Characterization and control of a cryogenic ion trap apparatus and laser systems for quantum computing

Abstract

Trapped ions are one of the leading approaches to build quantum computers. The required building blocks have all been demonstrated with high fidelity, recently culminating in elementary demonstrations of quantum error correction. The main challenge now is to scale up current-day systems without increasing the errors, but reducing them instead. The first part of this thesis is concerned with technical challenges faced when scaling up trapped-ion systems. We therefore present the design of a cryogenic ion trap setup and use it to evaluate new technological approaches. We perform a detailed characterization of the apparatus to identify its main limiting factors and to uncover unknown unknowns. The second part of this thesis is concerned with the use of feedback control, primarily for laser stabilization. We explore its benefits but also associated challenges and fundamental limitations in a number of scenarios motivated by the experimental apparatus and its characterization. A common theme in the latter is the occurrence of noise at discrete frequencies or confined to narrow frequency bands. We therefore investigate how to tailor feedback control loops to be particularly effective at these frequencies by incorporating resonant elements into the controller. The presentation in the second part is kept general enough such that it can serve as a reference on feedback control for a broad audience within atomic, molecular and optical physics. The goal is to provide a map of the control engineering literature to help physicists navigate the landscape, ultimately leading to better experiments.