  As superconducting qubit circuits become more complex, addressing a large
array of qubits becomes a challenging engineering problem. Dense arrays of
qubits benefit from, and may require, access via the third dimension to
alleviate interconnect crowding. Through-silicon vias (TSVs) represent a
promising approach to three-dimensional (3D) integration in superconducting
qubit arrays -- provided they are compact enough to support densely-packed
qubit systems without compromising qubit performance or low-loss signal and
control routing. In this work, we demonstrate the integration of
superconducting, high-aspect ratio TSVs -- 10 $\mu$m wide by 20 $\mu$m long by
200 $\mu$m deep -- with superconducting qubits. We utilize TSVs for baseband
control and high-fidelity microwave readout of qubits using a two-chip,
bump-bonded architecture. We also validate the fabrication of qubits directly
upon the surface of a TSV-integrated chip. These key 3D integration milestones
pave the way for the control and readout of high-density superconducting qubit
arrays using superconducting TSVs.
