Over the past two decades significant advances have been made in the research of superconducting quantum computing and quantum simulation, in particular of the device design and fabrication that leads to ever-increasing superconducting qubit coherence times and scales. With Google’s announcement of the realization of “quantum supremacy”, superconducting quantum computing has attracted even more attention. Superconducting qubits are macroscopic objects with quantum properties such as quantized energy levels and quantum-state superposition and entanglement. Their quantum states can be precisely manipulated by tuning the magnetic flux, charge, and phase difference of the Josephson junctions with nonlinear inductance through electromagnetic pulse signals, thereby implementing the quantum information processing. They have advantages in many aspects and are expected to become the central part of universal quantum computing. Superconducting qubits and auxiliary devices prepared with niobium or other hard metals like tantalum as bottom layers of large-area components have unique properties and potentials for further development. In this paper the research work in this area is briefly reviewed, starting from the design and working principle of a variety of superconducting qubits, to the detailed procedures of substrate selection and pretreatment, film growth, pattern transfer, etching, and Josephson junction fabrication, and finally the practical superconducting qubit and their auxiliary device fabrications with niobium base layers are also presented. We aim to provide a clear overview for the fabrication process of these superconducting devices as well as an outlook for further device improvement and optimization in order to help establish a perspective for future progress.
Over the past two decades significant advances have been made in the research of superconducting quantum computing and quantum simulation, in particular of the device design and fabrication that leads to ever-increasing superconducting qubit coherence times and scales. With Google’s announcement of the realization of “quantum supremacy”, superconducting quantum computing has attracted even more attention. Superconducting qubits are macroscopic objects with quantum properties such as quantized energy levels and quantum-state superposition and entanglement. Their quantum states can be precisely manipulated by tuning the magnetic flux, charge, and phase difference of the Josephson junctions with nonlinear inductance through electromagnetic pulse signals, thereby implementing the quantum information processing. They have advantages in many aspects and are expected to become the central part of universal quantum computing. Superconducting qubits and auxiliary devices prepared with niobium or other hard metals like tantalum as bottom layers of large-area components have unique properties and potentials for further development. In this paper the research work in this area is briefly reviewed, starting from the design and working principle of a variety of superconducting qubits, to the detailed procedures of substrate selection and pretreatment, film growth, pattern transfer, etching, and Josephson junction fabrication, and finally the practical superconducting qubit and their auxiliary device fabrications with niobium base layers are also presented. We aim to provide a clear overview for the fabrication process of these superconducting devices as well as an outlook for further device improvement and optimization in order to help establish a perspective for future progress.
Superconducting quantum circuits (SQCs), composed of superconducting capacitors, inductances, Josephson junctions, and transmission lines, exhibit macroscopic quantum effects at ultra-low temperatures. Due to the extremely low dissipation of superconductors, an important application of SQCs is superconducting qubits with long coherence time. Quantum computing based on superconducting qubits is a leading physical realization method of quantum computing, which is theoretically capable of accelerating many computational problems including quantum simulation, database searching, and optimization. In the long run, to use the power of quantum computing to its full extent, universal quantum computers should be constructed with quantum error correction. In the short term, one of the most pressing targets of the superconducting quantum computing community is to demonstrate quantum advantage using noisy intermediate-scale quantum chips. Both goals rely on the increasing of the number of coupled qubits in a quantum circuit and the quality of these qubits, which dictate the breadth and the depth of a problem it can solve, respectively. The fabrication of SQCs has adopted a few manufacturing processes from conventional integrated circuits. Quantum chips containing multiple superconducting qubits can be made and packaged on a large scale. “Quantum supremacy” has been demonstrated on a quantum chip with fifty-three qubits fabricated using the flip-chip bonding technology. However, to demonstrate quantum advantage, the quality and quantity of qubits need to increase simultaneously. Dissipative channels emerge in circuit design, material preparation, chip fabrication, and working environment, limiting the lifetime of superconducting qubits. For quantum chips with tens to hundreds of qubits, the integration will also introduce extra processing steps which may degrade the qubits, and unwanted coupling to loss channels. Most of these channels are microscopically related to materials constructing the quantum chips, chip surfaces, and material interfaces. Thus, it is important to investigate the material growth and device fabrication of SQCs, and to understand the material related loss mechanisms. This review paper investigates the material related topics in superconducting quantum circuits pertinent to the quantum computing application from a comprehensive perspective. The history of SQCs is briefly mentioned, followed by the peculiar characteristics as an engineerable quantum system, and a roadmap to quantum advantage and universal quantum computing in the next a few years. Technically, the superconducting quantum computer is compared to a Schrodinger’s cat, and can be boiled down into three parts, i.e., quantum chips, the environment, and the operating system, corresponding to the cat, the box, and the observer, respectively. The material related decoherence channels of the SQCs can be sorted into four categories; each will be discussed in details. To fabricate SQCs with minimal loss, the processing technologies are elaborated, including qubit design, material selection, surface treatments, film growth, and Josephson junction, etc. Though there are general guidelines that amorphous materials should be avoided and processing induced damage should be minimized, more is to be investigated due to the emerging of new materials and new fabrication methods. The last part presents the potential application of the superconducting quantum computer and its current state of play. SQC for quantum computing is a multi-discipline research topic, demanding joint efforts from different scientific and technical areas. Materials science and device processing will play important roles along the way to quantum practicality.
