Ultrasmall Silicon Quantum Dots For The Realization Of A Spin Qubit

"Electron spins in gate-defined quantum dots have emerged as a leading candidate for quantum-information-processing applications, including quantum computation. Long coherence times and compatibility with conventional semiconductor-manufacturing techniques contribute to the appeal of implementing these devices as quantum bits, or qubits. Recent research efforts have demonstrated many of the fundamental requirements for their utilization in a future quantum processor. Despite this, further development in the performance of these devices is necessary if the goal is truly to realize a universal quantum computer. Improvements will likely come in the form of both device-engineering advancements as well as novel qubit-operation and qubit-measurement schemes. This thesis describes a number of experiments carried out in gate-defined quantum dots in Si/SiGe, including demonstrations of high-fidelity spin-measurement, multiple studies of environmental noise, and coherent control of electron-spin qubits. This work represents the first realization of such devices in the Nichol Group at the University of Rochester. Together, the results represent the advancement of our understanding of silicon-based quantum dots and spin qubits"--Page xii.

The thesis gives the first experimental demonstration of a new quantum bit (“qubit”) that fuses two promising physical implementations for the storage and manipulation of quantum information – the electromagnetic modes of superconducting circuits, and the spins of electrons trapped in semiconductor quantum dots – and has the potential to inherit beneficial aspects of both. This new qubit consists of the spin of an individual superconducting quasiparticle trapped in a Josephson junction made from a semiconductor nanowire. Due to spin-orbit coupling in the nanowire, the supercurrent flowing through the nanowire depends on the quasiparticle spin state. This thesis shows how to harness this spin-dependent supercurrent to achieve both spin detection and coherent spin manipulation. This thesis also represents a significant advancement to our understanding and control of Andreev levels and thus of superconductivity. Andreev levels, microscopic fermionic modes that exist in all Josephson junctions, are the microscopic origin of the famous Josephson effect, and are also the parent states of Majorana modes in the nanowire junctions investigated in this thesis. The results in this thesis are therefore crucial for the development of Majorana-based topological information processing.

This thesis offers a comprehensive introduction to surface acoustic waves in the quantum regime. It addresses two of the most significant technological challenges in developing a scalable quantum information processor based on spins in quantum dots: (i) decoherence of the electronic spin qubit due to the surrounding nuclear spin bath, and (ii) long-range spin-spin coupling between remote qubits. Electron spins confined in quantum dots (QDs) are among the leading contenders for implementing quantum information processing. To this end, the author pursues novel strategies that turn the unavoidable coupling to the solid-state environment (in particular, nuclear spins and phonons) into a valuable asset rather than a liability.

A comprehensive review of cutting-edge solid state research, focusing on quantum dot nanostructures, for graduate students and researchers.

This book highlights state-of-the-art qubit implementations in semiconductors and provides an extensive overview of this newly emerging field. Semiconductor nanostructures have huge potential as future quantum information devices as they provide various ways of qubit implementation (electron spin, electronic excitation) as well as a way to transfer quantum information from stationary qubits to flying qubits (photons). Therefore, this book unites contributions from leading experts in the field, reporting cutting-edge results on spin qubit preparation, read-out and transfer. The latest theoretical as well as experimental studies of decoherence in these quantum information systems are also provided. Novel demonstrations of complex flying qubit states and first applications of semiconductor-based quantum information devices are given, too.

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.

This book highlights the most recent developments in quantum dot spin physics and the generation of deterministic superior non-classical light states with quantum dots. In particular, it addresses single quantum dot spin manipulation, spin-photon entanglement and the generation of single-photon and entangled photon pair states with nearly ideal properties. The role of semiconductor microcavities, nanophotonic interfaces as well as quantum photonic integrated circuits is emphasized. The latest theoretical and experimental studies of phonon-dressed light matter interaction, single-dot lasing and resonance fluorescence in QD cavity systems are also provided. The book is written by the leading experts in the field.

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.

Electrically-gated quantum dots in semiconductors is an excellent architecture on which to make qubits for quantum information processing. Silicon is attractive because of the potential for excellent manipulability, scalability, and for integration with classical electronics. This thesis describes several aspects of the theoretical issues related to quantum dot qubits in silicon. It may be broadly divided into three parts -- (1) the hybrid qubit and quantum gates, (2) decoherence and (3) charge transport. In the first part, we present a novel architecture for a double quantum dot spin qubit, which we term the hybrid qubit, and demonstrate that implementing this qubit in silicon is feasible. Next, we consider both AC and DC quantum gating protocols and compare the optimal fidelities for these protocols that can be achieved for both the hybrid qubit and the more traditional singlet-triplet qubit. In the second part, we present evidence that silicon offers superior coherence properties by analyzing experimental data from which charge dephasing and spin relaxation times are extracted. We show that the internal degrees of freedom of the hybrid qubit enhance charge coherence, and demonstrate tunable spin loading of a quantum dot. In the last part, we explain three key features of spin-dependent transport -- spin blockade, lifetime-enhanced transport and spin-flip cotunneling. We explain how these features arise in the conventional two-electron as well as the unconventional three-electron regimes, using a theoretical model that captures the key characteristics observed in the data.