Speaker
Description
Institut Néel, University Grenoble Alpes, CNRS, Grenoble INP, Grenoble, France
Gleb Wataghin Institute of Physic, State University of Campinas, Campinas, SP-Brazil
Départ. des accélérateurs, de la cryogénie et du magnétisme, Université Paris-Saclay-CEA, France
Superconducting qubit performance is fundamentally limited by decoherence mechanisms, notably dielectric losses. In this work, we investigate a transmon molecule design featuring an original coupling mechanism that enables a non-perturbative cross-Kerr interaction between the qubit and the readout microwave cavity [1]. This architecture has already demonstrated high readout fidelity (99.2% [2]) along with long coherence times (T₁ ≃ 120 μs and T₂ ≃ 23 μs [2]). Our objective is to implement such a transmon molecule using tantalum capacitive pads in conjunction with aluminum Josephson junctions. Recent advances indicate that tantalum (Ta) deposited on sapphire substrates can significantly enhance qubit performance, with relaxation times (T₁) reaching the millisecond range [3].
The tantalum thin films are grown on sapphire substrates by electron-beam evaporation in an ultra-high-vacuum chamber with a base pressure of approximately 5×10⁻¹⁰ mbar. To investigate the influence of growth conditions on superconducting and microwave properties, films are deposited at different substrate temperatures, including 350 °C and 580 °C.
We characterize the structural, morphological, and superconducting properties of the films using X-ray diffraction, atomic force microscopy (AFM), and resistance-versus-temperature measurements. To evaluate microwave losses, we fabricate coplanar waveguide resonators and measure their internal quality factors. We obtain values ranging from 6 × 10⁶ in the low-photon-number regime to above 6 × 10⁷ at high microwave drive power, placing our devices among state-of-the-art superconducting resonators. Finally, we perform systematic measurements of the internal quality factor and resonance frequency as functions of temperature and microwave drive power. The data are analyzed within the frameworks of two-level systems (TLS), Mattis–Bardeen theory, and Ginzburg–Landau theory to account for dielectric losses, quasiparticle contributions, and nonlinear behavior. Interestingly, from these measurements, we observe significant spatial variations of the superconducting critical temperature ($T_c$) across devices fabricated on the same wafer. These $T_c$ spatial variations depend on the substrate temperature during evaporation. They have been observed in several recent studies and their origins are still in debate [4]. By local probe using point contact tunnelling spectroscopy, we were able to relate them to superconducting gap variations.
[1] R. Dassonneville et al, Phys. Rev. X 10, 011045 (2020).
[2] C. Mori, V. Milchakov, et al., High-power readout of a transmon qubit using a nonlinear coupling, arXiv:2507.03642 (2025).
[3] A. Place et al. Nature communications 12.1 (2021).
[4] F. Bahrami et al. Phys. Rev. B 113, 054505 (2026).