A cryogenic trap for microwave-driven quantum logic using 43Ca+ ions

Thesis

The encoding and manipulation of information using the quantum states of trapped ions currently represents one of the most promising routes towards the realisation of a universal quantum computer. Although quantum gates in this system were originally driven by lasers, an alternative approach that uses the magnetic field gradients generated by current-carrying structures integrated directly into the trap has gained traction in recent years. This laser-free approach offers a few advantages, but the implementation of fast two-qubit gates with fidelities below the threshold required for quantum error correction remains challenging. In this thesis I describe progress in an ion trap system aimed at improving the fidelity and speed of two-qubit gates driven by magnetic field gradients oscillating near the transition frequency of the qubit. Work is based on a hyperfine qubit in 43Ca+ that has not been used for quantum logic before. We operate at a static magnetic field of 28.8 mT, at which point the qubit frequency of approximately 3.1 GHz becomes to first order insensitive to field fluctuations, enabling an internal coherence time that is orders of magnitude larger than the duration of quantum gates. The strong magnetic field results in a complex energy level structure, with adjacent states in the ground level being split by several multiples of the linewidths of the cooling lasers. Nevertheless, we demonstrate cooling to a temperature of 0.5mK for a single ion, close to the Doppler limit, by exploiting dark resonances that form between fine-structure levels. Furthermore, we implement resolved-sideband cooling using two laser beams driving a Raman transition, achieving an average occupation number ¯n = 0.08 for the two-ion motional mode of interest. Experiments are carried out in a surface trap that is suitable for operation at room and cryogenic temperatures. It features a U-shaped microwave electrode, which produces a microwave magnetic field such that the field component coupling the qubit states exhibits a strong gradient, but significantly reduced amplitude around the trap centre. I discuss measurements of parameters that will impact twoqubit gate fidelity, such as heating rates, drifts in motional frequencies, ac Zeeman shifts and the spatial dependence of the microwave magnetic field. Furthermore, we demonstrate spin-motion entanglement for a single ion using the Mølmer-Sørensen scheme and present the results of initial attempts at a twoqubit gate, for which we measure a fidelity of 0.77(2). I also provide a brief analysis of dominant sources of error that will have to be addressed in future experiments.

Authors: Löschnauer, C

• DOI: 10.5287/ora-agkgaqk86
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: calcium 43; two-qubit gates; microwave gates; surface trap; trapped ions; laser cooling; laser-free gates
• Subjects: Quantum computing; Physics; Trapped ions