UCLA

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.