Control Techniques In Spin Based Quantum Computation

Working on quantum systems entail different interests, for example, working on fundamental understanding of quantum systems also lay foundation for better quantum computation techniques. A test for whether a system is behaving quantum mechanically or classically is devised by Leggett and Garg in form of inequalities, called Leggett-Garg Inequalities (LGI). Such Inequalities are violated by a system whose evolution is governed by quantum mechanics. A precise experiment to violate LGIs require a guarantee that the measurement does not affect the system or its future dynamics. These Inequalities were proposed for dichotomic systems,systems which can have two outcomes. Here we present an LG experiment on a three-level quantum system, which theoretically have larger quantum upper bound than that of a two-level quantum system. This larger violation also provides a bigger buffer to taking in account of the various experimental imperfections. Performing a quantum computing task requires precise level of control to initialize, perform and measure the quantum system. With increasing size of the quantum processor the challenge is to maintain optimal control. Nuclear Magnetic Resonance (NMR) has always been a very faithful test-bed for quantum processing ideas. In NMR, we perform Radio Frequency (RF) pulses to control and steer the system to the desired state. Most used method to derive the exact frequency and amplitude of these pulses for a given task is based on gradient. Although systematic, one have to simulate these pulses on a classical computer first, which makes the task very inefficient. We report a a way of performing optimization with a hybrid quantum-classical scheme. This scheme helps us perform classically harder computational tasks on the quantum processor. We optimize pulses which drive our system from 7-coherence state to 12-coherence state on a 12-qubit NMR processor. Electron Spin Resonance (ESR) employs the same techniques as of NMR but having advantage in larger polarization compared to later. Although this does not imply better control, cause the frequency at which pulses are required to control an ESR system fall into microwave region. Microwave frequency are harder to control electronically, thus making it harder for performing ESR quantum computing. The hybrid scheme used in NMR experiment relies on some ideal pulses which are needed to be optimized classically. We alleviate this requirement by using finite difference method of calculating gradient. We compare these methods with the earlier methods to show the superiority of such a scheme. State-to-state transfer pulses are sufficient for most of the quantum computing task, but, an universal quantum information implementation requires state independent pulses. The techniques used in optimizing state-to-state pulses can be modified to optimize for a state independent pulse. We show that this methods scale polynomially with the number of qubits and is general in terms of its implementation. We further reduce the resource requirement by using a NMR related implementation.

This book addresses electron spin-qubit based quantum computing and quantum information processing with a strong focus on the background and applications to EPR/ESR technique and spectroscopy. It explores a broad spectrum of topics including quantum computing, information processing, quantum effects in electron-nuclear coupled molecular spin systems, adiabatic quantum computing, heat bath algorithmic cooling with spins, and gateway schemes of quantum control for spin networks to NMR quantum information. The organization of the book places emphasis on relevant molecular qubit spectroscopy. These revolutionary concepts have never before been included in a comprehensive volume that covers theory, physical basis, technological basis, applications, and new advances in this emerging field. Electron Spin Resonance (ESR) Based Quantum Computing, co-edited by leading and renowned researchers Takeji Takui, Graeme Hanson and Lawrence J Berliner, is an ideal resource for students and researchers in the fields of EPR/ESR, NMR and quantum computing. This book also • Explores methods of harnessing quantum effects in electron-nuclear coupled molecular spin systems • Expertly discusses applications of optimal control theory in quantum computing • Broadens the readers’ understanding of NMR quantum information processing

Over two volumes and 1500 pages, the Handbook of Spintronics will cover all aspects of spintronics science and technology, including fundamental physics, materials properties and processing, established and emerging device technology and applications. Comprising 60 chapters from a large international team of leading researchers across academia and industry, the Handbook provides readers with an up-to-date and comprehensive review of this dynamic field of research. The opening chapters focus on the fundamental physical principles of spintronics in metals and semiconductors, including an introduction to spin quantum computing. Materials systems are then considered, with sections on metallic thin films and multilayers, magnetic tunnelling structures, hybrids, magnetic semiconductors and molecular spintronic materials. A separate section reviews the various characterisation methods appropriate to spintronics materials, including STM, spin-polarised photoemission, x-ray diffraction techniques and spin-polarised SEM. The third part of the Handbook contains chapters on the state of the art in device technology and applications, including spin valves, GMR and MTJ devices, MRAM technology, spin transistors and spin logic devices, spin torque devices, spin pumping and spin dynamics and other topics such as spin caloritronics. Each chapter considers the challenges faced by researchers in that area and contains some indications of the direction that future work in the field is likely to take. This reference work will be an essential and long-standing resource for the spintronics community.

