  Building large-scale superconducting quantum computers requires two
complimentary elements: scalable wiring techniques and multiplex architectures.
In our previous work [B\'ejanin et al., Phys. Rev. Applied 6, 044010 (2016)],
we have introduced and characterized a truly vertical interconnect named the
quantum socket. In this paper, we exercise the quantum socket using
high-coherence flux-tunable Xmon transmon qubits. In particular, we test
potential qubit heating and one-qubit gate performance. We observe no heating
effects and time-stable gate fidelities in excess of 99.9%. We then propose and
experimentally characterize a demultiplexed gate technique based on flux pulses
and a common continuous drive signal: DemuXYZ. We discuss DemuXYZ's working
principle, show its operation, and perform quantum process tomography on a
selection of one-qubit gates to confirm proper operation. We obtain fidelities
around 93% likely limited by flux-pulse imperfections. We finally discuss
future solutions for wiring integration as well as improvements to the DemuXYZ
technique.
