  Initialization and readout of coupled quantum systems are essential
ingredients for the implementation of quantum algorithms. If the state of a
multi-qubit register can be read out in a single shot, this enables further key
resources such as quantum error correction and deterministic quantum
teleportation, as well as direct investigation of quantum correlations
(entanglement). While spins in solids are attractive candidates for scalable
quantum information processing, thus far single-shot detection has only been
achieved for isolated qubits. Here, we demonstrate preparation and measurement
of a multi-spin quantum register by implementing resonant optical excitation
techniques originally developed in atomic physics. We achieve high-fidelity
readout of the electronic spin associated with a single nitrogen-vacancy (NV)
centre in diamond at low temperature, and exploit this readout to project up to
three nearby nuclear spin qubits onto a well-defined state. Conversely, we can
distinguish the state of the nuclear spins in a single shot by mapping it onto
and subsequently measuring the electronic spin. Finally, we show compatibility
with qubit control by demonstrating initialization, coherent manipulation, and
single-shot readout in a single experiment on a two-qubit register, using
techniques suitable for extension to larger registers. These results pave the
way for the first test of Bell's inequalities on solid-state spins and the
implementation of measurement-based quantum information protocols.
