Superconducting gap spectroscopy in Ta films with different Ta/Sapphire interfaces

One of the major efforts in superconducting quantum computations is to improve the transmon qubits' coherence times by mitigating dissipation. A key source of decoherence arises from two-level systems hosted by a transmon capacitor metal-substrate interface [1]. Tantalum has been suggested as a promising replacement of Nb for the capacitor metal owing to its high coherence times and formation of favorable Ta oxides on the surface [2]. A recent study [3] on microwave loss in Ta films shows that films deposited at higher temperatures show more losses, which can be attributed to the Ta/sapphire interface. This issue can be addressed by growing a thin epitaxial Nb interlayer between Ta and the substrate. To fully understand the origin of these losses, we have performed the superconducting gap spectroscopy of Ta films on differently treated sapphire substrates. We observed significant differences in the deep in-gap quasiparticle distribution and excellent agreement with the measured quality factors. The Ta film deposited on a thin Nb interlayer showed the best result. Our results are supported by theoretical analysis. We demonstrated that the developed new quasiparticle spectroscopy offers a quick, non-destructive characterization of superconducting transmons.

[1] I. Siddiqi, Nat. Rev. Mater. 6, 875 (2021)

[2] C. Wang et al.,  npj Quantum Inf. 8, 3 (2022)

[3] McFadden et al., Phys. Rev. Materials 9, 096201 (2025)