Study of high 𝑸_i tantalum resonators for superconducting qubit implementation.

Superconducting qubits performance is limited by decoherence mechanisms such as dielectric losses. We are investigating a transmon molecule design featuring an original coupling mechanism that produces a non-perturbative cross-Kerr interaction between the qubit and the readout microwave cavity [1]. This setup has already achieved high readout fidelity (99.2%) and long coherence times (T₁ ≅120μs, T₂ ≅23μs) [2]. Our current objective is to implement such a transmon molecule, utilizing tantalum capacitive pads in conjunction with aluminum junctions. The tantalum film is grown on sapphire substrate using electron gun evaporation in an ultra-high vacuum chamber and a substrate temperature of 360°C and 580°C. We employ a combination of characterization techniques including x-ray diffraction, atomic force microscopy (AFM), and resistance versus temperature (R(T)) measurements, to assess the structural, surface, and superconducting properties of the Ta films. In order to evaluate microwave losses, we fabricated superconducting resonators. We measured quality factors ranging up 5×<span style="font-size:10.8333px">10⁶ in the low photon number limit to more than 60 million at large power, placing our results close to the current state-of-the-art. I will present and discuss systematic measurements of the internal quality factor and resonance frequency as functions of temperature and microwave power, providing insights into dielectric losses and superconducting properties.