Beta Tantalum Transmon Qubits with Quality Factors Approaching 10 Million

TL;DR

This study demonstrates that beta-phase tantalum can be used to fabricate low-loss transmon qubits with quality factors nearing 10 million, challenging previous beliefs about its suitability due to lower critical temperature.

Contribution

The paper shows that beta-phase tantalum films on sapphire can produce high-quality transmon qubits, with quality factors comparable to or exceeding those made from alpha-phase tantalum.

Findings

Mean quality factor of 5.6 million across 11 qubits

Best qubit quality factor of 10.1 million

Surface two-level systems are the main microwave loss channel

Abstract

Tantalum-based transmon qubits are a promising platform for building large-scale quantum processors. So far, these qubits have been made from tantalum films grown exclusively in the alpha phase ({\alpha}-Ta). The beta phase of tantalum (\{beta}-Ta) readily nucleates at room temperature, making it attractive for scalable qubit fabrication. However, \{beta}-Ta is widely believed to be detrimental to qubit performance because it has a lower superconducting critical temperature than {\alpha}-Ta. We challenge this prevailing belief by fabricating low-loss transmon qubits from \{beta}-Ta films on sapphire. Across 11 qubits, the mean time-averaged quality factor is (5.6 +/- 2.3) x 10^6, with the best qubit recording a time-averaged quality factor of (10.1 +/- 1.3) x 10^6. Resonator studies demonstrate that the dominant microwave loss channel is surface two-level systems, with the surface loss contribution for \{beta}-Ta being about twice that of {\alpha}-Ta. \{beta}-Ta films exhibit significant kinetic inductance, consistent with an estimated magnetic penetration depth of (1.78 +/- 0.02) {\mu}m. This work establishes \{beta}-Ta on sapphire as a material platform for realizing low-loss transmon qubits and other superconducting devices such as compact resonators, kinetic inductance detectors, and quasiparticle traps.