Designing A Quantum Computer Based On Pulsed Electron Spin Resonance

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

One of the first books to thoroughly examine the subject, Quantum Computing Devices: Principles, Designs, and Analysis covers the essential components in the design of a "real" quantum computer. It explores contemporary and important aspects of quantum computation, particularly focusing on the role of quantum electronic devices as quantum gates.

Quantum computing has been a new and challenging area of research since the concept was put forward in 1980s. A quantum computer is a computer that processes information encoded in systems that exhibit quantum properties and is proved in theory to be more powerful than classical computers. Various approaches to the implementation of the quantum computers have been studied over the decades, each of them having their own advantages and disadvantages in terms of the lifetime of the quantum information, processing time, and scalability of the implementation. Proposals for hybrid quantum processors are interesting because they benefit from the advantages of each comprising system, and thus providing a promising approach to a practical quantum computer. In this thesis, I demonstrate experimentally the principle of utilizing electron spin ensembles as a quantum memory for hybrid quantum processors. I demonstrate the storage and on-demand retrieval of multiple bits of quantum information into and from a single electron spin ensemble by applying magnetic field gradient pulses. I then study the coupling between an electron spin ensemble and a three-dimensional microwave cavity, in the aim of discussing the condition for the coherent information transfer between the excitations in solid-state matter and photons. As an alternative to the high power pulses in electron paramagnetic resonance (EPR), I study the possibility of controlling the electron spin states via adiabatic processes. I demonstrate the implementation of adiabatic geometric phase gates in electron spins and compare their performances to other phase gates achieved with microwave pulses in both simulation and experiment, verifying the robustness of the adiabatic gates against certain type of noises. Finally I present the simulation method developed for simulating the pulsed EPR experiments in this thesis, using a model more general than some currently-existing simulation packages.

The spins of phosphorus doped in silicon are potential candidates for a quantum computing device, with models based on the use of nuclear and/or electron spins suggested. For a quantum computing device, several essential criteria must be demonstrated before any physical implementation, and these include qubit control gates, long decoherence time and scalability. Scalability and compatibility with existing fabrication technologies are strong points in favour of a silicon based system. For spin based schemes, silicon has the potential to provide a host with zero nuclear spin (isotopically purifed 28Si) and also the phosphorus donor provides both nuclear and electron half integer spins (ideal case). In this work, a magnetic resonance method (electron spin resonance) was utilised to investigate these critical issues (controllable quantum gates and decoherence time) for the electron spins of phosphorus donors in silicon. Electron spin resonance (ESR) studies of an ensemble of phosphorus electron spins in silicon were conducted via both continuous wave and pulsed methods. For pulsed ESR operations, two low temperature (4 K and millikelvin) X-band pulsed ESR systems were built. They were designed especially to suit Si:P decoherence time measurements. The design, modelling, construction and evaluation of the probe heads are described. With the aid of computer simulations, the performance of the probe heads was optimised and a rectangular loop gap resonator was found to be the most suitable for wafer type samples. The resonant frequency, quality factor, and coupling coeffcient were calculated via simulation and are in reasonable agreement with experimental results. This demonstrates the effectiveness of such simulations as a tool for optimising the probe head performance. A millikelvin pulsed ESR system was set up through the combination of a dilution refrigerator, superconducting magnet and the in-house construction of a pulsed ESR spectrometer. This novel system allows pulsed ESR experiments on an ensemble system to be realised down to the millikelvin temperature range, hence providing conditions considered most favourable for quantum computing studies. The use of light in combination with the pulsed ESR systems was also explored in an endeavour to overcome the problem of very long spin-lattice relaxation time, T1, allowing the decoherence time to be measured more effciently. With these novel low temperature pulsed ESR units, two-pulse electron spin echo experiments were conducted on phosphorus donors in silicon (both natural silicon (natSi) and 28Si) with the phosphorus concentration in the range of 1015- 1016 P/cm3 and to lower temperatures than previously investigated. Decoherence times measured for both natSi:P and 28Si:P (with similar donor concentrations) were longer than previously reported. Discussions on several effective ways to obtain even longer Si:P decoherence times including variations to sample configurations and experimental conditions are presented. In addition to the pulsed ESR studies, the Si:P controllable quantum gate functions, A gate and J gate, were examined by the continuous wave technique via Stark shift and exchange interaction experiments respectively. Stark shift experiments on bulk samples were carried out to investigate possible manipulation of the spins by the applied electric field. Continuous wave ESR was also used to examine low energy ion implanted Si:P devices, both by single (P+) and dimer (P+2 ) implanted donors. The outcomes from these studies provide materials information useful in formulating a strategy toward the Si:P device fabrication via the top down approach.

