Superconducting microwave circuits provide a versatile platform for engineering controllable quantum systems, with applications in quantum information processing, quantum sensing, and microwave optomechanics. In this thesis, we investigate how key circuit elements can be improved by engineering new interactions, enhancing microwave readout, and reducing dissipation in superconducting resonators. We achieve this through the design, fabrication, and cryogenic characterization of superconducting circuits, focusing on three themes: longitudinal coupling, parametric amplification for microwave measurements, and reducing microwave loss through surface processing.

In Chapter 1 we introduce superconducting microwave circuits and motivate the main research directions of this thesis.

Chapter 2 describes the experimental workflow used throughout this thesis. Starting from circuit design and electromagnetic simulation, we explain how the devices are realized in the cleanroom, with particular attention to the practical details of the nanofabrication process. We then describe chip packaging, and give an overview of the cryogenic measurement setup used to characterize the devices.

Chapter 3 demonstrates how to implement an intrinsic longitudinal coupling between a transmon qubit and a linear microwave resonator by using the circuit quantum electrodynamics analog of radiation-pressure coupling. We discuss the experimental characterization of this interaction and highlight its prospects for high-connectivity quantum hardware and for experiments interfacing superconducting circuits with massive quantum systems.

Chapter 4 presents the design, fabrication, and characterization of a flux-tunable parametric amplifier based on the Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL). We describe the device design and wafer-scale fabrication, and characterize the amplifier response across its tunable operating range. We investigate the gain–bandwidth trade-off and quantify the dynamic range through gain-compression measurements.

Chapter 5 investigates microwave loss in tantalum superconducting coplanar-waveguide resonators. We study surface-related dielectric loss in the single-photon regime using devices patterned from a single tantalum film with varying surface participation ratios. By comparing different resonator geometries and post-fabrication surface treatments, we show that additional cleaning steps can significantly reduce loss and yield reproducible improvements in the internal quality factor.

Finally, Chapter 6 concludes this thesis by summarizing the main findings and providing an outlook for future research. ... Read more

This thesis summarises my work on the design, electromagnetic simulation and modelling, fabrication, and experimental characterisation of superconducting quantum circuits, with a particular focus on highly anharmonic fluxonium qubits. To this end, I have worked on complementary aspects of superconducting quantum-device development, ranging from predictive electromagnetic modelling and materials engineering to superinductive elements and multi-qubit architectures.

As a critical foundation for the design of highly anharmonic fluxonium qubits, I extended the energy participation ratio (EPR) approach to describe strongly nonlinear superconducting circuits beyond its conventional low-order treatment of nonlinear terms, which becomes inadequate for strongly anharmonic circuits such as fluxonium. This extension yields more accurate predictions of the fluxonium frequency, anharmonicity, and dispersive coupling, which we experimentally validated using a fluxonium qubit coupled to a microwave resonator. The predicted quantities show excellent agreement with the corresponding experimental measurements, demonstrating that the extended EPR framework can accurately capture the interaction between highly nonlinear circuits and their electromagnetic environment.

The implementation of high-coherence superconducting circuits requires careful material selection and fabrication-process engineering. Microwave resonators provide a useful platform for studying material-dependent loss mechanisms, as their performance can be quantified through the internal quality factor. In this work, different superconducting material stacks and substrates were investigated, with particular attention to tantalum-based resonators and the influence of deposition, etching, surface morphology, and structural properties on resonator performance. A growth recipe was developed for Nb-seed-layer tantalum films on silicon as an alternative to NbTiN for superconducting resonators. The resulting resonators showed reproducible power-dependent behaviour, while additional low-power measurements of the Nb/Ta-on-Si platform reached internal quality factors of up to ∼ 5 × 10^6. Across the investigated tantalum devices, the resonator performance varied substantially with the substrate, seed layer, and fabrication conditions. The Nb/Ta-on-Si devices achieved Q_i values exceeding 10^6 and outperformed the NbTiN reference, while the Ta-on-sapphire devices showed slightly higher performance than the NbTiN reference but did not reach 10^6. These results highlight tantalum as a promising material for high-coherence superconducting circuits, while also demonstrating the importance of controlling the growth and fabrication conditions.

In parallel, granular aluminium was investigated as a compact alternative to Josephson-junction arrays for fluxonium superinductors. GrAl nanowire resonators were fabricated and demonstrated to be superconducting, with power-dependent quality factors and a measurable resonance-frequency shift at high drive powers, indicating nonlinear behaviour. The deposition process was subsequently optimised to improve control over the deposition rate and its temporal stability, providing a more reproducible route towards the targeted kinetic inductance. Although the integration of the GrAl nanowire into a complete fluxonium device could not be completed within the scope of this thesis, these results establish the fabrication and deposition processes required for its future implementation.

