Cavity-coupled qubits in diamond for networked quantum computing

Point defects in wide band gap materials, such as the nitrogen vacancy (NV) centre in diamond, show strong potential as quantum bits (qubits) for future quantum information technologies. One of their key features is a coherent optical interface, which allows quantum information to be transferred between 'static' electronic and nuclear spin states and 'flying' photons, and which facilitates their use in quantum communications systems and the construction of networks for scalable quantum processors. However a significant bottleneck in recent years has been the low efficiency (~1%) of this spin-photon interface, which places limitations on scalability and therefore the impact that this approach has achieved. Research efforts to address this challenge focus on exploring new colour centres and on integrating NV centres into optical microcavities.

This project will investigate the use of a novel type of microcavity based on a monolithic Fabry Perot design, akin to those widely used in semiconductor devices such as vertical cavity surface emitting lasers which are grown by epitaxial techniques. Diamond does not support heteroepitaxial growth, so the cavity mirrors will be sputtered dielectrics (which are commercially available) deposited on either side of a thin diamond membrane. The student will develop techniques for forming the membranes to realise stable cavity modes with high quality factors, which are required to enhance the coherent spin-photon coupling strength. This will involve surface patterning to make microlens structures, using techniques such as focused ion beam milling and photolithography which are available in the host department. Key figures of merit will be the control over the shape of the surface as a means to determine mode geometry, and the surface roughness which determines the maximum mirror reflectivity as limited by scattering losses. These parameters will be measured using optical profilometry and atomic force microscope respectively. The student will characterise the resultant cavity devices using optical spectroscopy of the cavity modes, facilities for which exist within the host research group. The objective will be to achieve cavity modes with Purcell factors of 50 by the end of the project, which would be expected to translate to an interface efficiency of around 70%.

The project falls within the EPSRC Quantum Technologies research area, and the research group is part of the UK Hub in Quantum Computing and Simulation. Through this programme, the project will involve collaboration with UK companies Element Six Ltd, who supply high specification synthetic diamond, and Oxford Instruments Plasma Technologies, who provide bespoke diamond etching capabilities. A spin-out company developing diamond quantum technologies is also planned which, if realised, will interface with the project and will be a potential route for commercialisation of the project outputs.