Realizing Spin Qubits In 28si Sige

"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.

In this thesis, we study the spin and charge properties of a Si/SiGe double dot system and demonstrate coherent quantum control of few-electron quantum states. We use a pulsed magneto-spectroscopy method to measure the excited state spectrum of a Si/SiGe quantum dot containing two valence electrons. We extract the singlet-triplet energy splitting, an essential parameter for spin qubits, and we present calculations showing the data are consistent with a spectrum in which the first excited state of the dot is a valley-orbit state. We then propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. The proposed hybrid qubit consists of a double quantum dot with two electrons in one dot and one electron in the other. The hybrid qubit formed from three electrons in a double quantum dot has potential for great speed due to the presence of level crossings where the qubit becomes charge-like. Thus, as a stepping stone, we demonstrate coherent manipulations in a three-electron charge qubit. Finally, we demonstrate fast coherent manipulation of three-electron states in a double quantum dot as progress towards the implementation of the pulse-gated hybrid qubit. We demonstrate that tailored pulse sequences can be used to induce coherent rotations between 3-electron quantum states.

Since being proposed almost 40 years ago, scientists across many disciplines have made great progress in the fields of quantum computation and quantum information. Instead of a classical bit (0 or 1), a quantum computer uses a two-level quantum system as a quantum bit or qubit. By controllably manipulating the quantum-mechanical properties of these qubits, a quantum computer could, for example, be used to simulate other, less well understood quantum systems, or to run certain classes of quantum algorithms that cannot be run on classical hardware. In order to build a quantum computer, certain basic requirements must be met. As with a classical computer, logic gates are necessary to controllably manipulate qubits to perform calculations. One such requirement for a universal quantum computer is a two-qubit logic gate. This is an inherently quantum mechanical gate, which has no classical analog. For example, the controlled-not two-qubit gate will perform a not operation on the target qubit if and only if the control qubit is in the one state, else it does nothing to the target qubit. In either case, the control qubit is left unchanged and unmeasured. Being able to perform this gate with high fidelity is critical to creating a quantum computer. In this dissertation, I present progress towards fabricating, characterizing, and manipulating two-qubit devices in Si/SiGe heterostructures. First, I motivate the use of quantum dot qubits hosted in Si/SiGe as a suitable platform for quantum computing. Then, I present characterization of Si/SiGe substrates and discuss fabrication of a quantum dot device. Next, I outline the electronics set up for measuring a quantum dot device in a dilution refrigerator. I then present results of two, published experiments which explore multi-qubit systems: one which demonstrates controllable tunnel coupling between a quantum dot an a nearby localized impurity, and the other which demonstrates state-conditional Landau-Zener-Stückelberg oscillations between capacitively coupled double quantum dots in a quadruple quantum dot device. Next I discuss fabrication and characterization of micromagnets for spin qubit applications. I finally conclude by discussing future research avenues towards realizing a robust, multi-qubit device in silicon.

Zsfassung in dt. u. engl. Sprache

The project goals are to fabricate qubits in quantum dots in Si/SiGe modulation-doped heterostructures, to characterize and understand those structures, and to develop the technology necessary for a Si/SiGe quantum dot quantum computer. The physical qubit in our approach is the spin of an electron confined in a top-gated silicon quantum dot in a Si/SiGe modulation-doped heterostructure. Operations on such a qubit may be performed by controlling the voltages on gates in-between neighboring quantum dots. A quantum computer and qubits in silicon offer potential advantages, both fundamental and practical. Electron spins in silicon quantum dots are expected to have long coherence times. Silicon has an isotope, Si, which has zero nuclear spin and thus no nuclear magnetic moment. As a result, electron spins in silicon have longer coherence times than they would in the presence of a fluctuating nuclear spin background. From a practical perspective, modern classical computers are made in silicon, and one hopes that this will lead to synergy in the future with a silicon quantum computer. This QCCM includes both theory and experiment focusing on (i) the development of qubits in the form of electron spins in silicon quantum dots, (ii) the measurement and manipulation of those qubits, and (iii) the science essential for understanding the properties of such qubits.

"This book covers the current state-of-the-art theories and applications of neural networks with high-dimensional parameters"--Provided by publisher.

This Briefs volume describes the properties and structure of elementary excitations in isotope low-dimensional structures. Without assuming prior knowledge of quantum physics, the present book provides the basic knowledge needed to understand the recent developments in the sub-disciplines of nanoscience isotopetronics, novel device concepts and materials for nanotechnology. It is the first and comprehensive interdisciplinary account of the newly developed scientific discipline isotopetronics.