Advanced research reference examining the closed and open quantum systems Control of Quantum Systems: Theory and Methods provides an insight into the modern approaches to control of quantum systems evolution, with a focus on both closed and open (dissipative) quantum systems. The topic is timely covering the newest research in the field, and presents and summarizes practical methods and addresses the more theoretical aspects of control, which are of high current interest, but which are not covered at this level in other text books. The quantum control theory and methods written in the book are the results of combination of macro-control theory and microscopic quantum system features. As the development of the nanotechnology progresses, the quantum control theory and methods proposed today are expected to be useful in real quantum systems within five years. The progress of the quantum control theory and methods will promote the progress and development of quantum information, quantum computing, and quantum communication. Equips readers with the potential theories and advanced methods to solve existing problems in quantum optics/information/computing, mesoscopic systems, spin systems, superconducting devices, nano-mechanical devices, precision metrology. Ideal for researchers, academics and engineers in quantum engineering, quantum computing, quantum information, quantum communication, quantum physics, and quantum chemistry, whose research interests are quantum systems control.

"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 present book provides to the main ideas and techniques of the rapid progressing field of quantum information and quantum computation using isotope - mixed materials. It starts with an introduction to the isotope physics and then describes of the isotope - based quantum information and quantum computation. The ability to manipulate and control electron and/or nucleus spin in semiconductor devices provides a new route to expand the capabilities of inorganic semiconductor-based electronics and to design innovative devices with potential application in quantum computing. One of the major challenges towards these objectives is to develop semiconductor-based systems and architectures in which the spatial distribution of spins and their properties can be controlled. For instance, to eliminate electron spin decoherence resulting from hyperfine interaction due to nuclear spin background, isotopically controlled devices are needed (i.e., nuclear spin-depleted). In other emerging concepts, the control of the spatial distribution of isotopes with nuclear spins is a prerequisite to implement the quantum bits (or qbits). Therefore, stable semiconductor isotopes are important elements in the development of solid-state quantum information. There are not only different algorithms of quantum computation discussed but also the different models of quantum computers are presented. With numerous illustrations this small book is of great interest for undergraduate students taking courses in mesoscopic physics or nanoelectronics as well as quantum information, and academic and industrial researches working in this field.

This book presents the research and development-related results of the “FIRST” Quantum Information Processing Project, which was conducted from 2010 to 2014 with the support of the Council for Science, Technology and Innovation of the Cabinet Office of the Government of Japan. The project supported 33 research groups and explored five areas: quantum communication, quantum metrology and sensing, coherent computing, quantum simulation, and quantum computing. The book is divided into seven main sections. Parts I through V, which consist of twenty chapters, focus on the system and architectural aspects of quantum information technologies, while Parts VI and VII, which consist of eight chapters, discuss the superconducting quantum circuit, semiconductor spin and molecular spin technologies. Readers will be introduced to new quantum computing schemes such as quantum annealing machines and coherent Ising machines, which have now arisen as alternatives to standard quantum computers and are designed to successfully address NP-hard/NP-complete combinatorial optimization problems, which are ubiquitous and relevant in our modern life. The book offers a balanced mix of theory-based and experimentation-based chapters written by leading researchers. Extensive information is provided on Quantum simulation, which focuses on the implementation of various many-body Hamiltonians in a well-controlled physical system, Quantum key distribution, Quantum repeaters and quantum teleportation, which are indispensable technologies for building quantum networks with various advanced applications and require far more sophisticated experimental techniques to implement.

Quantum mechanics, the subfield of physics that describes the behavior of very small (quantum) particles, provides the basis for a new paradigm of computing. First proposed in the 1980s as a way to improve computational modeling of quantum systems, the field of quantum computing has recently garnered significant attention due to progress in building small-scale devices. However, significant technical advances will be required before a large-scale, practical quantum computer can be achieved. Quantum Computing: Progress and Prospects provides an introduction to the field, including the unique characteristics and constraints of the technology, and assesses the feasibility and implications of creating a functional quantum computer capable of addressing real-world problems. This report considers hardware and software requirements, quantum algorithms, drivers of advances in quantum computing and quantum devices, benchmarks associated with relevant use cases, the time and resources required, and how to assess the probability of success.

Processing information quantum mechanically allows the relatively efficient solution of many important problems thought to be intractable on a classical computer. A primary challenge in experimentally implementing a quantum information processor is the control and suppression of environmental noise that decoheres the quantum system and causes it to behave classically. Environmental errors may be dynamically suppressed by applying coherent control pulses to the qubits that decouple the environment. However, the pulses themselves are subject to implementation errors, which hinders the ability to robustly store a complete quantum state. This thesis details results on the use of optimal control theory, noise twirling, and logical qubit encodings to design high-fidelity control pulses and decoupling sequences that are robust to implementation errors. Results are also presented that demonstrate how high-fidelity inductive control of a quantum system may be obtained with limited resonator bandwidth, with a discussion of applications to actuator-based quantum information processors. In a multi-mode design for such a processor, which allows efficient removal of entropy, a new protocol is suggested that permits robust parallel information transfer between nodes. The results detailed in this thesis apply broadly to most implementations of quantum information processing and specifically enable a new design for a spin-based multinode quantum information processor based on single-crystal molecular monolayer electron-nuclear spin systems integrated with superconducting electronics.