from oxidation. This work provides valuable insights into the formation mechanism and functionality of surface tantalum oxide, as well as a new materials design principle with the potential to reduce dielectric loss in superconducting quantum materials. Ultimately, our findings pave the way for the realization of large-scale, high-performance quantum computing systems.