Measurements of fast dynamics of hole spins in silicon
The development of quantum computing promises to revolutionise the technology of the 21st century. Single spins trapped in silicon quantum dots (QDs) have become an increasingly promising hardware platform for quantum computing due to their long coherence times, high-fidelity control and industry compatibility for scaling purposes. In particular, the use of hole spin system, displaying strong spin-orbit interactions (SOI), has enabled fast electrical control, however, requires a richer understanding of the physics of interacting spins in the presence of SOI both for control as well as readout of the spin states. In this thesis, I explore the spin dynamics of holes in silicon utilizing fast microwave readout circuitry. High-fidelity readout, a requirement for fault-tolerant quantum computing, requires long measurement times when compared to typical operation times, creating a bottleneck in the operational speed of quantum computers. To address this challenge, I develop a series of multi-module superconducting resonators that enable charge readout at the state-of-the-art while allowing for multiplexing the readout of many QDs, by sensitively detecting the capacitance changes associated with single charge movements between QDs and reservoirs. This result, in conjunction with spin-to-charge conversion methods, should reduce the time needed for spin readout in scaled up architectures. Next, I explore spin-to-charge conversion in the presence of SOI. The presence of SOI can lead to spin projection errors under standard readout methodologies, limiting the maximum achievable fidelity. To overcome this challenge, I develop a new protocol based on in-situ dispersive readout techniques making use of the energy detuning dependence of the charge polarizability of a hybrid DQD system to achieve spin-selective readout. I use this readout method to measure spin dynamics of holes shared between QDs and single boron atoms. Further, I explore the resonant interaction between the hybrid DQD system and the readout resonator to demonstrate electrically-driven spin resonance. I do so with both the resonator and an external microwave excitation to explore the incoherent dynamics of the system and obtain the T$_1$ and T$_2^*$. Finally, I explore the impact of strong driving on cyclic tunnelling events between the QD and reservoir commonly used as charge sensors to probe nearby spin states. In order to extract the maximum signal, these systems are driven at high powers and frequencies, conditions that break the semiclassical approximations typically used to describe their response. I discover a previously unexplored phenomenon arising from the interaction of the Floquet modes of a strongly-driven QD scattering off the reservoir, questioning the validity of the semiclassical models while providing a new handle on measuring charge dynamics in QDs. This unexplored regime can be utilized as a tool to characterize the QD-reservoir electrostatic properties while it enables exploring the coherence of the QD dressed states. It further encourages studies to explore its impact in readout of QDs using strong drive tones.
Quantum computing, Quantum dots, Hole spin, Spin-orbit coupling, Radiofrequency reflectometry, Superconducting resonators, Floquet theory
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