  Large-scale quantum computers with more than $10^5$ qubits will likely be
built within the next decade. Trapped ions, semiconductor devices, and
superconducting qubits among other physical implementations are still confined
in the realm of medium-scale quantum integration ($\sim 100$ qubits); however,
they show promise toward large-scale quantum integration. Building large-scale
quantum processing units will require truly scalable control and measurement
classical coprocessors as well as suitable wiring methods. In this blue paper,
we introduce a fully vertical interconnect that will make it possible to
address $\sim 10^5$ superconducting qubits fabricated on a single silicon or
sapphire chip: Pin-chip bonding. This method permits signal transmission from
DC to $\sim 10$ GHz, both at room temperature and at cryogenic temperatures
down to $\sim 10$ mK. At temperatures below $\sim 1$ K, the on-chip wiring
contact resistance is close to zero and all signal lines are in the
superconducting state. High-density wiring is achieved by means of a fully
vertical interconnect that interfaces the qubit array with a network of
rectangular coaxial ribbon cables. Pin-chip bonding is fully compatible with
classical high-density test board applications as well as with other qubit
implementations.
