Copper waveguide cavities with reduced surface loss for coupling to superconducting qubits

TL;DR

This paper explores methods to improve copper waveguide cavity quality factors for superconducting qubits, including electropolishing and tin coating, leading to promising coherence performance.

Contribution

It introduces novel surface treatment techniques for copper cavities and demonstrates their effectiveness in enhancing qubit coherence.

Findings

Tin-plated copper cavities show improved surface quality.

Coherence times in copper cavities approach 0.1 ms.

Robustness of tin-plating across multiple cooldowns.

Abstract

Significant improvements in superconducting qubit coherence times have been achieved recently with three-dimensional microwave waveguide cavities coupled to transmon qubits. While many of the measurements in this direction have utilized superconducting aluminum cavities, other recent work has involved qubits coupled to copper cavities with coherence times approaching 0.1 ms. The copper provides a good path for thermalizing the cavity walls and qubit chip, although the substantial cavity loss makes conventional dispersive qubit measurements challenging. We are exploring various approaches for improving the quality factor of three-dimensional copper cavities, including electropolishing and coating with superconducting layers of tin. We have characterized these cavities on multiple cooldowns and found the tin-plating to be robust. In addition, we have performed coherence measurements on transmon qubits in these cavities and observed promising performance.