Building on the advanced circuit-simulation and fabrication capabilities developed throughout this work, increasingly complex fluxonium systems were designed and characterised. To enable fast and high-fidelity two-qubit operations, a fluxonium-transmon-fluxonium architecture was developed, with electromagnetic and circuit-level simulations identifying suitable operating conditions and optimised gate protocols. To further improve the speed of qubit-state assignment, flux-pulse-assisted readout was demonstrated experimentally by temporarily moving the fluxonium away from its sweet spot to a readout point with a larger dispersive shift, enabling faster qubit-state assignment than conventional readout at the fluxonium sweet spot.

Together, these results demonstrate the importance of integrating modelling, materials engineering, fabrication, device design, and quantum control in the development of scalable fluxonium-based quantum processors.
... Read more

Quantum computing has seen remarkable progress over the past decade, with superconducting qubits emerging as one of the leading hardware platforms for its physical realisation. Among the various superconducting qubit architectures, the transmon has been one of the most widely adopted candidates, owing to its relatively simple design and compatibility. However, the transmon suffers from key limitations, most notably its weak anharmonicity and multi-qubit gates using transmon being difficult to execute faster without errors.
In this context, alternative qubit architectures have attracted interest. The fluxonium qubit, with its large anharmonicity and long coherence times, has emerged as a promising candidate for overcoming shortcomings of the transmon. Similarly, the cos(2φ) qubit offers an intriguing design with inherent noise protection, making it a compelling subject of investigation for next-generation superconducting hardware.
Beyond the question of which qubit to build, a central challenge in quantum computing is the realisation of high-fidelity two-qubit gates. Higher gate fidelity directly translates to better quantum operations and is essential for the implementation of error correction protocols. Equally important is the question of how to perform these gates in a fast and practical manner—ideally using simple pulse techniques that minimise the time and effort required for calibration.
This thesis addresses these themes across two broad directions. First, we explore the fabrication of fluxonium and cos(2φ) qubits, developing improved processes and cleanroom techniques to achieve more reliable and reproducible devices. Second, we investigate two-qubit gate operations using analytic pulse techniques, with the goal of realising faster and simpler gates that reduce the overhead associated with pulse calibration. Together, these contributions advance the development of superconducting qubit platforms towards more practical and scalable quantum computing. ... Read more

Since the early 2000s, High Electron Mobility Transistors (HEMTs) have been the preferred choice for the first-stage amplification of qubit readout resonators at cryogenic temperatures. However, Josephson Parametric Amplifiers (JPAs) have recently emerged as a more attractive alternative due to their quantum-level noise performance. Despite their advantages, JPAs face significant challenges such as narrow bandwidths, low dynamic ranges, and limited gains, which hinder their widespread adoption. By presenting an optimized JPA chip design with a Kerr coefficient of 1000 Hz and a Kappa value of 100 MHz that operates in the 4-8 GHz range, this thesis seeks to address these issues. The design methodology involves an iterative process using Comsol and Microwave Office softwares to refine the design and the resulting Kappa and Kerr coefficients at each phase of development. This approach results in a JPA design that overcomes the traditional limitations, making it a viable candidate for broader applications in quantum technology. ... Read more

Nanotechnology enables the study of various quantum phenomena on real hardware. For instance, semiconducting and superconducting nanostructures can define single-electron transistors, quantum dots, Josephson junctions, and many other examples of quantum devices. It's a wonderful sandbox.
In this thesis, we exploit such a technology to bring the Kitaev chain model to life. The Kitaev Hamiltonian, discussed in the second chapter of this dissertation, describes a chain of N fermionic sites coupled by a standard tunneling and a more exotic superconducting pairing. It is one of the simplest models able to bring the concept of topology into condensed matter physics. Proposed more than twenty years ago, it attracted many experimental groups around the world, due to the promise of realizing a topologically protected qubit. This would be encoded into the Majorana bound states predicted to appear at the ends of the chain. However, such a qubit was never made, due to the difficulty of reproducing the Kitaev model with realistic, hence imperfect, materials.
Here, we demonstrate that engineering Kitaev chains with state-of-the-art materials is possible, by compensating imperfections with fine tuning. As opposed to top-down approaches, this requires building the chain site-by-site and tuning carefully each of them. In this work, each site is represented by a semiconducting quantum dot, while short semiconducting-superconducting hybrids mediate the inter-dot couplings. First, we describe minimal arrays of two quantum dots, show how to control every term of the Kitaev Hamiltonian, and detect the appearance of Majorana bound states. Then, we generalize the tuning procedure to three-site Kitaev chain devices. We also study the additional complications caused by multiple superconductors on the same device.
The main downside of a few-site Kitaev chain is the lack of topological protection. Nevertheless, we demonstrate that its Majorana bound states already exhibit partial protection (against some parameter perturbations), which increases substantially from two- to three-site chains. In the outlook, we propose to generalize the techniques described here to realize a rudimentary Majorana qubit and scale up to even longer Kitaev chains, whose partial protection evolves into topological as N grows. ... Read more

