  We fabricate and characterize superconducting through-silicon vias and
electrodes suitable for superconducting quantum processors. We measure internal
quality factors of a million for test resonators excited at single-photon
levels, on chips with superconducting vias used to stitch ground planes on the
front and back sides of the chips. This resonator performance is on par with
the state of the art for silicon-based planar solutions, despite the presence
of vias. Via stitching of ground planes is an important enabling technology for
increasing the physical size of quantum processor chips, and is a first step
toward more complex quantum devices with three-dimensional integration.
