Reducing surface loss in 3D microwave copper cavities for superconducting transmon qubits

The recent implementation of three-dimensional microwave cavities coupled to superconducting transmon qubits has led to dramatic improvements in qubit coherence times [1]. Besides the superconducting aluminum cavities that have been used in many such measurements, other recent experiments have utilized copper cavities [2] with coherence times now approaching 0.1 ms. We are investigating the effects on the quality factor for three-dimensional copper cavities that have the cavity-wall surfaces electropolished and coated with a superconducting tin layer. The copper base provides a good path for thermalizing the cavity walls and qubit chip, while the surface treatment reduces the microwave loss. We have measured a superconducting transmon qubit in such a cavity and we report our low-temperature coherence results on the same qubit and cavity measured in two different labs.