In working towards a quantum internet, nodes based on nitrogen-vacancy (NV) centres in diamond have shown great potential. A key challenge in scaling these networks is the low entanglement generation rate due to low coherent photon emission (≈ 3%) and limited collection efficiency (≈ 15% using state-of-the-art solid immersion lens setups). Both can be improved by integrating NV centres in an optical cavity. In this thesis NV centres coupled to an open Fabry-Pérot microcavity are investigated. The NV centres are integrated into the cavity by bonding a μm-thin diamond sample to one of the mirrors.
The goal of this thesis is to move towards the realisation of an efficient spin photon interface of NV centres in an open microcavity. To this end, short optical pulses for eventual spinphoton entanglement creation and microwave electronics for spin control are implemented.
A cavity is formed and characterised. A finesse of 3.3×103 is found, along with a quality factor of (3.14 ± 0.03)×105 and a mode volume of 83 𝜆3. From this, a theoretical outcoupled coherent photon fraction of 14% is determined. NV centres are found in the cavity, and their coupling strength is determined using off-resonant lifetime measurements. From this, the actual outcoupled coherent photon fraction is determined to be (12 ± 1) %. Which represents a more than 25 times improvement over NV centres in solid immersion lenses.
The electron spin resonance (ESR) of an NV centre is measured and a magnetic field strength aligned with its spin axis of (36 ± 1) G is found. A lifetime measurement of an NV centre using pulsed resonant excitation is shown. The last two measurements can be extended to achieve coherent control and resonant readout of the NV spin. Paving the way towards a more efficient spin-photon interface. ... Read more

Quantum Batteries (QBs) are quantum-mechanical devices for energy storage, gaining interest due to a potential quantum advantage in power. Recently, the first experimental implementations of QBs were realised. This study characterises the superconducting transmon qubits of Starmon-5 as QBs using the Quantum Inspire platform. In addition to direct charging of a QB, our focus is on charger-mediated energy transfer and parallel charging of an array of transmon qubits. The figures of merit include the average stored energy, the charging time and the charging power.

The results from direct charging of the qubits align with existing literature. Charger-mediated energy transfer is demonstrated through the characterisation of the CNOT gate as an interaction gate, gaining the same amount of stored energy, but with a significant increase in the charging time, resulting in a lower charging power. Furthermore, our findings demonstrate that parallel charging of an array of qubits preserves the quality of direct charging of the individual qubits.

To our knowledge, this work presents the first results of charger-mediated energy transfer in real quantum devices. Charger-mediated energy transfer can be interesting for specific applications such as quantum metrology, where preserving the quantum state is critical. Additionally, this is the first demonstration of parallel charging of superconducting transmon qubits in the QB context, giving promising results for the scalability of superconducting transmon qubits as QB. Our study paves the way forward to implementing quantum batteries for energy management in quantum technologies, a near-term future application of quantum batteries. ... Read more

Superconducting circuits in cryogenic environments form an excellent material platform for the realization and study of quantum systems.
In this thesis, we continue the exploration of novel types of circuit elements which expand the circuit quantum electrodynamics toolbox to enable exotic, and potentially better circuit implementations. To this end, we combine the study of condensed matter systems and circuit quantum electrodynamics in what is called hybrid cQED experiments to arrive at the implementation of gate-tunable kinetic inductances for superconducting circuits. This discovery shed new light on the physics of gate-tunable kinetic inductances and enabled the observation of emergent phenomena in gate-tunable metamaterials, in particular the phase transition in a bosonic Su-Schrieffer-Heeger chain. Moreover, as gate-tunable kinetic inductances became available we realized tunable resonators and parametric amplifiers for enhanced control and readout of superconducting circuits. ... Read more

The Josephson effect is a quintessential topic of condensed matter physics. It has stimulated decades of fundamental research, leading to a plethora of applications from metrology to outer space. In addition, it is set to play a crucial role in the development of quantum computers, forming the dissipationless non-linear inductance that lies at the core of superconducting qubits.