In the past years, superconducting quantum computation has received much attention and significant progress of the device design and fabrication has been made, which leads qubit coherence times to be improved greatly. Recently, we have successfully designed, fabricated, and tested the superconducting qubits based on the negative-inductance superconducting quantum interference devices (nSQUIDs), which are expected to have the advantages for the fast quantum information transfer and macroscopic quantum phenomenon study with a two-dimensional potential landscape. Their quantum coherence and basic physical properties have been demonstrated and systematically investigated. On the other hand, a new type of superconducting qubit, called transmon and Xmon qubit, has been developed in the meantime by the international community, whose coherence time has been gradually increased to the present scale of tens of microseconds. These devices are demonstrated to have many advantages in the sample design and fabrication, and multi-qubit coupling and manipulation. We have also studied this type of superconducting qubit. In collaboration with Zhejiang University and the University of Science and Technology of China, we have successfully fabricated various types of the coupled Xmon devices having the qubit numbers ranging from 4 to 10. Quantum entanglement, quantum algorithm of solving coupled linear equations, and quantum simulation of the many-body localization problem in solid-state physics have been demonstrated by using these devices. Also, we have made significant achievements in the studies of the macroscopic quantum phenomena, quantum dissipation, quantum microwave lasing, and some other quantum optics problems. In particular, Autler-Townes splitting under strong microwave drive, electromagnetically induced transparency, stimulated Raman adiabatic passage, microwave mixing, correlated emission lasing, and microwave frequency up-and-down conversion have been successfully studied, both experimentally and theoretically.
The aim of this thesis was to realize superconducting quantum bits by using novel fabrication techniques and materials in order to enhance the coherence time of the former devices. For the system to be studied we chose the ux-biased phase qubit, which consists of a Josephson junction integrated in a superconducting loop. This qubit realization has some advantages, which make it an ideal test-bed for different fabrication technologies and materials. Relatively large Josephson junctions can be used in the phase qubit realization, which are easy to be fabricated by using standard lithographic technology with the flexibility of employing various fabrication techniques and materials. The coherence of current-generation of superconducting qubits seems to be limited by intrinsic sources of noise and energy loss, related to materials' defects (two-level systems, TLSs) on the surfaces and interfaces of the superconducting lms, and in the bulk of the dielectric films used for the microelectronic realization of the circuits. For this reason, we aimed our research activities on the realization of low loss dielectrics (a-Si:H), on its integration with Josephson junction technologies, on the protection of the superconducting films' surfaces from unwanted contamination, and on the optimization of the circuit design and fabrication processes.
Abstract Superconducting qubits are a promising quantum computing platform, but significant challenges remain, such as achieving sufficiently long and stable coherence times and precise control of circuit parameters. Further improvements in material processing are essential to overcome these challenges. This paper introduces our research on superconducting quantum circuit technology based on nitride superconductors, where the core technologies are epitaxial growth of titanium nitride (TiN) on silicon (Si) substrates and epitaxial Josephson junctions using niobium nitride (NbN), aluminum nitride (AlN), and magnesium oxide (MgO). We have established a fabrication process of NbN/AlN/NbN epitaxial junctions on Si (100) substrates using a TiN film as a buffer layer and are working on applying this process to superconducting qubits. We describe recent results on the effect of junction annealing and discuss the advantages and potential of epitaxial Josephson junctions for realizing large-scale superconducting quantum circuits.
Quantum computing is rapidly evolving from theoretical frameworks to functional hardware, with physical qubits now being realized across multiple experimental platforms. Precision nanofabrication has enabled the development of low-loss, high-coherence quantum devices. Among the various approaches, superconducting circuits and trapped-ion systems have emerged as the most advanced and experimentally successful platforms for realizing controllable and scalable qubits. This review presents an overview of nanoscale fabrication techniques employed in these two architectures. We first outline the operating principles that define qubit behavior, followed by a discussion of representative device designs and fabrication workflows. In superconducting circuits, we examine processes such as thin-film deposition, lithographic patterning, and Josephson-junction fabrication that determine key parameters including qubit frequency, anharmonicity, and coherence. For trapped-ion systems, we describe the transition from bulk electrode assemblies to microfabricated surface traps produced using photolithography, metal deposition, and dielectric layering, where electrode geometry and surface treatment directly influence ion confinement and motional heating. We further discuss fabrication-related challenges, including dielectric loss, surface contamination, and material defects that limit coherence and reproducibility, along with emerging mitigation strategies such as advanced cleaning, low-loss materials, and improved surface engineering. By drawing parallels between these two leading platforms, this review highlights that continued progress in nanofabrication remains central to realizing scalable, high-fidelity quantum processors.
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)