  Medium-scale ensembles of coupled qubits offer a platform for near-term
quantum technologies including computing, sensing, and the study of mesoscopic
quantum systems. Atom-like emitters in solids have emerged as promising quantum
memories, with demonstrations of spin-spin entanglement by optical and magnetic
interactions. Magnetic coupling in particular is attractive for efficient and
deterministic entanglement gates, but raises the problem of individual spin
addressing at the necessary nanometer-scale separation. Current
super-resolution techniques can reach this resolution, but are destructive to
the states of nearby qubits. Here, we demonstrate the measurement of individual
qubit states in a sub-diffraction cluster by selectively exciting spectrally
distinguishable nitrogen vacancy (NV) centers. We demonstrate super-resolution
localization of single centers with nanometer spatial resolution, as well as
individual control and readout of spin populations. These measurements indicate
a readout-induced crosstalk on non-addressed qubits below $4\times10^{-2}$.
This approach opens the door to high-speed control and measurement of qubit
registers in mesoscopic spin clusters, with applications ranging from
entanglement-enhanced sensors to error-corrected qubit registers to multiplexed
quantum repeater nodes.
