With the rapid progress in nanofabrication, scientists and researchers are now able to make lateral quantum dots in semiconductor materials. The few electrons confined in these quantum dots provide the possibility of realizing a qubit, the building block of a quantum computer. Tremendous effort has been put in the solid state quantum information field in the last ten years of making single electron spin qubit or singlet triplet qubit based on two electron spin. However, the operation of these types of qubit requires additional engineering by either integrating a microwave loop or an external magnet to creat field difference. This thesis project was inspired by DiVincenzo's proposal of developing qubit based on three electrons controlled by Heisenberg exchange interactions only, which is called "exchange-only" qubit. All the qubit operation can be done in principle via electrical pulses only. We proposed to make the triple quantum device in silicon system. This type of device will have small qubit decoherence, easy integration to industry infrustructure and great chance of scaling up to a real quantum computer. We developed and fabricated the electrostatically defined triple quantum dot (TQD) device in a silicon metal-oxide-oxide structure. We characterized its electrostatic properties using a quantum point contact charge sensing channel nearby. We are be able to obtain the charge stability diagram in the last few elelctron regime that provides the experimental basis of forming a exchange only qubit. We demonstrated the tunability of the TQD by acheiving the quadruple points where all three dots are on resonance. This is the first experimental demonstration of well controlled triple quantum dot device in silicon system. The constant interaction model and the hubbard model for triple quantum dot system are developed to help understand the electrostatic dynamics. Tunnel couplings between quantum dots, which determines the exchange interactions, are extracted using various fitting methods. We implemented the qubit manipulation with three quantum dots in both a linearly and a triangularly arranged geometry. For the first time, we observed coherent oscillation in the Si MOS based triple quantum dot device with oscillation frequency of 2MHz and 7MHz. We suspect the these oscillations are related with spin dynamics in our system. These experimental investigations demonstrate that we have the ability to develop triple quantum dot device for exchange ony qubit and the potential to perform qubit operation in the future.

Quantum computing in nanoscale silicon heterostructures has received much attention, both from the scientific community and private industry, largely due to compatibility with highly-developed silicon-based device fabrication and design present in essentially all aspects of modern life. Breakthroughs in quantum control and coupled qubit systems in silicon in the last five years have accelerated scientific research in this area, with gate-defined quantum dots at the forefront of this effort.

As techniques for quantum control become more sophisticated, subtle details of the silicon band structure are now of vital importance for the ultimate success of silicon quantum computing. Chief among these band features are the valley states, regions of the conduction band that form the ground state and a nearly degenerate excited state in quantum dot heterostructures. These valley states and their effects on electron dynamics can lead to quantum information loss and qubit decoherence, and so detailed characterization of the valleys is of great importance.

In this work, I first describe a spectroscopic technique utilizing fast voltage pulses on one or two gates in a double quantum dot device to precisely measure the relevant valley state energies in both quantum dots as well as the coupling between valley states and electron orbital states. With this information, the valley states are leveraged to form a novel qubit basis with innate protection against decoherence from charge noise. Sub-nanosecond operations on this "valley qubit" are used to demonstrate complete quantum control. Finally, using real-time read-out of energy-selective tunneling in a single quantum dot, pure valley state coherence in the form of intervalley relaxation is directly probed. This relaxation is subsequently linked to spin-valley electron dynamics and the observance of a valley-dependent tunneling process is discussed theoretically using tight-binding formalism.

