Title:Oral: High Q-factor superconducting resonators fabricated from ultra-thin Au/Nb epitaxial heterostructures

Title: High Q-factor superconducting resonators fabricated from ultra-thin Au/Nb epitaxial heterostructures

Authors: Daniel Morales1, Anthony McFadden2, Raymond Simmonds2, Varrick Suezaki 1, Nathan Ng 1, Joe Aumentado2 & Peng Wei1

1. Department of Physics and Astronomy, University of California, Riverside, CA 92521
   
   National Institute of Standards and Technology, Boulder, CO 80305

Abstract: Recent advances in superconducting quantum computing demand materials platforms that can simultaneously achieve longer coherence and scalability. We demonstrate a unique approach utilizing ultra-thin epitaxial Au(111)/Nb bilayers for fabricating superconducting circuit components, specifically distributed element superconducting microwave resonators [1]. The epitaxial Au(111) serves as an inert layer that suppresses the formation of Nb oxides, which is a main source responsible for two-level system (TLS) defects, possible trapping sites for quasiparticles, and additional dielectric losses in commonly used superconducting qubits. At the same time, the Au(111) layer acquires a superconducting gap via the proximity effect, resulting in a completely gapped Au(111)/Nb heterostructure. Our resonators, operating in the 4-8 GHz range at 35 mK, achieve an internal quality factor exceeding 10⁶ in the high-power regime while maintaining 10⁵ in the single-photon regime [1]. These metrics are comparable to the performance of traditional thick-film (>100 nm) Nb resonators, despite our implementation using significantly thinner (~10 nm) Nb layers, which is possibly beneficial for scalability. Furthermore, because Au(111) is epitaxial, its Shockley surface states can acquire a distinct superconducting gap compared to the bulk. Through high-resolution point contact tunneling spectroscopy, we show that thicker Au(111) layers exhibit a much reduced surface superconducting gap, creating a natural gap profile across the material's thickness. This gives rise to a unique material that can be potentially useful for removing quasiparticle excitations through its surface states, potentially which could increase the coherence of superconducting qubits.

[1] Chen et al., Sci. Adv. 10, eado4875 (2024)