Towards quantum computing using nitrogen vacancy centers: optimising photon collection

Thesis

Nitrogen vacancy centres in diamond have shown strong potential for use as qubits in quantum computing. However, nitrogen vacancy centres have a low percentage of overall emission emitted through the Zero Phonon Line (ZPL). In addition, the fabrication of NV centres with controlled positioning is limited. This thesis presents the ZPL linewidth characterisation of NV centres fabricated using laser writing and laser diffusion. Only one out of the six NV centres characterised showed a <100 MHz average single scan linewidth, which suggests that the laser diffusion process needs to improved. The low ZPL emission can be enhanced by cavity coupling to Fabry-Pérot microcavities. Fabry-Pérot microcavities are susceptible to vibrations, which will decouple the cavity from the NV centre and reduce the photon enhancement. A cavity stabilisation system based on cavity fringe stabilisation is implemented, and the results of the stability are presented. The Fabry-Pérot microcavity can be stabilised with 0.13 nm using fringe stabilisation. High collection efficiency of the emission is also desired for maximising the entanglement rate and is reduced by optical aberrations. For NV centres in bulk diamond, using Solid Immersion Lenses (SILs) removes the spherical aberration that is introduced when collecting NV emission through a planar interface. This thesis presents the results of modelling the aberrations for NV centres cavity coupled to a Fabry-Pérot microcavity and for hemispherical SILs on diamond. The wavefronts of the aberrations introduced by the planar mirror of the microcavity and the aberrations introduced by the positioning error of the SIL are modelled numerically. A deformable mirror is implemented into a confocal microscope to monitor the aberrations of the photonic devices and the results are compared with the numerical model. For Fabry-Pérot microcavities, the intensity loss caused by the aberrations introduced by the planar mirror was predicted to be less than 10 \% due to the thin mirror substrate and low numerical aperture of the cavity mode. This prediction was consistent with the experimental results where an effective 3 \% increase in light collection was observed with aberration correction. For the SILs, the model predicts that the collection efficiency is more sensitive to the lateral positioning of the SIL relative to the NV centre than the axial positioning error.

Authors: Mai, ZF

• DOI: 10.5287/ora-jneywrdwq
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: diamond; optical aberration; NV centre; solid immersion lens; fabry-pérot microcavity
• Subjects: Optics; Quantum computing