In this thesis, we explore the physics and control protocols enabled by hopping spins in the quantum dot arrays. Our approach consists of experimental studies as well as numerical simulations. In chapter 2 we provide the background information relevant to the work in this thesis, including the theoretical description of spins in germanium quantum dots, as well as the measurement setup used in the experiments. In chapter 3 we study the control protocols for spin-spin exchange interactions, and quantify the individual coupling strength in a configuration of 2×2 array when all the nearest neighboring coupling are turned on. We can tune to a regime of equal coupling strength, in which the four-spin entangled states emerge as the resonating valence bond states. In chapter 4 we model the qubit frequency susceptibility to charge noise and predict the optimal magnetic field orientation for extended qubit coherence time. In chapter 5 we show multi-photon transitions in a two-spin system, and discuss opportunities to use this method for qubit addressability in large qubit array based on shared control architecture. In chapter 6 we demonstrate coherent spin qubit shuttling through quantum dots. We quantify the loss of the quantum information during the process. In chapter 7 we demonstrate high-fidelity baseband control of single qubit and two qubit gates, with extended coherence times by operating at low magnetic field. The single qubit gate is realized based on shuttling between the quantum dots, resembling the original spin qubit proposal by Loss and Divincenzo. The two-qubit CPhase gate achieves average fidelity 99.3% in the randomized benchmarking experiments. In chapter 8 we conclude and provide an outlook on the near future for germanium spin-qubits operations.
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Spin qubits in semiconductor quantum dots hold great promises for quantum information processing thanks to their small footprint, long coherence time, and similarities with classical transistors. However, such a new technology comes with new challenges and requires considering new metrics to develop proof-of-principle devices into a technological platform at scale.

Here, we study Si/SiGe heterostructures developed to host single electron spin qubits. We characterize the heterostructure and material stack using different structural techniques and measure the performances of multiple quantum devices with statistical significance. We use classical and quantum metrics to identify the performance-limiting mechanisms and improve them upon modification of selected parameters of the material stack to enable the next generation of spin qubit devices.

The first experiment is about the electrostatics of undoped Si/SiGe heterostructures. We study the semiconductor/dielectric interface between the epitaxial SiGe spacer and the SiOx and AlOx dielectrics. Against the mainstream approach, we grow heterostructures without an epitaxial Si cap. We find an improved interface from a structural characterization and in the two-dimensional electron transport at low temperatures.

The second experiment concerns the charge noise in few-electron quantum dots. We build on the previous results and focus our attention on the thickness of the Si quantum well. In thin quantum wells without a sacrificial Si cap, we find lower charge noise that we attribute to decreased density of remote impurities and misfit dislocations at the SiGe/Si and Si/SiGe interfaces arising from the local quantum well strain relaxation.

The third experiment finds the balance between disorder and the energy splitting of the nearly degenerate conduction band valleys (valley splitting) by fine-tuning the thickness of the Si quantum well. We challenge the apparent dichotomy between these two parameters and demonstrate heterostructures with simultaneously low disorder and high valley splitting. Besides, we give a quantitative estimation of the amplitude of the strain fluctuations in the quantum well arising from the virtual substrate.

The advancements reported in this thesis confirm the steady progress of the Si/SiGe platform towards realizing a full-scale quantum computer.
We summarize the results in the conclusion chapter, where we also highlight the general trends in the spin qubit community and suggest a few knobs to tweak to further improve the material platform.@en