Active Length Stabilisation of an Open Fabry-Pérot Microcavity for Colour Centres in Diamond

Abstract

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.