Engineering Strong Intrinsic Longitudinal Coupling in Circuit Quantum Electrodynamics
R.C. Dekker (TU Delft - Applied Sciences)
G.A. Steele – Promotor (TU Delft - Applied Sciences)
C.K. Andersen – Copromotor (TU Delft - Applied Sciences, TU Delft - QRD/Andersen Lab)
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Abstract
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.