As quantum computing advances towards practical implementation, preserving coherence in superconducting circuits remains a central challenge in developing scalable systems. Advances in materials science and fabrication techniques have significantly improved component quality over the past decade. In particular, treatments such as chemical etching and protective coatings have demonstrated reductions in two-level system (TLS) losses by reducing oxide-related dissipation. However, these methods are insufficient to prevent re-oxidation when devices are exposed to the atmosphere between fabrication and measurement, introducing decoherence channels and hindering reproducibility across cooldown cycles. This thesis addresses these issues by fabricating and evaluating a hermetically sealed three-dimensional aluminum cavity capable of preserving vacuum and preventing oxidation during cooldown cycling. We begin by fabricating the cavity, after which we test its vacuum integrity using a helium leak detector and its structural integrity under repeated thermal changes. After this, we characterize test samples and assess the cavity’s suitability for standardized quality factor and two-level system loss measurements. The results show that the cavity remains sealed across varying temperatures and supports reliable characterization measurements. These results demonstrate that hermetically sealed cavities present a promising strategy for reducing decoherence and improving the reproducibility of superconducting qubit experiments, although further research is warranted to evaluate the long-term viability of the device.
As quantum computing advances towards practical implementation, preserving coherence in superconducting circuits remains a central challenge in developing scalable systems. Advances in materials science and fabrication techniques have significantly improved component quality over the past decade. In particular, treatments such as chemical etching and protective coatings have demonstrated reductions in two-level system (TLS) losses by reducing oxide-related dissipation. However, these methods are insufficient to prevent re-oxidation when devices are exposed to the atmosphere between fabrication and measurement, introducing decoherence channels and hindering reproducibility across cooldown cycles. This thesis addresses these issues by fabricating and evaluating a hermetically sealed three-dimensional aluminum cavity capable of preserving vacuum and preventing oxidation during cooldown cycling. We begin by fabricating the cavity, after which we test its vacuum integrity using a helium leak detector and its structural integrity under repeated thermal changes. After this, we characterize test samples and assess the cavity’s suitability for standardized quality factor and two-level system loss measurements. The results show that the cavity remains sealed across varying temperatures and supports reliable characterization measurements. These results demonstrate that hermetically sealed cavities present a promising strategy for reducing decoherence and improving the reproducibility of superconducting qubit experiments, although further research is warranted to evaluate the long-term viability of the device.
Superconducting qubits are a leading platform for scalable quantum computation. This platform offers high gate speeds and can make use of readily scalable complementary metal-oxide semiconductor (CMOS) fabrication techniques. However, superconducting qubits are susceptible to a variety of losses that arise in systems with superconductors, such as due to ubiquitous two-level systems, quasiparticles, and more. Designing fabrication processes to mitigate and manage these loss sources is critical to achieving scalable quantum computation. However, many of these loss sources are set during fabrication processing, of which there can be many different steps, each with its own parameter space. Exploring this space with qubits would be prohibitively time-consuming. Coplanar waveguide resonators provide a suitable proxy system for minimizing loss while costing less time and money to fabricate and measure. In this thesis, we explore the results of such an optimization performed on niobium resonators. Niobium is a widely used material platform for superconducting qubit circuits. We report highly consistent state-of-the-art resonator quality factors, with clear improvements over the previous fabrication recipe, and correlate the observed improvements to the internal quality factors with microstructural changes induced by hydrofluoric acid treatment, photoresist stripping, and choice of sputter deposition conditions. Additionally, we provide a detailed description of our fab process to ensure reproducibility in other lab facilities. Finally, we address some intriguing anomalies uncovered in temperature-dependent quality factors of the niobium resonators and show a correlation of the quality factors with compressive film stress.
While significant strides have been made to increase the coherence time of superconducting qubits, further advancements are essential for realizing scalable quantum computing. Decoherence is often a result of loss and noise stemming from two-level systems and excess quasiparticles, arising due to material defects, fabrication processes, and ambient exposure, particularly at surfaces and interfaces. Our recent efforts to mitigate these decoherence mechanisms have employed a variety of strategies, including low-loss surface encapsulation materials, advanced substrate preparation techniques, modifications to metal film growth, and the development of novel fabrication processes. The structural and chemical properties of materials, surfaces, and interfaces are studied using scanning probe microscopy, electron microscopy, photoelectron spectroscopy, mass spectrometry, and X-ray diffraction, which is correlated to device performance metrics, including superconducting resonator internal quality factor and qubit T1 time. This information is used to identify and understand material sources of loss and their origins in the device fabrication process. Through multi-institution efforts within SQMS we have identified the loss mechanism of interstitial hydrogen in niobium-based devices and shown how standard fabrication processes introduce these hydrides, developing strategies to mitigate their formation.1 Furthermore, we have characterized the metal-substrate interface, including the loss of niobium-silicides formed at that interface, and developed silicon surface treatments that reduce atomic scale roughness and oxygen content at the metal-substrate and Josephson junction interfaces.2-4 By developing the connection between materials properties and the overall performance of superconducting quantum circuitry, we can develop fabrication strategies to mitigate material losses, thus supporting the ongoing efforts to enhance coherence time in superconducting quantum devices. 1. Torres-Castanedo, C. G.*, Goronzy, D. P.*, et al., Adv. Funct. Mater., 2401365 (2024) 2. Lu, X., et al., Phys. Rev. Materials 6, 064402 (2022) 3. Berti, G., Appl. Phys. Lett. 122, 192605 (2023) 4. Kopas, C. J., Goronzy, D. P., et al., arXiv:2408.02863 (2024)
Significant improvements in superconducting qubit coherence times have been achieved recently with 3-D microwave waveguide cavities coupled to transmon qubits. While many of the measurements in this direction have utilized superconducting aluminum cavities, other recent work has involved qubits coupled to copper cavities with coherence times approaching 0.1 ms. The copper provides a good path for thermalizing the cavity walls and qubit chip, although the substantial cavity loss makes conventional dispersive qubit measurements challenging. We are exploring various approaches for improving the quality factor of 3-D copper cavities, including electropolishing and coating with superconducting layers of tin. We have characterized these cavities on multiple cooldowns and found tin plating to be robust. In addition, we have performed coherence measurements on transmon qubits in these cavities and observed promising performance.
