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Abstract: In this paper, we propose the development of superconductive combinational logic circuits. One of the difficulties in designing superconductive single-flux-quantum (SFQ) digital circuits can be attributed to the fundamental nature of the SFQ circuits, in which all logic gates have latching functions and are based on sequential logic. The design of ultralow-power superconductive digital circuits can be facilitated by the development of superconductive combinational logic circuits in which the output is a function of only the present input. This is because superconductive combinational logic circuits do not require determination of the timing adjustment and clocking scheme. Moreover, semiconductor design tools can be used to design digital circuits because CMOS logic gates are based on combinational logic. The proposed superconductive combinational logic circuits comprise a magnetically coupled SQUID array. By adjusting the circuit parameters and coupling strengths between neighboring SQUIDs, fundamental combinational logic gates, including the AND, OR, and NOT gates, can be built. We have verified the accuracy of the operations of the fundamental logic gates by analog circuit simulations. [ABSTRACT FROM AUTHOR]

Abstract: High-frequency microwave applications, such as filters, delay lines, and resonators, are quite important for superconducting electronic devices. In order to design the superconducting microwave devices, circuit parameters should be precisely extracted from the physical structure of the devices. A 3-dimentional electromagnetic-field simulator is very useful for designing microwave devices. However, designing of superconducting microwave devices using a conventional 3D electromagnetic-field simulator is difficult because most of commercially available 3D electromagnetic-field simulators can not exactly characterize electromagnetic phenomena of superconductors. In this study, a novel calculation method of superconducting microwave devices, which can be applicable to a conventional 3D electromagnetic-field simulator, has been proposed. Calculation results of characteristic impedance of superconducting microstrip lines show very good agreements with the theoretically calculated vales. The frequency response of a superconducting Nb microwave filter designed by the proposed calculation method agrees well with the experimental results. This calculation method enables us to precisely estimate microwave characteristics of superconducting devices. [Copyright &y& Elsevier]

Abstract: Ultra-fast operation with ultra-low power consumption and ballistic transmission of signals are attractive features of single-flux-quantum (SFQ) circuits for building large-scale digital systems. We have been designing SFQ circuits based on the bit-serial architecture, which can reduce the circuit area and timing error induced by clocking skew. In the bit-serial circuits, a serial–parallel converter, which is one of key circuits, is used for I/O interfaces, address decoders, multipliers and so on. In this paper, we propose a novel architecture for the SFQ serial–parallel converter whose latency is very small compared with conventional one. And, we have designed some SFQ circuits suitable for the proposed circuit. As an example, we have implemented and tested SFQ 4-bit multifunctional buffers using the SRL 2.5kA/cm2 niobium standard process. We have successfully confirmed all functions of the multifunctional buffer at low speed. [Copyright &y& Elsevier]

Abstract: A new on-chip microwave generator for manipulating superconductive quantum bits (qubits) have been proposed. To perform practical quantum computation, on-chip microwave generators, which can be controlled by single flux quantum (SFQ) circuits, are desirable because multiple qubits can be driven simultaneously with high accuracy by an SFQ control circuit and the number of wires between qubits and room-temperature electronics can be reduced. The proposed on-chip microwave generator is composed of an SFQ clock generator, which generates a periodic SFQ pulse train, and a band-pass filter. A sinusoidal microwave can be obtained by filtering an output of the SFQ clock generator. Simulation results indicate that the generated microwave has a sharp spectrum, which can be chopped by controlling the SFQ clock generator. Moreover, a phase of the generated microwave can be accurately controlled by adding a phase control circuit, which is composed of cascaded toggle flip-flops, between the SFQ clock generator and the filter. The results indicate that we can perform fast and accurate quantum gate operations using the proposed on-chip microwave generator. [Copyright &y& Elsevier]

Abstract: In this paper we have considered how to perform logic synthesis of the single-flux-quantum (SFQ) logic circuits by using logic synthesis tools for semiconductor logic circuits. Fundamental difference of the SFQ logic gates from the semiconductor logic gates in terms of logic synthesis is that typical SFQ gates are clocked gate, where gates have to be clocked by SFQ pulses. In our approach, we divided the procedure into two steps; logic synthesis and clock distribution. In the logic synthesis step, the logic function is optimized using a logic synthesis tool without considering the clock network. The Design Analyzer provided from Synopsys is used in this study. We have developed an SFQ cell library for the logic synthesis. In the clock distribution step, clock networks for every clocked SFQ gates are designed taking account of timing. We used a clock-followed-data clocking scheme in this study. We have classified every clocked gate into stages, where the clocked gates in the same stage are clocked simultaneously. To demonstrate the validity of our approach, we have designed a controller of an 8-bit SFQ microprocessor. [Copyright &y& Elsevier]

