Visible Emitters in Hexagonal Boron Nitride as an Optically Addressable Spin Qubit Platform

Abstract

Optically active spins in solid-state systems serve as excellent spin-photon interfaces, a crucial building block for quantum technologies such as information networks and sensing. This thesis presents a new spin qubit platform: visible emitters embedded in epitaxially grown hexagonal boron nitride (hBN) films, displaying bright single-photon emission and quantum coherent spins under ambient conditions. Two optically addressable electronic spin manifolds in these defects are described, investigated using optically detected magnetic resonance (ODMR) techniques. The first manifold corresponds to a spin signature with small zero-field splitting (D/h < 30 MHz). Interestingly, the sign of ODMR contrast for this resonance can be positive or negative, likely arising from variability in the balance of optical rates across defects.The second manifold is a spin triplet with large zero-field splitting (D/h=1.97 GHz, E/h=62 MHz), displaying positive ODMR contrast that reaches over 90% for some defects, indicating a high degree of optically initialised spin polarisation. Analysis of the ODMR response under angle-resolved magnetic fields reveals that the principal spin axis of these spin defects lies parallel to the hBN sample plane, suggesting a defect symmetry of C2v or lower. Combined with favourable optical dynamics and strongly spin-selective intersystem crossing dynamics, this low symmetry enables the ODMR contrast to persist under high magnetic field of arbitrary orientation, with ~μT/Hz^(1/2) magnetic-field sensitivity for bias magnetic field exceeding 50 mT, demonstrating the potential of these defects for vector magnetometry at the nanoscale. The identification and characterisation of optically addressable electronic spin resonances of single defects in hBN presented in this work represents an important milestone towards enabling a room-temperature spin-photon interface in a two-dimensional host crystal and opens exciting routes for the development of novel nanoscale sensing devices.