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High-quality superconducting tantalum resonators with beta phase defects

Dhundhwal, Ritika, Duan, Haoran, Brauch, Lucas, Arabi, Soroush, Fuchs, Dirk, Haghighirad, Amir-Abbas, Welle, Alexander, Scharwaechter, Florentine, Pal, Sudip, Scheffler, Marc, José M. Palomo, Zaki Leghtas, Anil Murani, Hahn, Horst, Aghassi-Hagmann, Jasmin, Kübel, Christian, Wulf Wulfhekel, Ioan M. Pop, Reisinger, Thomas

Published Jan 1, 2025• 0 citations
Publisher
DOI
10.5445/ir/1000187642

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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 ααα-tantalum, avoiding the meta-stable βββ-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 βββ-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors (QiQ_\text{i}Qi​) up to (5.0±2.5)×106(5.0 \pm 2.5) \times 10^6(5.0±2.5)×106 in the single photon regime. They outperform resonators with no sign of the βββ-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of βββ-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits.
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 (α-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 α-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 α-Ta. \{beta}-Ta films exhibit significant kinetic inductance, consistent with an estimated magnetic penetration depth of (1.78 +/- 0.02) μm. 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.