While they are traditionally realized using oxide based tunnel barriers, in this thesis we construct Josephson junctions from non-insulating materials such as semiconducting nanowires and quantum dots. We investigate how their highly nontrivial interplay with superconductivity can lead to new effects, both of fundamental interest and of relevance for quantum applications. To study these effects we make use the exhaustive toolbox available for superconducting circuits, allowing us to probe the junction behavior to beyond what is possible with conventional transport techniques.

The first experimental chapter of this thesis examines the behaviour of a transmon that hosts a highly transparent semiconducting weak-link as the Josephson junction. In this system we find spectroscopic evidence for the predicted vanishing of Coulomb effects in open superconducting islands, in accordance with theoretical predictions from 1999.

In the second experiment we deterministically place a quantum dot inside the junction of a transmon circuit. We then demonstrate that by using microwave spectroscopy we are able to accurately probe the energy-phase relationship of the Josephson junction over a vast regime of parameter space. This reveals the remnants of a quantum phase transition, and allows us to probe the time dynamics of the junction parity.

We subsequently use the same type of device to reveal the predicted spin-splitting of the Andreev bound states in a quantum dot with superconducting leads, as brought about by the spin-orbit interaction. When combined with a magnetic field, this is shown to result in the anomalous Josephson effect. Furthermore, we demonstrate that transitions between the spin-split quantum dot states can be directly driven with microwaves.

This motivated the investigation of a novel superconducting spin qubit, performed in the fourth experiment. Here we demonstrate rapid, all-electric qubit manipulation in addition to detailed coherence characterization. We ultimately show signatures of strong coherent coupling between the superconducting spin qubit and the transmon into which it is embedded, setting the stage for future research of this nascent qubit platform.

In the fifth and final experiment, we utilize a different approach compared to the preceding chapters. While we once-more construct transmons based on semiconducting weak-links, we now do so to leverage the intrinsic magnetic field resilience of semiconducting nanowires. This allows us to use a single device to study the mitigation of phonon-induced quasiparticle losses by trapping the phonons using both super and normal-state conductors.

This thesis concludes by discussing several ideas and proposals that aim to leverage the alternative Josephson junctions studied in this thesis. Combined with the results of the preceding chapters, this shows that hybrid superconducting circuits can be used to obtain deep insights into the fundamental physics governing their constituent junctions, and opens avenues towards building better qubits. ... Read more

Superconducting parametric amplifiers have been developed for fast and high
fidelity single shot readout of superconducting qubits. The nonliniarity re-
quired for those amplifiers is either based on various types of Josephson
junctions or high kinetic inductance materials. The Andersen Lab currently
develops a novel type of superconducting parametric amplifiers based on
superconducting-semiconducting hybrid nanowires, which will allow for in-
situ tuning of the amplifier characteristics with a gate voltage.
To prepare for the required performance tests of nanowire-based para-
metric amplifiers, we characterize a commercially available travelling wave
parametric amplifier in terms of 5 figures of merit: gain, bandwidth, satu-
ration power, noise temperature and quantum efficiency. We find a state of
the art amplifier performances, but also detect several hardware problems
in the measurement setup, which must be solved prior to further test. The
method implemented here can be generalized to the characterization of all
types superconducting parametric amplifiers.
... Read more

Resonators are useful structures due to their simplicity in modeling, design, fabrication and measurement. They can be measured over a wide range of frequencies, power and temperatures, making them convenient for many experiments. Recently, superconducting resonators started being used to investigate microscopic phenomena which require applied magnetic fields of the order of 1T, such as Majorana physics. For this kind of applications it is important that resonators maintain high quality factors in the presence of magnetic fields. A suitable candidate easy to fabricate, resilient to applied fields, and simple capacitive coupling is the lumped element resonator. Such a type of resonator has already been used for some experiments that require magnetic fields, but a systematic study on the resonator’s optimal geometry is yet to be undertaken. The aim of this thesis is to investigate the role of lumped element resonator geometry in its quality factor. That is how to optimize the resonator’s design for enhanced performance of the resonator even in the presence of applied magnetic fields. ... Read more