  In circuit-based quantum computing, the available gate set typically consists
of single-qubit gates acting on each individual qubit and at least one
entangling gate between pairs of qubits. In certain physical architectures,
however, some qubits may be 'hidden' and lacking direct addressability through
dedicated control and readout lines, for instance because of limited on-chip
routing capabilities, or because the number of control lines becomes a limiting
factor for many-qubit systems. In this case, no single-qubit operations can be
applied to the hidden qubits and their state cannot be measured directly.
Instead, they may be controlled and read out only via single-qubit operations
on connected 'control' qubits and a suitable set of two-qubit gates. We first
discuss the impact of such restricted control capabilities on the quantum
volume of specific qubit coupling networks. We then experimentally demonstrate
full control and measurement capabilities in a superconducting two-qubit device
with local single-qubit control and iSWAP and controlled-phase two-qubit
interactions enabled by a tunable coupler. We further introduce an iterative
tune-up process required to completely characterize the gate set used for
quantum process tomography and evaluate the resulting gate fidelities.
