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International Journal of Nanotechnology

Superconducting quantum nano-circuits

Wiebke Guichard, Laurent-Patrick Lévy, B. Pannetier, T. Fournier, O. Buisson, F. W. J. Hekking

Published Jan 1, 2010• 1 citation
Publisher

Abstract

Superconducting circuits are described by a small number of macroscopic variables and behave as simple quantum mechanical systems. They can be manufactured and integrated by extending micro-electronics techniques with suspended masks and shadow evaporation techniques. Superconducting circuits can be designed to study novel quantum phenomena and explore some of their applications. In this paper, the physics of some superconducting qubits circuits (phase and coupled charge-phase qubits) is reviewed. Other interesting applications involving quantised superconducting pumps and the current-phase relation of Josephson rhombi networks are also presented. Presently, applications of superconducting circuits to quantum computing are limited by decoherence phenomena. Prospects in material development and circuit integration to overcome these difficulties are also discussed in this review.

DOI
10.1504/ijnt.2010.031731

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Superconducting circuits are described by a small number of macroscopic variables and behave as simple quantum mechanical systems. They can be manufactured and integrated by extending micro-electronics techniques with suspended masks and shadow evaporation techniques. Superconducting circuits can be designed to study novel quantum phenomena and explore some of their applications. In this paper, the physics of some superconducting qubits circuits (phase and coupled charge-phase qubits) is reviewed. Other interesting applications involving quantised superconducting pumps and the current-phase relation of Josephson rhombi networks are also presented. Presently, applications of superconducting circuits to quantum computing are limited by decoherence phenomena. Prospects in material development and circuit integration to overcome these difficulties are also discussed in this review.
Quantum information processing systems can be used in a variety of applications. Among proposed realizations of quantum bits nano-electronic circuits appear most promising for integration in electronic circuits and large-scale applications. We discuss Josephson junction circuits in two regimes where they can be used for quantum computing. In one limit, two logic states of the quantum bit differ by a Cooper pair charge, 2e, on a superconducting island. In the other limit different magnetic fluxes penetrate a superconducting loop in two states. Single-qubit and two-qubit logic operations can be performed by means of voltage or current pulses. Weak coupling to the environment allows a series of these elementary steps before the phase coherence is lost. To read out the result of the computation a quantum measurement is needed. For charge qubits this can be accomplished by coupling a single-electron transistor (SET) capacitively to the qubit. Monitoring the current in the transistor one can extract the information about the state of the qubit. We discuss recent experiments and present a suitable set of circuit parameters available with the present-day technology.
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.
During the last ten years, superconducting circuits have passed from being interesting physical devices to becoming contenders for near-future useful and scalable quantum information processing (QIP). Advanced quantum simulation experiments have been shown with up to nine qubits, while a demonstration of quantum supremacy with fifty qubits is anticipated in just a few years. Quantum supremacy means that the quantum system can no longer be simulated by the most powerful classical supercomputers. Integrated classical-quantum computing systems are already emerging that can be used for software development and experimentation, even via web interfaces. Therefore, the time is ripe for describing some of the recent development of superconducting devices, systems and applications. As such, the discussion of superconducting qubits and circuits is limited to devices that are proven useful for current or near future applications. Consequently, the centre of interest is the practical applications of QIP, such as computation and simulation in Physics and Chemistry.
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.
Electrons in a superconductor form Cooper pairs and thus condense into a ground state which can be described by a macroscopic quantum wavefunction. The phase of the wavefunction represents a collective movement of millions of Cooper pairs. Using devices based on Josephson tunnel junctions we are able to control and detect the phase and number of Cooper pairs in a superconductor. Josephson devices are therefore excellent systems for demonstrating macroscopic quantum phenomena. We present a short review of the macroscopic quantum phenomena in superconducting Josephson devices, as well as their applications and prospects in quantum computationl.
Superconducting integrated circuits incorporating Josephson junctions are an attractive candidate for scalable quantum information processing in the solid state. The strong nonlinearity of the Josephson effect enables one to tailor an anharmonic potential and thus to realize an artificial quantum two-level system (“qubit”) from a macroscopic superconducting circuit. Josephson qubits can be made to interact strongly and controllably, and it should be straightforward to fabricate circuits incorporating hundreds or even thousands of Josephson qubits using standard thin-film processing techniques. Work over the last several years has shown that qubit performance is limited by spurious coupling of the qubit to microscopic defect states in the materials that are used to implement the circuit. Here we discuss the materials origins of dissipation and dephasing in superconducting qubits. A deeper understanding of the underlying materials physics that governs decoherence in superconducting quantum circuits will guide the search for improved, low-noise materials and fuel continued progress in the field of superconducting quantum computing.