Microscopic relaxation channels in materials for superconducting qubits

Abstract

Despite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties and qubit coherence are poorly understood. Here, we combine measurements of transmon qubit relaxation times (T1) with spectroscopy and microscopy of the polycrystalline niobium films used in qubit fabrication. By comparing films deposited using three different techniques, we reveal correlations between T1 and intrinsic film properties such as grain size, enhanced oxygen diffusion along grain boundaries, and the concentration of suboxides near the surface. Qubit and resonator measurements show signatures of two-level system defects, which we propose to be hosted in the grain boundaries and surface oxides. We also show that the residual resistance ratio of the polycrystalline niobium films can be used as a figure of merit for qubit lifetime. This comprehensive approach to understanding qubit decoherence charts a pathway for materials-driven improvements of superconducting qubit performance.

Fabrication and surface treatment of electron-beam evaporated niobium for low-loss coplanar waveguide resonators

“…Finally, the films resulted in a superconducting transition temperature T c = 9.20 ± 0.06 K and a residual resistivity ratio (RRR) of 4.8 (Fig. 1(e)) exhibiting the high quality of the evaporated Nb, in accord with recent results with sputtered Nb 36 .…”

“…Due to its various crystalline phases and physical properties, Nb 2 O 5 has been considered as one of the main sources of defects present on the surface of Nb 41 . NbO 2 also contributes to the loss due to oxygen vacancies 36 , which has the lowest participation in the deposited films reported here.…”

“…1(d)) exceeding previous values reported using the same deposition technique 31 . Correlation between the grain sizes and materials loss has been recently studied 36 , substantiating the advantage of larger grain sizes by treating the grain boundaries as Josephson weak-links 37 . Finally, the films resulted in a superconducting transition temperature T c = 9.20 ± 0.06 K and a residual resistivity ratio (RRR) of 4.8 (Fig.…”

TOF-SIMS Analysis of Decoherence Sources in Nb Superconducting Resonators

“…Based on previous findings, it is likely that this oxygen is mainly localized at grain boundaries in the Nb film. [40] Following the lithographic patterning of the qubit architecture, TOF-SIMS depth profiles are taken from the Nb capacitance pads deposited with HiPIMS to once again probe the distribution of oxides and hydrocarbons in the film. In this case, we find that the concentration of hydrogen, carbon, and oxygen increase uniformly.…”

Nitrogen Plasma Passivated Niobium Resonators for Superconducting Quantum Circuits

“…In theory, this method can be adapted to make planar superconducting devices, however the effect of high temperature is not yet well understood. Treatment at high temperature can change the stress and grain size of the deposited metal and allow Nb to diffuse into a substitutional site inside the Si substrate, which all could potentially affect microwave loss [23][24][25] . Baking at high temperature can also introduce dislocation and vacancy defects in the Si substrate, whose effects on microwave loss have not yet been systematically explored.…”