Superconductor devices are promising for building large-scale quantum computing systems. This article reports the current status and issues of the superconductor classical (digital and mixed-signal) circuit technology for quantum computing applications. The single-flux-quantum-based circuits provide advanced signal processing featuring several gigahertz clock frequencies, high sensitivity, and ultra-low power operation at a millikelvin-level temperature and are helpful for control and readout of quantum bits. The superconductor cryogenic peripheral circuits are expected to be a key to breakthrough in system scalability.
Superconductor devices are promising for building large-scale quantum computing systems. This article reports the current status and issues of the superconductor classical (digital and mixed-signal) circuit technology for quantum computing applications. The single-flux-quantum-based circuits provide advanced signal processing featuring several gigahertz clock frequencies, high sensitivity, and ultra-low power operation at a millikelvin-level temperature and are helpful for control and readout of quantum bits. The superconductor cryogenic peripheral circuits are expected to be a key to breakthrough in system scalability.
Single-flux quantum logic (SFQ) circuits, in which a flux quantum is used as an information carrier, have the possibility for opening the door to a new digital system operated at over 100-GHz clock frequency at extremely low power dissipation. The SFQ logic system is a so-called pulse logic, which is completely different from the level logic for semiconductors like CMOS, so circuit design technologies for SFQ logic circuits have to be newly developed. Recently, much progress in basic technologies for designing SFQ circuits and operating circuits at high speeds has been made. With advances in these design tools, large-scale circuits including more than several thousand junctions can be easily operated with the clock frequency of more than several tens of gigahertz. High-end routers and high-end computers are possible applications of SFQ logic circuits because of their high throughput nature and the low power dissipation of SFQ logic. In this paper, recent advances of SFQ circuit design technologies and recent developments of switches for high-end routers and microprocessors for high-end computers that are considered possible applications for SFQ logic will be described.
Rapid Single-Flux-Quantum (RSFQ) logic is a superconductor IC technology that, with only a modest number of researchers worldwide, has produced some of the world's highest performance digital and mixed-signal circuits. This achievement is due, in part, to a constellation of characteristics that manifest themselves at the circuit level – namely, high-speed digital logic at low-power, ideal interconnects, quantum accuracy, scalability, and simplicity of fabrication. A necessary key to translating these advantages to the system-level involves understanding the I/O, synchronization, and packaging issues associated with a cryogenic technology. The objective of this paper is to review the status of current RSFQ circuit-level infrastructure components and their potential impact on system-level applications.
Interface circuits are the key components that enable the hybrid integration of superconductor and semiconductor digital electronics. The design requirements of superconductor-semiconductor interface circuits vary depending on the application, such as high-performance classical computing, superconducting quantum computing, and digital signal processing. In this survey, various interface circuits are categorized based on the working principle and structure. The superconducting output drivers are explored, which are capable of converting and amplifying, e.g., single flux quantum (SFQ) voltage pulses, to voltage levels that semiconductor circuits can process. Several trade-offs between circuit- and system-level design parameters are examined. Accordingly, parameters such as the data rate, output voltage, power dissipation, layout area, thermal/heat load of cryogenic cables, and bit-error rate are considered.
A systematic research on the development of cryogenic complementary metal-oxide semiconductor (cryo-CMOS) circuits, for implementing the required control electronics to manipulate the quantum bit (qubit) state, is performed over the last few years. Scalability constitutes a key term regarding the evolution of quantum computing from theory to practical application and CMOS technology has been proven to be a promising candidate for implementing the coveted scalable next-generation quantum computers (QCs). Mixed-signal blocks, used for uniting the analog and digital domains, play a key role in the efficient functionality of the qubit control/readout system, thus there is an ever-increasing interest in their high-performance circuit realization. The critical challenge in this venture is to achieve efficient cryogenic operation at low temperatures, i.e., close to the qubit around 4 K, simultaneously keeping power requirements at low values. An overview and comparison of the cryo-CMOS Digital-to-Analog converter (DAC) and Analog-to-Digital converter (ADC) circuit implementations for quantum computing applications that heretofore have been proposed in the literature is presented in this work. A discussion on the challenges and future strategic steps that are henceforth required to proceed toward the development of a functional scalable quantum computer is also conducted.
Ultrafast switching speed, low power, natural quantization of magnetic flux, quantum accuracy, and low noise of cryogenic superconductor circuits enable fast and accurate data conversion between the analog and digital domains. Based on rapid single-flux quantum (RSFQ) logic, these integrated circuits are capable of achieving performance levels unattainable by any other technology. Two major classes of superconductor analog-to-digital converters (ADCs) are being developed - Nyquist sampling and oversampling converters. Complete systems with digital sampling at rates of /spl sim/20 GHz and above have been demonstrated using low-temperature superconductor device technology. Some ADC components have also been implemented using high-temperature superconductors. Superconductor ADCs have unique applications in true digital-RF communications, broadband instrumentation, and digital sensor readout. Their designs, test results, and future development trends are reviewed.