  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.
