Experiments on quantum knowledge and reality with spins in silicon

Abstract

Electron and nuclear spins are highly controllable and coherent quantum objects. They are therefore an excellent platform to study both fundamental physics and quantum information. Semiconductor quantum devices leverage the vast infrastructure that currently exists to produce our everyday electronics. With spins integrated into semiconductor devices, coherent control of individual electrons and nuclei has been demonstrated. Further development of these devices is essential to propel novel quantum technologies, such as quantum computers, beyond the lab. This thesis focuses on three themes: spin physics, quantum information processing and the foundations of quantum theory. We explore these topics with donors in silicon, phosphorus (31 P) and antimony (123 Sb). With a ‘Maxwell’s demon’ observing a single electron spin, its knowledge of the spin state heralds high-fidelity electron spin initialisation without requiring additional quantum resources. We benchmark the electron initialisation with high-fidelity nuclear spin readout by first mapping the electron state to the nucleus. We then motivate further improvements to the measurement apparatus to further enhance electron spin initialisation and readout. Recent advances have demonstrated embryonic two-qubit gates between donor-bound electrons. Alongside the high-fidelity readout afforded by nuclear spins, we are rapidly approaching the fault-tolerant threshold for some error-correcting codes, e.g. the surface code. We also discuss an electrical technique to coherently control quadrupolar nuclei, which we discovered for the first time in silicon with 123 Sb. With electrical control of donor electrons, this discovery could pave the way to an all-electrical donor spin quantum computer. Finally, with the highly-coherent ionised nuclear spin 123 Sb, we explore a foundational question in quantum mechanics. Originating from the Einstein-Podolsky-Rosen paradox, we discuss the reality of the quantum state. The quantum state is an incredibly accurate predictive tool, however the predictions are inherently probabilistic. The key question we seek to answer: is the probabilistic nature of the quantum state a representation of our (lack of) knowledge of the true state of reality? We propose an experimental test with 123 Sb that constrains the degree to which the quantum state can only represent knowledge.