Superconducting Resonators for Hybrid Semiconducting and Magnetic Quantum Systems

Doctoral thesis, 2026

Quantum computers are widely seen as the next frontier in computing technology. Unlike conventional computers, which process information as bits that are either 0 or 1, quantum computers use qubits, which can exist in a superposition of both states at once. This allows them, in principle, to tackle certain problems far more efficiently than classical machines, including simulating molecules for medicine or optimising complex systems.

Among the many approaches to building qubits, superconducting circuits are currently one of the most advanced. They are fast, relatively scalable, and can be fabricated using techniques like those used in modern microelectronics. However, they face a fundamental challenge: quantum states are extremely fragile, and space to cool them down to the required cryogenic temperatures is limited. The very tools used to control and measure them, electromagnetic signals, wiring, and even the local environment, can easily disturb and destroy the quantum information they carry.

At the heart of these systems are superconducting resonators, which act as intermediaries between qubits and the outside world. These devices confine particles of light at gigahertz frequencies, much like a musical instrument confines sound. By carefully designing these resonators, researchers can read out qubit states and mediate interactions between different parts of a quantum circuit.

The standard design used today is called a coplanar waveguide (CPW) resonator. It consists of a central wire flanked by large metallic ground planes. While highly effective in most settings, this geometry becomes problematic in more advanced experiments. The large metal areas tend to trap tiny whirlpools of magnetic flux, called vortices, which move under microwave currents and dissipate energy. This introduces noise and reduces the performance of the quantum circuit. At the same time, these ground planes make it difficult to apply local electric fields, which are essential for tuning many quantum devices.

This thesis introduces an alternative: the coplanar stripline (CPS) resonator. Instead of relying on a central wire and surrounding ground planes, CPSs uses two closely spaced conductors carrying opposite signals. This seemingly small change has large consequences. By eliminating the ground planes, the design suppresses vortex formation and allows the device to operate reliably even in strong magnetic fields, up to one Tesla. At the same time, it opens space for electrostatic gates, enabling precise control of embedded quantum systems based on semiconductors.

This improved control is essential for building hybrid quantum systems, where different types of quantum materials are combined to leverage their individual strengths. One example explored in this work is the integration of superconductors with semiconducting nanowires. In these systems, electrons can form exotic quantum states known as Andreev bound states, which behave like tiny, tuneable artificial atoms. These states are defined at the microscopic level and can be controlled using simple voltages, offering a new route towards more flexible and potentially more scalable quantum devices.

The thesis also explores an even more unconventional direction: using magnetism to carry quantum information. In certain magnetic insulators, such as thulium iron garnet, information can propagate as collective waves of spin rather than motion of electric charge: so-called magnons. Because no charge is transported, these excitations can move with extremely low energy loss. By interfacing such materials with superconducting resonators, it becomes possible to envision a “magnonic quantum bus,” where information is transferred efficiently between different parts of a quantum processor.

Taken together, this work addresses a central bottleneck in quantum technology: how to combine fast, controllable superconducting circuits with materials that offer new functionality and improved robustness. By developing resonators that remain stable under magnetic fields and compatible with electrostatic control, it lays the foundation for a new generation of hybrid quantum devices: systems that are not only more powerful, but also more versatile and resilient.