A single flux quantum (SFQ) circuit has been designed to demonstrate an energy levels quantization in a Josephson Junction. The high-speed operation of SFQ circuits realizes applying a high-speed ramped bias current to a Josephson junction, which allows the noiseless experiment at higher temperature. A high-speed ramped bias current (dI/dt=2000 A/s) is applied to a Josephson junction using the proposed circuit. Although the energy levels quantization has not been observed yet, the obtained escape rate was high enough to observe the energy levels quantization. The proposed circuit indicates the effectiveness of SFQ circuits as experimental apparatuses. [Copyright &y& Elsevier]

We have been developing a high-resolution superconducting time-of-flight mass spectrometry (TOF-MS) system, which utilizes a superconducting strip ion detector (SSID) and a single-flux-quantum (SFQ) time-to-digital converter (TDC). The SFQ TDC can measure time intervals between multiple input signals and directly convert them into binary data. In our previous study, 24-bit SFQ TDC with a 3 × 24-bit First-In First-Out (FIFO) buffer was designed and implemented using the AIST Nb standard process 2 (STP2), whose time resolution and dynamic range are 100 ps and 1.6 ms, respectively. In this study we reduce the jitter of the TDC by using two different approaches: one uses an on-chip clock generator with an on-chip low-pass filter for reducing the noise in the bias current, and the other uses a low-jitter external clock source at room temperature. We confirmed that the jitter is reduced to less than 100 ps in the latter approach. [ABSTRACT FROM AUTHOR]

Highlights: [•] Balanced Josephson comparator decisions depend on previous decisions at high frequencies. [•] The maximum effective data rate is lower than the clock frequency. [•] Tradeoff between maximum effective data rate and minimum comparator gray-zone. [ABSTRACT FROM AUTHOR]

Abstract: We have been studying a superconducting quantum-computing system where superconducting Josephson-junction quantum bits (qubits) are controlled and read out by rapid single-flux-quantum (RSFQ) circuits. In this study, we designed and fabricated an on-chip RSFQ microwave pulse generator (MPG), which generates microwave pulses with the time resolution of sub-ns for precise control of qubit states. The output microwave amplitude of the MPG can be amplified to more than 350μV using a multi-flux-quantum (MFQ) driver, which increases the number of the propagating single-flux-quantum (SFQ) pulses. Fundamental properties of the MFQ driver and the RSFQ MPG were measured at 4.2K. It was confirmed that the MFQ driver can amplify an SFQ pulse up to six MFQ pulses and the irradiation time and the amplitude of the output microwave of the RSFQ MPG can be controlled adequately. [ABSTRACT FROM AUTHOR]

We performed yield analysis of large-scale adiabatic-quantum-flux-parametron (AQFP) circuits using circuit simulations based on the Monte Carlo method assuming the distribution of the critical current of Josephson junctions. Based on the simulation results, we also made an analytical model for the circuit yield of large-scale AQFP circuits. The yield analysis indicates that AQFP integrated circuits containing 1 million gates can be realized when the interconnect inductance is about 40 pH and the normalized standard deviation of the critical current is less than 1%. [ABSTRACT FROM AUTHOR]

Abstract: We present design of single flux quantum (SFQ) cells for a 10-Nb-layer process which has been developed to fabricate SFQ VLSI circuits. The device fabricated by the process has a structure of an active layer on the top, two passive transmission line (PTL) layers in the middle, and a DC power (DCP) layer at the bottom. We have determined a unit cell size of 30μm×30μm and a unit cell structure by taking accounts of the design rules and the experimental data on the PTLs. This cell size enables us to draw two PTLs of each PTL layer. PTL driver and receiver cells have a unit cell size, whereas a half unit cell with a size of 15μm×15μm is used for PTL segments and vias. On the active layer, circuit parameters in analog simulation level are based on those of CONNECT cells, except for junction parameters with McCumber parameter β c of 2.0. Major cells including logic cells and PTL driver/receiver cells have been developed. We have designed a 2×2 switch on-chip test circuit using the cells and successfully tested them at high speed. [Copyright &y& Elsevier]

Abstract: We have demonstrated the components of datapath toward single-flux-quantum microprocessors based on our new architecture called the forwarding architecture. In the forwarding architecture, we improve the performance by utilizing multiple ALUs to conceal the inefficiency of bit-serial processing. The key components of the datapath are a register file and cascaded ALUs. We have designed the register file to hold four bit-serial data with two read ports and one write port. In each ALU, we have implemented six functionalities: addition, subtraction, logical AND, OR and Exclusive OR operations, and comparison with zero. We have tested the register file and the ALU using high-speed on-chip testing, and confirmed the correct operations up to 18GHz and 23GHz, respectively. [Copyright &y& Elsevier]