(color online) Elements of the loophole-free Bell inequality violation experiment.^[88] (a) The lambda transition makes it is possible to entangle the NV electron spin state with the polarization of an emitted photon. For example, the detection of a photon indicates that the electron spin is at the state.^[91] (b) A measurement-based scheme to entangle two remote spins, with the assistance of flying qubits. Adapted from Ref. [92], MRS. (c) Energy levels and (d) typical results of single-shot readout of the NV electron spin state under resonant excitation (left). The spin-selective transitions in panel (c) can be used to polarize the spin state with high fidelity (right). Reproduced from Ref. [27], NPG.