UCLA

Quantum computing has received growing interest not only from the research community, but also in the general public. One reason for this focus is due to the ability of a quantum computer to solve problems that cannot be computed classically, creating a pathway for solutions to complex problems that would benefit numerous aspects of society. One of the popular proposals for a quantum computer harnesses semiconductor quantum dots to define qubits within electron or hole states. Due to the compatibility of this technology with existing nanoscale industrial fabrication facilities, it has a unique advantage of being able to scale to the millions of qubits required for a commercially practical quantum computer.

Within this work, I will describe two approaches to building a qubit using semiconductor quantum dots. One exploits the valley degree of freedom of conduction electrons in silicon. While these valley states are usually seen as obstacles to spin encoding schemes, we demonstrate the ability to encode a qubit within them. This valley qubit comes with the advantage of sub-nanosecond operation times and protection from charge noise during operation. We further characterize this qubit using quantum process tomography to find fidelities ranging from $79\%-93\%$.

The second approach leverages the spin states of holes in a germanium double quantum dot. This area of research has grown rapidly over the past few years, partially because the strong spin-orbit coupling and site-dependent $g$-tensors of these holes allows for all-electrical control of the qubit states without the need for micromagnets. I will characterize the evolution between singlet and triplet states and describe how the hole $g$-tensors can be modified, which are vital to qubit manipulation. By adjusting the voltage applied to the barrier separating the two quantum dots, we have found a $g$-factor that can be increased by approximately an order of magnitude, revealing a sensitivity and tunability these $g$-tensors have to the the local electrostatic environment.