Princeton University

School of Engineering & Applied Science

Decoherence of 31P Donor Spins in Silicon

Evan Petersen
Prof. Lyon
Engineering Quadrangle B327
Friday, July 13, 2018 - 1:30pm to 3:00pm

Spin coherence is important for the fields of electron spin resonance, nuclear magnetic resonance, magnetic resonance imaging, and quantum devices. For donor spins in silicon, coherence both quantifies their potential as qubits and measures environmental processes. By understanding those processes, we can counteract them and extend coherence times. Silicon crystals are uniquely suited to this task, benefiting from decades of advancements in purification. I use that quality to study nuclear spin decoherence due to flip-flops of 29Si nuclei and 31P donor electrons, as well as electron spin decoherence due to instantaneous diffusion when applying dynamical decoupling.
By measuring nuclear spin echo decays in crystals with varying 29Si concentrations, I reveal a range of 29Si flip-flop rates, both fast and slow compared to the timescales of experiments. These measurements also imply a “frozen core” picture, where the donor electron spin protects the nuclear spin by detuning 29Si atoms. I also measure and model nuclear spin echo decays in crystals with varying 31P concentrations, revealing the rates of electron spin flip-flops. By reducing both 29Si and 31P concentrations, a new limit is reached where an unknown mechanism appears to dominate decoherence.
Additionally, I investigate how dynamical decoupling sequences composed of pi rotations, a popular method for counteracting magnetic field noise, can also suppress the dipolar interaction between electron spins. In silicon crystals where electron spin coherence is known to be limited by instantaneous diffusion, instead of field noise, I model how such sequences can still lead to longer measured echo decay times.