High quality superconducting tantalum resonators with beta phase defects
2025-02-28 10:23
14 浏览Source: https://arxiv.org/abs/2502.17247
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 α-tantalum, avoiding the meta-stable β-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 β-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors (Qi) up to (5.0±2.5)×106 in the single photon regime. They outperform resonators with no sign of the β-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of β-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits.
Conclusion: In summary, we have demonstrated that low-loss superconducting resonators can be fabricated from Ta films with a significant amount of β-phase defects at the metal-substrate interface. These mixed-phase Ta resonators with a 50µm capacitor gap and 10% volume fraction of β-phase achieved internal quality factors of (5.0±2.5)×106 in the single-photon regime. For resonators fabricated from high quality single phase Ta films, single-photon quality factors were inferior in general. For one of these α-phase resonators similarly high internal quality factor was measured. However, the temperature dependence of resonance frequencies and quality factors suggest that low-Tc defects are present in the film, possibly associated with trace amounts of the β-phase not detectable in XRD analysis, or introduced by sample fabrication processes. The correlation between β-phase concentration and Qi is more complex than anticipated. We conclude that a small amount of β-phase in the film can be beneficial, enhancing critical magnetic fields and its concentration might even be a possible optimization parameter leading to improved coherence in Ta based superconducting circuits.
