Development Of Spin Qubits In Silicon Germanium Heterostructures

Quantum computing - leveraging quantum phenomena to perform complex and otherwise intractable computational problems - has rapidly progressed from a theoretical aspiration to a potential reality. Currently, there are many competing approaches to the way the physical qubits (quantum bits) are built, from trapped ions, to superconducting circuits, to semiconductor quantum dots, and beyond. Here, we focus on quantum dots, where electrons or holes are confined within a semiconductor and the quantized nature of charge and spin are utilized for computation. Within the field of quantum dots, heterostructures made of silicon and silicon-germanium are especially enticing due to their low density of defects and nuclear spin. Although quantum dots are a promising avenue for quantum computation because of their intrinsically small size and similarity to classical transistors, nearly every aspect of their design, realization, and control has yet to be fully optimized.This thesis explores modifications to the heterostructure, fabrication, and measurement of Si/SiGe quantum dots in the pursuit of improved quantum dot qubits. The valley splitting in silicon quantum dots, a near degeneracy of the lowest lying energy states, is critical to the formation and performance of silicon qubits. In this work, we present several modifications to the Si/SiGe heterostructure in an effort to enhance this splitting. In particular, we investigate the effects of introducing germanium to the silicon quantum well by the inclusion of a single spike in germanium concentration or an oscillatory concentration throughout the well. We present experimental measurements of the energy spectrum arising from both modifications and, coupled with theoretical support, demonstrate enhancements to the valley splitting. Next, we present several fabrication techniques with the goal of improved quantum dot functionality and lowered charge noise, a major barrier to higher quality devices. We report a new strategy for etched-palladium fabrication and discuss the current progress. Finally, we present work towards the automation of quantum dot tuning. As quantum dot devices increase in the number of qubits, so do the number of electrostatic gates which control the device. We discuss the development of automated tuning procedures and present a procedure for the formation of well-controlled quantum dots from initial voltage settings.

Here is a discussion of the state of the art of spin resonance in low dimensional structures, such as two-dimensional electron systems, quantum wires, and quantum dots. Leading scientists report on recent advances and discuss open issues and perspectives.

Quantum computing is radically different from the conventional approach of transforming bit-strings from one set of zeros and ones to another. With quantum computing, everything changes. The physics used to understand bits of information and the devices that manipulate them are vastly different. Quantum engineering is a revolutionary approach to quantum technology. Technology Road Mapping for Quantum Computing and Engineering explores all the aspects of quantum computing concepts, engineering, technologies, operations, and applications from the basics to future advancements. Covering topics such as machine learning, quantum software technology, and technology road mapping, this book is an excellent resource for data scientists, engineers, students and professors of higher education, computer scientists, researchers, and academicians.

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.

This book provides a comprehensive summary of nanowire research in the past decade, from the nanowire synthesis, characterization, assembly, to the device applications. In particular, the developments of complex/modulated nanowire structures, the assembly of hierarchical nanowire arrays, and the applications in the fields of nanoelectronics, nanophotonics, quantum devices, nano-enabled energy, and nano-bio interfaces, are focused. Moreover, novel nanowire building blocks for the future/emerging nanoscience and nanotechnology are also discussed.Semiconducting nanowires represent one of the most interesting research directions in nanoscience and nanotechnology, with capabilities of realizing structural and functional complexity through rational design and synthesis. The exquisite control of chemical composition, morphology, structure, doping and assembly, as well as incorporation with other materials, offer a variety of nanoscale building blocks with unique properties.

A timely reference from leading experts on semiconductor nanowires and their applications.