Tuning Electron Spin Coherence in Carbon Nanospheres through Defect Engineering

This study investigates spin decoherence in large disordered carbon nanosphere (CNS) particles with a focus on predicting and extending the electron spin qubit decoherence time (T₂). We present and experimentally validate a simple analytical model that predicts T₂ and reveal how defects─such as carbon vacancies, hydrogen chemisorption, and substitutional impurities─along with their concentration and distribution, impact T₂. This model offers insights into the interplay among spin density distribution, structural defects, and isotopic composition, demonstrating the role of defect minimization through controlled annealing in enhancing spin coherence. Through comparison with experimental data, we validate our model and demonstrate that spin polarization in the CNS is likely evenly distributed over a characteristic region of approximately 5 nm for T₂ ∼ 200 ns. Based on these findings, we propose a synthetic protocol involving a confined annealing procedure that extends the spin lifetime in the CNS to up to 362 ns. With observed improvements in T₂, our findings provide valuable guidelines for optimizing electron spin coherence time in quantum devices and spintronic applications.