Investigating Valley States And Their Interactions In Silicon Silicon Germanium Quantum Dots

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.

The pursuit of a quantum computer has been driven both by the prospect of solving otherwise intractable problems as well as the desire to master the quantum control of mesoscopic systems. Lateral quantum dots in semiconductors, particularly in silicon, have garnered considerable attention as a potential host for a quantum bit formed from the states of confined electrons. The conduction states of electrons in silicon are characterized by an additional degree of freedom, known as the valley degree of freedom. Valley states have certain properties which represent both a complication and an opportunity for the use of electrons in silicon as a qubit. Valley states are 6-fold degenerate, reducing to being very nearly 2-fold degenerate when confined to a quantum well, meaning they have the potential to disrupt many qubit readout schemes for spin based on the Pauli exclusion principle. Also, they are rapidly oscillating relative to one another, making their interaction highly sensitive to atomic scale properties of the confining potential, which are difficult to control when fabricating devices. However, they share many spin-like properties leading to the possibility of familiar control protocols and long coherence times. We have developed techniques for the fabrication of lateral double quantum dots and experimental procedures for the coherent manipulation and readout of quantum states of confined electrons. Through the use of fast pulses, quantum oscillations between two valley states are induced, allowing the energy splitting between these valley states to be probed to a resolution not possible with conventional methods of valley spectroscopy. The fabrication techniques and experimental procedures have led to the detection of quantum oscillations in multiple devices, demonstrating the utility and generality of the techniques. In the course of refining these experimental techniques, we developed software allowing for the linear compensation of multiple experimental parameters. These improvements led to the ability to efficiently adjust the inter-dot tunneling rate, the dot-reservoir tunneling rate and the charge sensing channel sensitivity, control parameters whose fine tuning was vital to the success of the quantum manipulation and readout ultimately achieved.

Semiconductor nanocrystals, also known as quantum dots (QDs), have gained much popularity in recent years not only in the researching community but also in general public. Within researching circles, there are many areas of application that are studies are honing in from the QDs utilization in cellular imaging to light-emitting diodes and beyond. In this particular study, the energy storage characteristics are investigated along with the role that cations may play. The process of ion diffusion of group I cations (H+, Li+, and Na+) through various anionic, (MX)−84, nanocrystals (with M=Zn, Cd and X=S, Se) were studied. Utilizing the discrete variable representation (DVR) method, the one-dimensional nuclear Schr ̈odinger equation bound solutions for the cations were analyzed. Utilizing these results, further investigation was conducted to determine whether or not the intercalating ions were quantum or classical in nature.

The potential power of quantum algorithms to solve problems that are currently computationally intractable has launched investigations into a wide range of quantum systems with the goal of developing quantum hardware. Among these candidate systems for encoding quantum bits (qubits), electron spins localized in gate-defined quantum dots are promsing. In particular, quantum dots defined in Si/SiGe heterostructures have a number of advantages, including a low defect density and a low intrinsic density of interfering nuclear spins. This work presents a series of investigations into gate-defined quantum dots formed in Si/SiGe heterostructures. A key parameter for the performance of quantum dot qubits hosted in Si is the valley splitting energy. In Si/SiGe heterostructures, controlling the valley splitting remains an outstanding challenge. Here we present measurements of valley splitting in Si/SiGe heterostructures with varied Ge concentration profiles. The measured scaling of the valley splitting with vertical electric field is compared with tight-binding simulations of heterostructures with interfacial steps, and the agreement in scaling provides evidence for the important role played by interfacial disorder in setting the valley splitting in these samples. Additionally, for gate-defined quantum dots, a key device element is the dielectric layer which mediates the electric fields between the gate electrodes and the gate-defined dots. We discuss experiments to test a novel method for generating a gate dielectric for Si/SiGe quantum dots using thermal oxidation at 700 degrees C. Another critical component towards developing quantum processors based on quantum dots is the engineering of strong and controllable inter-qubit coupling. For double quantum dot qubits with an effective charge dipole moment, a capacitive dipole-dipole interaction can generate coherent coupling between neighboring qubits. Here we also discuss investigations into the capacitive interaction between double quantum dots in a quadruple quantum dot device. This includes a demonstration of the tuning of the capacitive coupling energy over a wide range using barrier gate voltages and analysis of the dependence of that coupling energy on device geometry. Finally, we present procedures for fabricating quadruple quantum dot devices in Si/SiGe using an architecture based on overlapping self-oxidized Al gates.

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