Optical NMR Study of 31P Donor Spins in Isotopically Enriched 28Si

A quantum computer requires a quantum system that is isolated from its environment, but can be integrated into devices, and whose states can be measured with high accuracy. Nuclear spin qubits using shallow neutral donors in semiconductors have been studied extensively as quantum memories due to their promise of long coherence lifetimes. However, the nuclear spins of neutral donors are not only difficult to initialize into known states and detect with high sensitivity, they are limited to use at cryogenic temperatures. The nuclear spins of ionized donors, on the other hand, have the potential for high-temperature operation. In this thesis, I show how the distinctive optical properties of enriched 28Si enable the use of hyperfine-resolved optical transitions of donor bound excitons, as previously applied to great effect for isolated atoms and ions in vacuum. Together with efficient Auger photoionization, these optical transitions permit rapid nuclear hyperpolarization and electrical spin readout. These techniques are combined to detect nuclear magnetic resonance from dilute 31P in an isotopically purified 28Si sample, at concentrations inaccessible to conventional NMR techniques. Dynamical decoupling is used to measure cryogenic coherence times of over 180 seconds and 3 hours for an ensemble of neutral and ionized 31P nuclear spins in 28Si, respectively. A room-temperature coherence time of over 39 minutes is demonstrated in the latter system, which is more than an order of magnitude longer than the previous solid-state coherence time record. I further show that a coherent spin superposition can be cycled from 4.2 Kelvin to room temperature and back.