Quantum gates, sensors, and systems with trapped ions

Abstract

Quantum information science promises a host of new and useful applications in communication, simulation, and computational algorithms. Trapped atomic ions are one of the leading physical systems with potential to implement a large-scale quantum information system, but many challenges still remain. This thesis describes some experimental approaches to address several such challenges broadly organized under three themes: gates, sensors, and systems. Quantum logic gates are the fundamental building blocks for quantum algorithms. Although they have been demonstrated with trapped ions previously, scalability requires miniaturizing ion traps by using a surface-electrode geometry. Using a single ion in a surface-electrode trap, we perform a two-qubit entangling gate and fully characterize it via quantum process tomography, as an initial validation of surface-electrode ion traps for quantum information processing. Good logic gates are often good sensors for fast fluctuations and energy changes in their environment. Trapped ions are sensitive to fluctuating and static charges, leading to motional state decoherence (heating) and instabilities, problems exacerbated by the surface-electrode geometry. We investigate the material dependence of heating, specifically with aluminum and superconducting traps, to elucidate the physical origin of these fluctuating charges. Static charging is hypothesized to be caused by the trapping and cooling lasers due to the photoelectric effect. We perform systematic experiments with aluminum, gold, and copper traps with lasers at various wavelengths to validate this hypothesis. Realizing quantum processors at the system level requires models and tools for predicting system performance, demonstration of good classical and quantum control, and techniques for integrating different quantum systems. We develop a modeling system for trapped ion quantum computing experiments and simulate the effect of physical and technical noise sources on practical realizations of quantum algorithms in a trapped ion system. We experimentally demonstrate several such algorithms, including the quantum Fourier transform, order-finding, and Shor's algorithm on up to 5 ions. These experiments highlight several unique advantages of ion trap systems and help identify needs for further development. Finally, we explore the integration of ion traps with optical elements including mirrors and photon detectors as key elements in creating future hybrid quantum systems.