Quantum computing has become a thriving field over the past several decades. Although many candidate systems exist, this dissertation will focus on quantum dots as a quantum computing implementation, specifically lateral quantum dots in silicon based heterostructures. Lateral quantum dots use trapped electrons in semiconducting heterostructures to form qubits, the basic building block of a quantum computer. There are several potential qubit implementations using quantum dots and new qubit schemes, such as the valley qubit presented in Chapter 4, are still being investigated. Many of these implementations have already been successfully demonstrated. In this sense, research into quantum dots is a maturing field, having successfully demonstrated proof of concept for multiple qubit implementations. If quantum dots are to succeed as a quantum computing platform research needs to focus on improving the qubits themselves. Decoherence and dephasing need to be improved, but also yield and reproducibility. In this work I describe experiments intended to help understand and improve the performance of lateral quantum dots. I fabricated multiple lithographically identical devices on Si/SiO2 and Si/SiGe heterostructures to compare charge noise on the two Silicon based substrates. I describe the first conclusive observation and characterization of a valley based qubit. The noise characteristics of the valley qubit are particularly attractive as it's operation is resistant to charge noise, the primary source of noise in Silicon based qubits. Finally I present the ongoing development of a novel gate architecture for lateral quantum dots. Called a hybrid architecture, this design possesses good tunability along with simple fabrication and a reduced number of total gates relative to other leading architectures; this has the potential to dramatically improve yield and scalability.

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.

The pursuit of a quantum computer has been driven both by the prospect of solving otherwise intractable problems as well as the desire to master the quantum control of mesoscopic systems. Lateral quantum dots in semiconductors, particularly in silicon, have garnered considerable attention as a potential host for a quantum bit formed from the states of confined electrons.

The conduction states of electrons in silicon are characterized by an additional degree of freedom, known as the valley degree of freedom. Valley states have certain properties which represent both a complication and an opportunity for the use of electrons in silicon as a qubit. Valley states are 6-fold degenerate, reducing to being very nearly 2-fold degenerate when confined to a quantum well, meaning they have the potential to disrupt many qubit readout schemes for spin based on the Pauli exclusion principle. Also, they are rapidly oscillating relative to one another, making their interaction highly sensitive to atomic scale properties of the confining potential, which are difficult to control when fabricating devices. However, they share many spin-like properties leading to the possibility of familiar control protocols and long coherence times.

We have developed techniques for the fabrication of lateral double quantum dots and experimental procedures for the coherent manipulation and readout of quantum states of confined electrons. Through the use of fast pulses, quantum oscillations between two valley states are induced, allowing the energy splitting between these valley states to be probed to a resolution not possible with conventional methods of valley spectroscopy. The fabrication techniques and experimental procedures have led to the detection of quantum oscillations in multiple devices, demonstrating the utility and generality of the techniques.

In the course of refining these experimental techniques, we developed software allowing for the linear compensation of multiple experimental parameters. These improvements led to the ability to efficiently adjust the inter-dot tunneling rate, the dot-reservoir tunneling rate and the charge sensing channel sensitivity, control parameters whose fine tuning was vital to the success of the quantum manipulation and readout ultimately achieved.

Magnetism has been utilized since ancient civilizations to build tools for day to day life for example, magnetic compass. Understanding of nature behind magnets has ever since evolved passing significant milestones such as, discovery of interplay between magnetism and electricity leading to invention of electric motor and dynamo and, microscopic insight developed following discovery of quantum mechanics. About twenty years ago we passed arguably the most recent milestone with the prediction of the ability to manipulate a nano sized magnet with the aid of the spin of an electronic current. With the advent of magnetic tunnel junctions (MTJ), one of the main applications of this has been spin transfer torque nano oscillators (STNO) that have attracted great attention. Recently, a new emerging field regarding magnetic skyrmions in MTJs has shown even more promising aspects such as higher energy efficiency for real world applications. In the first half of this thesis, we present a tri- layer MTJ based STNO offering 6 GHz microwave emission for both current polarities at zero external magnetic field which is the highest frequency achieved in absence of any bias field to-date, to the best of our knowledge. We have investigated into spin dynamics of this STNO discussing about out of plane (OOP) precessions along with micromagnetic simulations. In the second half of this thesis, we present observation of skyrmionic signature in an MTJ at cryogenic temperature producing random telegraph signal (RTS). We have studied the dependencies of this RTS upon various physical parameters. RTS seen in MTJs involving ferromagnetic states has attracted attention in the research community as it is an ideal candidate to realize neuromorphic computers which are inspired on human brain offering tremendous improved performance in specific tasks over conventional boolean computing. We believe this investigation into RTS involving skyrmionic states in an MTJ will further electrify this on going expedition.