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.

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.

Simulating the quantum properties of superposition and entanglement can be highly inefficient with digital computers. To allow for exploration and verification of complex quantum systems, a new tool must be developed. Quantum computers, leveraging discrete operations and error correction, offer potential speedups in factoring, unsorted database search and machine learning as well. However, quantum computers rely on large numbers of qubits that can be controlled with high accuracy to outperform classical computing and key challenges include managing interconnects, achieving uniformity and maintaining isolation from the environment.

Semiconductor spin qubits are promising because of their affinity for mass production and small footprint. However, a statistical understanding of the devices and materials is needed to ensure sufficient yield, which requires high measurement throughput. Additionally, the number of quantum dots per device must increase, transitioning from small linear systems to large 2D architectures which can support the millions of qubits required for fault-tolerant quantumcomputing.

Measurement throughput can be enhanced by mitigating bottlenecks in the feedback cycle through the development of a cryogenic multiplexing platform, using commercial off-the-shelf CMOS for sub-kelvin temperature measurements. This approach increases the number of wires available at cryogenic temperature, enabling efficient statistical characterization of mobility and percolation density in natSi/SiGe heterostructure Hall bars fabricated in an industrial CMOS fab.

The increased throughput enables systematic studies of performance-critical device parameters. Activation energy measurements reveal that conduction band valley energy splitting increases linearly with magnetic field but is independent of Hall density. Instead, it depends on incremental density changes across quantum Hall edge strips, aligning with theoretical predictions for a near-perfect Si quantum well top interface. By combining multiplexing with a crossbar architecture that offers quadratic scaling of unit cells with cryogenic interconnects, 648 narrow-channel field-effect transistors on an industrial 28Si-MOS stack are measured at 100% yield. The device scale allows for comparison between macroscopic pinch-off differences across the crossbar and local differences within each unit cell. These distributions can be converted to energy using addition voltages and compared to proposed 2D architecture uniformity requirements. Additionally, unit cell geometry variations can be used to characterize the impact of device design compared to intrinsic material stack variance.

Finally, the conclusion highlights how advances in this thesis contribute to both the quality and quantity of quantum devices, necessary for future quantum computers to solve real problems. Quality can be further improved with high throughput characterization of efficient measurement structures, using statistical data to deepen understanding viamodeling and exploration of alternative material stacks like Ge/SiGe heterostructures. Increasing quantity calls for further system integration and industry participation.

This dissertation describes a set of experiments with the goal of creating a super-conductor-semiconductor hybrid circuit quantum electrodynamics architecture with single electron spins. Single spins in silicon quantum dots have emerged as attractive qubits for quantum computation. However, how to scale up spin qubit systems remains an open question. The hybrid architecture considered here could provide a route to realizing large networks of quantum dot–based spin qubit registers. The first experiment in this thesis is aimed at achieving strong coupling between a single electron spin and a single microwave photon. The electron is trapped in a gate-defined double quantum dot in a Si/SiGe heterostructure and the photon is stored in an on-chip superconducting high-impedance NbTiN cavity. The photon is coupled directly to the electron charge, and indirectly to the electron spin, mediated through a synthetic spin-orbit field. We observe a vacuum Rabi splitting that depends on the spin-charge hybridization. The ratio of spin-photon coupling strength to decoherence rates of the spin and cavity combined is larger than unity, confirming the strong coupling regime has been reached. In addition, we find an optimal degree of spin-charge hybridization for which this ratio is maximized. The demonstration of strong spin-photon coupling not only opens a new range of physics experiments, but fulfills also a crucial requirement for coupling spin qubits at a distance via a cavity. The second experiment is focused on spin readout with the on-chip cavity. Instead of the direct dispersive readout of a single spin, we use the cavity to detect whether the electron is allowed to tunnel between the two dots or not. We benchmark the charge sensitivity and bandwidth of the detector and find that rapid detection of the electron charge with high SNR is possible. In the two-electron regime, electron tunneling is contingent on the total spin state (Pauli spin blockade). This spin-to-charge conversion scheme enables single-shot detection of singlet states with high-fidelity. The demonstration of single-shot spin readout with a cavity is an essential step towards readout in dense spin qubit arrays, such as the crossbar network, where it is not possible to integrate electrometers and accompanying reservoirs adjacent to the qubit dots. In the third experiment, we develop on-chip microwave filters to suppress microwave photon leakage from the cavity through the gate electrodes that are necessary to form quantum dots. We introduce a new cavity design that is compatible with long-distance connectivity between spins, but is also more susceptible to microwave leakage. We test and compare two low-pass filter variations in terms of performance, footprint and integrability. They use the same nanowire inductor, but different implementations of the capacitor: one with a planar interdigitated capacitor and one novel design with an overlapping thin-film capacitor. We find that both approaches are effective against microwave leakage. However, the large footprint of the interdigitated capacitor makes this solution inconvenient as the number of gate lines increases. The thin-film capacitor, with its much smaller footprint, is better suited for our devices. The final part of this dissertation contains concluding remarks and possible future directions are proposed.