  Projective measurements are a powerful tool for manipulating quantum states.
In particular, a set of qubits can be entangled by measurement of a joint
property such as qubit parity. These joint measurements do not require a direct
interaction between qubits and therefore provide a unique resource for quantum
information processing with well-isolated qubits. Numerous schemes for
entanglement-by-measurement of solid-state qubits have been proposed, but the
demanding experimental requirements have so far hindered implementations. Here
we realize a two-qubit parity measurement on nuclear spins in diamond by
exploiting the electron spin of a nitrogen-vacancy center as readout ancilla.
The measurement enables us to project the initially uncorrelated nuclear spins
into maximally entangled states. By combining this entanglement with
high-fidelity single-shot readout we demonstrate the first violation of Bells
inequality with solid-state spins. These results open the door to a new class
of experiments in which projective measurements are used to create, protect and
manipulate entanglement between solid-state qubits.