Quantum computing set a goal to harness the quantum laws of physics and create computers more powerful than ever imagined. Different technologies can be chosen to implement quantum bits (qubits), each with their advantages and drawbacks. The idea of combining different technologies then seems natural in order to come up with an optimal quantum computer. In this sense, Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR) seem to be the perfect marriage. Indeed, while electron spins can perform quantum gates within nanoseconds, they have to fight very fast decoherence phenomena, the nuclear spins, on the other hand, require longer electromagnetic pulses to be rotated but can be controlled longer without loss of quantum information. Using electron spins as actuators and nuclear spins as memory then appears as the optimal use of this hybrid system. Another fact accounting for this association is that the control of the system through the electron spin requires techniques very similar to the well-known NMR ones. This work focuses on characterizing as precisely as possible the Hamiltonian of a hybrid spin system in a solid-state single crystal, especially the electron-nuclear interactions, to perform high-fidelity control in a home-built pulsed ESR spectrometer. Using this knowledge, we show that we can choose the orientation of the magnetic field with respect to our crystal to obtain optimal experimental conditions. Indeed, with a good knowledge of the Hamiltonian of the system, we want demonstrate high-fidelity quantum control. The final aim of this work is to dynamically supply highly polarized ancilla qubits that can be used in a Quantum Error Correction (QEC) experiment by implementing heat bath algorithmic cooling using a cold electron spin bath. This is an important step towards demonstrating the viability of spin systems for building quantum computers.

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.

The past few decades of research and development in solid-state semicon ductor physics and electronics have witnessed a rapid growth in the drive to exploit quantum mechanics in the design and function of semiconductor devices. This has been fueled for instance by the remarkable advances in our ability to fabricate nanostructures such as quantum wells, quantum wires and quantum dots. Despite this contemporary focus on semiconductor "quantum devices," a principal quantum mechanical aspect of the electron - its spin has it accounts for an added quan largely been ignored (except in as much as tum mechanical degeneracy). In recent years, however, a new paradigm of electronics based on the spin degree of freedom of the electron has begun to emerge. This field of semiconductor "spintronics" (spin transport electron ics or spin-based electronics) places electron spin rather than charge at the very center of interest. The underlying basis for this new electronics is the intimate connection between the charge and spin degrees of freedom of the electron via the Pauli principle. A crucial implication of this relationship is that spin effects can often be accessed through the orbital properties of the electron in the solid state. Examples for this are optical measurements of the spin state based on the Faraday effect and spin-dependent transport measure ments such as giant magneto-resistance (GMR). In this manner, information can be encoded in not only the electron's charge but also in its spin state, i. e.

This book provides an overview on physical realizations of quantum computing by means of molecular systems. It will be useful for graduate students and researchers interested in quantum computing from different areas of physics, physical chemistry, informatics and computer science. Each chapter is written in a self-contained manner and hence can be accessible for researchers and graduate students with even less background in the topics.

This book targets computer scientists and engineers who are familiar with concepts in classical computer systems but are curious to learn the general architecture of quantum computing systems. It gives a concise presentation of this new paradigm of computing from a computer systems' point of view without assuming any background in quantum mechanics. As such, it is divided into two parts. The first part of the book provides a gentle overview on the fundamental principles of the quantum theory and their implications for computing. The second part is devoted to state-of-the-art research in designing practical quantum programs, building a scalable software systems stack, and controlling quantum hardware components. Most chapters end with a summary and an outlook for future directions. This book celebrates the remarkable progress that scientists across disciplines have made in the past decades and reveals what roles computer scientists and engineers can play to enable practical-scale quantum computing.