High quality superconducting tantalum resonators with beta phase defects

TL;DR

This study demonstrates that incorporating small amounts of beta-phase tantalum in superconducting resonators can enhance their quality factors, potentially leading to better coherence in quantum circuits, challenging previous assumptions about its detrimental effects.

Contribution

The paper shows that beta-phase tantalum inclusions can improve the internal quality factors of superconducting resonators, offering a new approach to optimize quantum hardware performance.

Findings

Resonators with beta-phase tantalum achieve Q_i up to 5.0 million.

Beta-phase tantalum inclusions can be beneficial, not detrimental, to resonator performance.

Small beta-phase concentrations may enhance critical magnetic fields in superconducting circuits.

Abstract

For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials. Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from $\alpha$-tantalum, avoiding the meta-stable $\beta$-phase that forms when depositing tantalum at room temperature, and has been previously identified to be a source of microwave losses. In this work, we show lumped element resonators containing $\beta$-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors ($Q_\text{i}$) up to $(5.0 \pm 2.5) \times 10^6$ in the single photon regime. They outperform resonators with no sign of the $\beta$-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of $\beta$-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits.