Efficient network interfaces for tuneable-cavity-coupled diamond spin qubits

Thesis

To enable the transformative potential of quantum information processing, it is essential to develop robust quantum networks capable of sharing quantum states via entanglement swapping. A highly efficient network node is critical for coupling qubit states into quantum channels. The nitrogen-vacancy (NV) centre in diamond is a promising qubit candidate but suffers from a low branching ratio of photons coupling into the zero-phonon line (ZPL). Embedding the NV centre in a resonant cavity can enhance this coupling, but existing open-air cavity designs lack mechanical stability. This work re- ports the first monolithic Fabry-Pérot microcavities fabricated in diamond membranes. By employing focused ion beam milling to form convex features and enclosing the diamond with distributed Bragg reflector (DBR) mirrors, we achieve a mechanically robust cavity structure. Addressing the challenge of tunability, we introduce a novel method using the strong electro-optic material Sr_0.75Ba_0.25Nb₂O₆ (SBN). SBN, synthesised via pulsed laser deposition, can alter the cavity optical path length under an applied electric field. We characterise the microstructure, optical properties, and electro-optic response of SBN, proposing a fabrication method for freestanding, releasable membranes suitable for optical integration. A numerical model incorporating these material properties examines the potential impact on cavity resonance tuning. Prototype cavities in this work are currently limited by anomalous absorption in one DBR mirror. However, improving the mirror coatings and integrating the SBN layer is expected to enable substantial Purcell enhancement and cavity tunability. This approach provides a significant advancement towards efficient and scalable quantum network nodes.

Authors: Jones, GS

• DOI: 10.5287/ora-00gpm8pby
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: quantum optics; electro-optic; SBN; quantum computing; quantum; cavity
• Subjects: Photonics; Quantum entanglement; Nanophotonics; Quantum computing