Optical Pumping Experiments to Increase the Polarization in Nuclear-spin Based Quantum Computers

Nuclear-magnetic-resonance (NMR) quantum-computer architectures have been by far the leading architectures in implementing small-scale proof-of-principle quantum computations. However, for an efficient computer initialization, nuclear spin polarizations of 100% are required. The low thermal nuclear-spin polarizations therefore prevent the realization of large-scale NMR quantum computers. It has been the goal of this thesis work to address this initialization problem by applying existing nuclear polarization-enhancement techniques to NMR-based quantum-computer architectures. To this purpose xenon optical-pumping techniques have been applied to liquid-state NMR quantum computers and optical pumping of single-crystal silicon has been studied to address the requirements of an all-silicon NMR quantum-computer proposal. First we have proven that the use of nuclear polarization-enhancement techniques is compatible with NMR quantum computing, by the realization of the first fully-functional polarization-enhanced NMR quantum computer. Then we have explored the polarizationenhancement capabilities of optical pumping in bulk silicon at liquid helium temperatures and high magnetic fields, by studying a 29Si-enriched silicon sample. Our study has resulted in a new 29Si polarization record of 0.25%, nearly an order of magnitude larger than the previous record. Our data modeling has stimulated the investigation of above-bandgap excitations where further increases in 29Si polarization are expected. Preliminary experiments in this regime look promising. It therefore remains hopeful that the all-silicon NMR quantum-computer architecture can be made scalable. Further research should especially focus on a demonstration of the 29Si polarizations achieved when exciting far above the bandgap. Our thesis work also includes a new technique to measure liquid 129Xe hyperpolarizations as well as an NMR study of 29Si-enriched and naturally-abundant single-crystal silicon.