The superconducting qubit technology based on Josephson junctions is a promising platform for realizing quantum computers. Since being first demonstrated, the coherence time of superconducting qubits has improved from 10 ns to 100 μs over the past 18 years. As a result, high-fidelity qubit operations in a scalable architecture are now becoming possible. In this article, we introduce recent progress in the fabrication process of superconducting qubits for large-scale quantum computation.
The superconducting qubit technology based on Josephson junctions is a promising platform for realizing quantum computers. Since being first demonstrated, the coherence time of superconducting qubits has improved from 10 ns to 100 μs over the past 18 years. As a result, high-fidelity qubit operations in a scalable architecture are now becoming possible. In this article, we introduce recent progress in the fabrication process of superconducting qubits for large-scale quantum computation.
Superconducting qubits are Josephson junction-based circuits that exhibit macroscopic quantum behavior and can be manipulated as artificial atoms. Benefiting from the well-developed technology of microfabrication and microwave engineering, superconducting qubits have great advantages in design flexibility, controllability, and scalability. Over the past decade, there has been rapid progress in the field, which greatly improved our understanding of qubit decoherence and circuit optimization. The single-qubit coherence time has been steadily raised to the order of 10 to 100 μs, allowing for the demonstration of high-fidelity gate operations and measurement-based feedback control. Here we review recent progress in the coherence and readout of superconducting qubits.
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 circuits based on Josephson junctions are regarded as one of the most promising technologies for the implementation of scalable quantum computers. This review presents the basic principles of superconducting qubits and shows the progress of quantum computing and quantum simulation based on superconducting qubits in recent years. The experimental realization of gate operations, readout, error correction codes, as well as some quantum algorithms are summarized, followed by an introduction of quantum simulation. And then some important applications in fields including condensed matter physics, quantum annealing, and quantum chemistry are discussed.
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.
In the upcoming decades quantum technologies, especially quantum computation, will have a strong influence on industrial, technological, economic, and social progress and are, therefore, considered to be one of the key technologies. The fundamental building blocks for quantum computation are quantum bits (qubits). The superconducting platform offers one of the most promising implementations due to its monolithic character and potential for scalability. Present research around the world primarily focusses on scaling up the number of qubits, so reliable wafer-scale fabrication technologies are mandatory. In this paper, we present our recent developments on wafer-scale fabrication of superconducting quantum circuits including Josephson junctions by the Manhattan-type technology. We discuss fabrication parameters as well as electrical characterization. For arrays out of 50 Josephson junctions with structure dimensions of 200 nm × 200 nm, we achieve a critical current standard deviation of approx. 3% on-chip and 7% on-wafer. We also show first measurement results of so fabricated transmon-type qubits with relaxation and decoherence times ofT1= 12.5 µs andT2= 15.0 µs, respectively. With an increasing number of superconducting qubits and, thus, their input and read-out lines, new challenges arise: Crosstalk between qubits, unwanted modes in the qubit environment, and required interconnections. One way to counteract is the implementation of low inductance connections across coplanar waveguides. This can be realized by aluminum air bridges. In this paper we discuss the air bridges’ fabrication parameters and the impact on the integrated circuits’ elements, e.g., Josephson junctions. We show electrical and superconducting measurements with critical temperatures and critical currents ofTc> 1.0 K andIc> 0.6 mA, respectively.
The superconducting qubit quantum computer is one of the most promising quantum computing architectures for large-scale integration due to its maturity and close proximity to the well-established semiconductor manufacturing infrastructure. From an education perspective, it also bridges classical microwave electronics and quantum electrodynamics. In this paper, we will review the basics of quantum computers, superconductivity, and Josephson junctions. We then introduce important technologies and concepts related to DiVincenzo's criteria, which are the necessary conditions for the superconducting qubits to work as a useful quantum computer. Firstly, we will discuss various types of superconducting qubits formed with Josephson junctions, from which we will understand the trade-off across multiple design parameters, including their noise immunity. Secondly, we will discuss different schemes to achieve entanglement gate operations, which are a major bottleneck in achieving more efficient fault-tolerant quantum computing. Thirdly, we will review readout engineering, including the implementations of the Purcell filters and quantum-limited amplifiers. Finally, we will discuss the nature and review the studies of two-level system defects, which are currently the limiting factor of qubit coherence time. DiVincenzo's criteria are only the necessary conditions for a technology to be eligible for quantum computing. To have a useful quantum computer, large-scale integration is required. We will review proposals and developments for the large-scale integration of superconducting qubit devices. By comparing with the application of electronic design automation (EDA) in semiconductors, we will also review the use of EDA in superconducting qubit quantum computer design, which is necessary for its large-scale integration.