The Nitrogen-Vacancy (NV) center has demonstrated great potential as a quantum networks platform. While many milestones have been reached, the current hardware implementations have reached their limit in terms of remote entanglement generation rates, which hinders the scalability of the platform. The most dominant limits are that only ∼ 3% of the emission is coherent, and the outcoupling efficiency in state-of-the-art network experiments is ∼ 15%. The coherent emission can be increased via the Purcell effect, which can be achieved by the use of an open Fabry-Pérot microcavity. In such a setup the NV center is embedded in a micrometer-thin diamond membrane, thereby retaining its favourable optical properties. The open microcavity has the ability of in situ spectral and spatial tunability. However, this is accompanied by a high susceptibility to vibrations, which needs to be considered in the experimental design. The goal of this thesis is to simultaneously increase the coherent emission and the outcoupling efficiency by coupling NV centers to an open Fabry-Pérot microcavity. An optimized cavity is found by characterizing different fiber tip mirrors, reaching a bare cavity finesse of 9500. The vibrations in the bare cavity are analyzed, revealing a root mean square cavity length detuning of 22 pm while operating at low temperature. A hybrid cavity is formed with a diamond membrane with NV centers, resulting in cavity with a finesse of 2100, quality factor of 266000, mode volume of 108 λ 3 , and outcoupling efficiency ∼ 30%. Due to a different setup configuration, an increased root mean square cavity length detuning of 190 pm is measured, resulting in a lowered Purcell factor. Off-resonant measurements demonstrate the coupling of NV centers to the cavity. Moreover, excited state lifetimes are measured to obtain a Purcell factor of 2.6 ± 0.5, with a corresponding enhanced coherent emission ratio of 0.07 ± 0.01. This result is in good agreement with simulations that include the effect of vibrations. The ZPL branching ratio and outcoupling efficiency are improved by a factor two compared to state-of-the-art confocal microscope setups. For the first time in open microcavities, the ability to drive the spin of NV centers is shown by an optically detected magnetic resonance measurement with a contrast of (28 ± 2)%. By improving the setup to the already obtained vibration level, and a realistic finesse of 5000, an expected Purcell factor of ∼ 15.7 and outcoupling efficiency of ∼ 80% can be reached, paving the way to the next-generation of quantum network nodes.

In order to create a quantum network for distributed quantum computing and secure communication, quantum nodes are required which can share an entangled state. Colour centres in diamond are a very promising set of nodes that can be used to create entanglement between remote locations. A well-studied colour centre in diamond is the Nitrogen Vacancy (NV) centre. The NV centre is a promising candidate because the electron spin can be readout optically and the spin state can be manipulated using microwaves. Furthermore, the NV-centre can be coupled to closely located nuclear spins, where the nuclear spins grant access to multiqubit protocols and the creation of a linked qubit register. A problematic downside of the NV centre is its entanglement rate which is currently limited to ∼ 39𝐻𝑧. The entanglement rate can be increased by coupling the colour centre to an optical cavity. One approach is to use open fibre-based Fabry-Pérot microcavities, with the advantage that it can be fibre coupled, achieve high quality factors and its spectral tunability. An important downside to the spectral tunability, is its sensitivity to vibrations. Therefore the goal of this thesis is to characterise a novel fibre-based microcavity setup, investigate the vibrations influencing the microcavity and actively reduce the vibrations with a feedback loop. When a stable optical microcavity is established, a diamond membrane can be placed in the microcavity to enhance its coherent emission. The novel setup uses a new, low vibration cryostat design together with a passive vibration isolator to suppress vibrations inducing a change in microcavity length. A moveable mirror coated fibre and a flat mirror form the microcavity, where the transmitted light of the microcavity is collected with an objective. We first show control over the microcavity by measuring the linewidth and the length of the microcavity. With this setup we obtain a typical finesse ∼ 6000 (for a bare microcavity), which results in a quality factor of around 80.000 and a mode volume of 27𝜆 3 for a microcavity length of 5𝜇𝑚. Moreover, we show that the microcavity length dependence on temperature, is roughly 3 1 3 𝜇𝑚 per ℃. And we demonstrate a measurement which allows us to measure the angle of the fibre. The angle of can cause a large decrease in the quality factor and is important for mode coupling. Next, vibrations causing a detuning in microcavity length are characterised. The vibrations can be categorised into multiple frequency ranges which are further analysed. The vibrations for certain frequency ranges can be heavily suppressed by changing the conditions of the passive vibration isolator. By characterising the vibrations, we come to the conclusion that the passive vibration isolation is not optimal, resulting in a high level of vibration. Improving the passive vibration isolation is required in order to proceed to a regime where active feedback can stabilise the microcavity length when the cryostat is on. Finally, we demonstrate active length stabilisation under vacuum at room temperature. The transmitted light of the microcavity is used to actively feedback a voltage to a coarse piezo and a fine piezo. The coarse piezo counteracts microcavity length drifts at lower frequencies and the fine piezo counteracts faster length fluctuations due to vibrations. With this active feedback method, a fundamental mode of the microcavity can automatically be found, and the microcavity length can be locked to that mode. An estimation of the maximum root mean square (RMS) vibration level with the cryocooler off, at room temperature and with active stabilisation is 10𝑝𝑚. This is at least a factor 2 lower than the RMS vibration level without active stabilisation. Multiple suggestions are made to lower the vibration level in the novel setup. For example, how to improve the performance of the passive vibration isolator and how to implement a different active length stabilisation scheme like the Pound-Drever-Hall lock.