Aluminum (Al) remains the central material for superconducting qubits, and considerable effort has been devoted to optimizing its deposition and patterning for quantum devices. However, post-processing strategies focused on oxide removal of niobium (Nb) and tantalum (Ta) -based resonators using buffered oxide etch (BOE), which can not be used for Al. This challenge becomes particularly relevant for industry-scale fabrication with multi-chip bonding, where delays between sample preparation and cooldown require surface treatments that preserve low dielectric loss during extended exposure to ambient conditions. In this work, we investigate surface modification approaches for Al resonators subjected to a 24-hour delay prior to cryogenic measurement. Passivation using self-limiting oxygen and fluorine chemistries was evaluated utilizing different plasma processes. Remote oxygen plasma treatment reduced dielectric losses, in contrast to direct oxygen plasma. A fluorine-based plasma process was developed that passivated the Al surface for subsequent BOE treatment. However, the fluorine content in the surface resulted in higher loss, identifying fluorine as an unsuitable passivation material for Al resonators. Above all, selective oxide removal using HF (hydrogen fluoride) vapor and phosphoric acid yielded median dielectric losses as low as δ~LP=5.7×10−7 (QLP≈1.7M) with δ~TLS=3.6×10−7 (QTLS≈2.8M) in the single photon regime. Selective oxide removal provides a promising pathway for robust Al-based qubit fabrication, as it preserves low dielectric losses for a 24-hour delay before cooldown.
Abstract Quantum bits (qubits) with long coherence times are an important element for the implementation of medium- and large-scale quantum computers. In the case of superconducting planar qubits, understanding and improving qubits’ quality can be achieved by studying superconducting planar resonators. In this paper, we fabricate and characterize coplanar waveguide resonators made from aluminum thin films deposited on silicon substrates. We perform three different substrate surface treatments prior to aluminum deposition: one chemical treatment based on a hydrofluoric acid clean; one physical treatment consisting of a thermal annealing at 880 °C in high vacuum; and one combined treatment comprising both the chemical and the physical treatments. The aim of these treatments is to remove the two-level state (TLS) defects hosted by the native oxides residing at the various samples’ interfaces. We first characterize the fabricated samples through cross-sectional tunneling electron microscopy, acquiring electron energy loss spectroscopy maps of the samples’ cross sections. These measurements show that both the chemical and the physical treatments almost entirely remove native silicon oxide from the substrate surface and that their combination results in the cleanest interface. Additionally, we analyze the effects of the various substrate treatments on the roughness of the silicon surface by means of atomic force microscopy surface morphology mapping. We then study the quality of the resonators by means of microwave measurements in the ‘quantum regime’, i.e., at a temperature T ∼ 10 mK and at a mean microwave photon number 〈 n ph 〉 ∼ 1 . In this regime, we find that both surface treatments independently improve the resonator’s intrinsic quality factor by ≈172%. The highest quality factor is obtained for the combined treatment, Q i ≈ 0.82 million, corresponding to an improvement by ≈256%. Finally, we find that the TLS quality factor averaged over a time period of 3 h is ∼3 million at 〈 n ph 〉 ∼ 10 , indicating that substrate surface engineering can potentially reduce the TLS loss below other losses such as quasiparticle loss and flux noise.
A quantum computer will potentially solve far-reaching problems which are currently intractable on any classical computer. Many technological obstacles have prevented the realization of a quantum computer, the main obstacle being decoherence, which is the loss of quantum information. Decoherence arises from the undesired interaction between qubits and their environment. Isolated qubits have better coherence but are more difficult to control. Superconducting qubits are a promising platform since their macroscopic size allows for easy control and coupling to other qubits. While the coherence of superconducting qubits has substantially improved over the past two decades, further improvements in coherence are required.We have repeatedly and reliably increased the coherence times of superconducting qubits. Currently decoherence in these devices is dominated by coupling to material defects. These defects are present in the dielectrics used to fabricate these devices or introduced during fabrication. Using simpler resonators as a testbed, we individually isolate, characterize, and then improve each step of the more complicated fabrication of superconducting qubits. We increased the quality factor of resonators by a factor of four by first identifying the surfaces and interfaces as a major source of loss and then by optimizing the substrate preparation. Furthermore, we measure and subsequently mitigate additional defect loss, which is dependent on the position of ground plane holes used to limit the loss from magnetic vortices. Implementing these improvements led to an increase of our qubit coherence times by more than an order of magnitude.The progress made in coherence while maintaining a high degree of connectivity and controllability has been directly used in more complex circuits. One such device is a fully connected three qubit ring with both tunable qubit frequencies and adjustable qubit-qubit couplings. The considerable level of control allows us to generate the quantum dynamics resulting from a collision between Na and He atoms by encoding the problem into a time-dependent multi-qubit interaction. We compute a scattering cross section by measuring the probability of exciting the Na atom over a wide range of collision parameters. The scattering cross-section shows that there is a velocity where the excitation probability is maximized, in agreement with scattering experiments.