Crucial to the behaviour of a Nitrogen-Vacancy (NV) centre are its excited state cyclicities, determining the ability to perform high fidelity readout, spin-photon entanglement generation or fast spinpumping. This work presents a unified model to predict these excited state cyclicities over a wide range of strain, electric field, magnetic field and temperature, a useful tool for a model based predictive engineering approach towards the development of new NV qubits.
A set of measurements was performed to verify the prediction made by the model and improve the model over a wide range of perpendicular strain up to 7.5 GHz. An automated photon detection efficiency measurement was implemented, showing an average PSB photon detection efficiency of 2.8(3)% in the given setup. The cyclicities of the |Ex⟩ and |Ey⟩ excited states were measured over the given strain range for a single NV centre, showing a severe drop in cyclicity towards higher strain for both transition, a convergence of cyclicities towards zero strain, and a consistently higher cyclicity for |Ex⟩ compared to |Ey⟩, as predicted by the model. However, clear differences were discovered between predictions and measurements, with the recommendation to repeat the measurement at a lower sample temperature of 4K where temperature mixing is negligible. A framework was developed to measure the excited state branching ratios into and out of the singlet states, and were measured for the |E1,2⟩ states, showing that significant singlet branching remains present at high strain. However, a broken optical device prevented accurate measurements.
Furthermore, an optimisation framework was developed, implemented and verified for a mirror coupling light out of a quantum frequency converter into a telecom fibre; one of the final steps towards an operationally autonomous NV quantum network node.

Generation of high fidelity entanglement between quantum nodes is a key component of a future quantum internet. Heralded entanglement generation of two spatially separated qubit nodes can be established by interference and measurement of two photons, each entangled with one qubit state. The two-node entanglement fidelity is limited by the degree of indistinguishability of the photons, which can be measured in a Two-Photon Quantum Interference (TPQI) experiment. In this thesis, a TPQI experiment has been performed with photons emitted by a single Nitrogen-Vacancy (NV) center. This self-interference experiment shows a visibility of V=0.91±0.02 and a photon indistinguishability of J=0.945 in the Monte-Carlo method obtained 1σ-confidence interval of [0.920, 0.966] after correction for system imperfections, demonstrating near-perfect indistinguishability of zero-phonon line photons emitted by a single NV center. Furthermore, an extensive TPQI model was developed that includes possible arrival time- and frequency differences of the photons. This model predicts a dark- and noise count limited V=0.79±0.06 for a future two-node NV TPQI experiment with quantum frequency-converted photons, at a distinguishable photon coincidence rate of 1.2mHz, allowing for an experimentally feasible double-click two-node entanglement fidelity of 0.89±0.03.