| Summary: | 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.
|
|---|