42 research outputs found

    Single-Flux-Quantum Bipolar Digital-to-Analog Converter Comprising Polarity-Switchable Double-Flux-Quantum Amplifier

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    We present a single-flux-quantum (SFQ)-based digital-to-analog converter (DAC) generating bipolar output voltages, in which the key component is a polarity-switchable double-flux-quantum amplifier (PS-DFQA). The DAC comprised a dc/SFQ converter, an 8-bit variable pulse-number-multiplier (PNM), and a 8-fold PS-DFQA integrated on a single chip. SFQ pulse-frequency modulation was employed to realize variable output voltage amplitude, for which the multiplication factor of the variable-PNM was controlled by a commercial data generator situated at room temperature. The variable-PNM realized 8-bit resolution with a multiplication factor between 0 and 255. Bias currents fed to the 8-fold PS-DFQA were polarity-switched in synchronization with the digital code for the variable-PNM. The whole circuits including I/O elements were designed using SFQ cell libraries, and fabricated using a niobium integration process. Sinusoidal bipolar voltage waveform of 0.38 mVpp was demonstrated using a reference signal source of 43.94 MHz

    Numerical Simulation of Single-Electron Tunneling in Random Arrays of Small Tunnel Junctions Formed by Percolation of Conductive Nanoparticles

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    We numerically simulated electrical properties, i.e., the resistance and Coulomb blockade threshold, of randomly-placed conductive nanoparticles. In simulation, tunnel junctions were assumed to be formed between neighboring particle-particle and particle-electrode connections. On a plane of triangle 100×100 grids, three electrodes, the drain, source, and gate, were defined. After random placements of conductive particles, the connection between the drain and source electrodes were evaluated with keeping the gate electrode disconnected. The resistance was obtained by use of a SPICE-like simulator, whereas the Coulomb blockade threshold was determined from the current-voltage characteristics simulated using a Monte-Carlo simulator. Strong linear correlation between the resistance and threshold voltage was confirmed, which agreed with results for uniform one-dimensional arrays

    Hardware Random Number Generator Using Josephson Oscillation and SFQ Logic Circuits

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    A hardware random number generator using Josephson oscillation and a few single flux quantum (SFQ) logic gates is presented. The logic circuit of the random number generator consists of one toggle flip flop and one and gate. A prototype random number generator is designed by logic cells based on a 2.5-kA/cm^2 Nb/AlOx/Nb integration process. The fundamental operation at a few hundred megahertz of the random number sampling frequency is confirmed by numerical simulation when a DC/SFQ converter is used for generating trigger signals. An additional delay line using an overdamped Josephson transmission line is used for increasing the timing jitter to get random numbers. The delay line makes it possible for the random number generator to operate over 1 GHz. To confirm the fundamental operation of the circuit, a primitive SFQ random number generator is fabricated using the AIST standard process with 2.5-kA/cm^2 Nb/AlOx/Nb junctions and the standard logic cell library. A random number generation based on a low-speed functional test is successfully confirmed

    1000-Fold Double-Flux-Quantum Voltage Multiplier Employing Directional Propagation of Flux Quanta Through Asymmetrically Damped Junction Branches

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    Precise voltage generation is a unique feature of single-flux-quantum (SFQ) circuits, in addition to their high-speed digital signal processing with low power consumption. We investigated SFQ pulse-frequency modulation D/A converters for metrological applications. In our SFQ-based D/A converters, the maximum output voltage is determined by the maximum SFQ pulse-frequency at the pulse number multiplier, and by the voltage multiplication factor at the voltage multiplier. In this study, we present our new design for a double-flux-quantum amplifier (DFQA) that works as a quantum voltage multiplier. In the new parameter set, we tuned the damping parameters of the Josephson junctions to realize proper propagation of SFQ pulses. A 1000-fold DFQA designed with the new parameter set was fabricated using a 25-μ A/μ m 2 Nb/AlOx/Nb integration technology. A 1000-fold voltage multiplication was confirmed for the input voltage up to 43 μV, with a corresponding SFQ repetition frequency of 21 GHz. That is, the output voltage reached 43 mV

    4-bit Bipolar Triangle Voltage Waveform Generator Using Single-Flux-Quantum Circuit

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    AbstractSFQ digital-to-analog converters (DACs) are one of the candidates for AC voltage standards. We have proposed SFQ-DACs based on frequency modulation (FM). Bipolar output is required for applications of AC voltage standards, while our previous SFQ-DACs generated only positive voltages. In this paper, we present our design of a 4-bit bipolar triangle voltage waveform generator comprising an SFQ-DAC. The waveform generator has two output ports. Synthesized half-period waveforms are alternately generated in one of the output ports. The bipolar output is realized by observing the differential voltage between the ports. We confirmed a 72-μVPP bipolar triangle voltage waveform at the frequency of 35.7Hz

    Enhanced operation frequencies of bipolar double-flux-quantum amplifiers fabricated using 10 kA cm−2 Nb/AlOx/Nb integration process

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    We have demonstrated digital-to-analog (D/A) operations using single-flux-quantum (SFQ) pulse-frequency modulation (PFM) D/A converters for future AC voltage standards. In this paper, for the improvement of SFQ-PFM D/A converters, we investigated a double-flux-quantum amplifier (DFQA) and a magnetically-coupling SFQ driver/receiver circuit (MC-SFQ-DR) fabricated using a 10 kA cm−2 Nb/AlOx/Nb integration process. The critical current density Jc of 10 kA cm−2 was four times larger than that of the integration process we had used. A DFQA and an MC-SFQ-DR included unshunted Josephson junctions, and therefore, it was unclear if the high-Jc process improved their performances. We measured test chips cooled in a liquid helium bath. The maximum input voltages for a +20 fold and a −20 fold DFQA were 147 and 126 μV, for which the corresponding Josephson frequencies were 70.9 and 61.1 GHz. It was confirmed that the operation frequencies of the DFQAs and MC-SFQ-DR were improved approximately two-fold

    Rapid Single-Flux-Quantum NOR Logic Gate Realized through the Use of Toggle Storage Loop

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    Recently, we demonstrated a rapid-single-flux-quantum NOT gate comprising a toggle storage loop. In this paper, we present our design and operation of a NOR gate that is a straightforward extension of the NOT gate by attaching a confluence buffer. Parameter margins wider than ±28% were confirmed in simulation. Functional tests using Nb integrated circuits demonstrated correct NOR operation with a bias margin of ±21%

    Fabrication of resistively-coupled single-electron device using an array of gold nanoparticles

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    We demonstrated one type of single-electron device that exhibited electrical characteristics similar to those of resistively-coupled SE transistor (R-SET) at 77 K and room temperature (287 K). Three Au electrodes on an oxidized Si chip served as drain, source, and gate electrodes were formed using electron-beam lithography and evaporation techniques. A narrow (70-nm-wide) gate electrode was patterned using thermal evaporation, whereas wide (800-nm-wide) drain and source electrodes were made using shadow evaporation. Subsequently, aqueous solution of citric acid and 15-nm-diameter gold nanoparticles (Au NPs) and toluene solution of 3-nm-diameter Au NPs chemisorbed via decanethiol were dropped on the chip to make the connections between the electrodes. Current–voltage characteristics between the drain and source electrodes exhibited Coulomb blockade (CB) at both 77 and 287 K. Dependence of the CB region on the gate voltage was similar to that of an R-SET. Simulation results of the model based on the scanning electron microscopy image of the device could reproduce the characteristics like the R-SET
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