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Journal of Physics Conference Series

Thermally-fluctuated single-flux-quantum pulse intervals reflected in input-output characteristics of a double-flux-quantum amplifier

Yoshinao Mizugaki, Yoshiaki Urai, Hiroshi Shimada

Published Jul 1, 2017• 3 citations
Publisher

Abstract

A double-flux-quantum amplifier (DFQA) is a voltage multiplier of quantum accuracy, which we have employed at the final stage of a single-flux-quantum (SFQ) digital-to-analog converter (DAC). We recently found that experimental input–output (IO) characteristics of DFQAs were always slightly different from numerical results assuming ideally-periodic SFQ pulse trains. That is, experimental IO characteristics obtained using an over-biasing method were gradually deteriorated near their maximum operation voltages. Numerical simulation including the over-biasing method at a finite temperature suggested that the difference was likely to be attributed to thermally-fluctuated intervals of input SFQ pulses.

DOI
10.1088/1742-6596/871/1/012066

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Journal of Physics Conference Series•Jul 1, 2018•2 citations
A double-flux-quantum amplifier (DFQA) is a voltage multiplier of quantum accuracy, which we have employed at the final stage of a single-flux-quantum (SFQ) digital-to-analog converter (DAC). We recently found that experimental input–output (IO) characteristics of DFQAs were always slightly different from numerical results assuming ideally-periodic SFQ pulse trains. That is, experimental IO characteristics obtained using an over-biasing method were gradually deteriorated near their maximum operation voltages. Numerical simulation including the over-biasing method at a finite temperature suggested that the difference was likely to be attributed to thermally-fluctuated intervals of input SFQ pulses.
We evaluate the input–output characteristics of a 1000-fold voltage multiplier designed for single-flux-quantum (SFQ) digital-to-analog converters. The voltage multiplication is based on the double-flux-quantum amplifier (DFQA) that was originally proposed by Herr in 2005. We have designed and fabricated a 999-stage DFQA using a Nb/AlO x /Nb integration technology. In experiments, we have used two methods of feeding the input SFQ pulse train. The one is an overbiasing method, i.e., the input SFQ pulse train is generated at an overbiased input junction. A 1000-fold voltage multiplication with errors less than ±1% has been confirmed for input voltages of up to 27 µV, of which the corresponding Josephson frequency is 13 GHz. In the other method, a dc/SFQ converter is used for feeding the input SFQ pulse train. A 1000-fold voltage multiplication with an error of −0.03% has been obtained for an input frequency of 600 MHz, of which the corresponding Josephson voltage is 1.2 µV. In experiments, errors in measured voltages are likely to limit the accuracy in evaluation of the multiplication factor.
In addition to their fast switching with low power dissipation, precise voltage generation is a unique feature of single-flux-quantum (SFQ) circuits. This feature is attractive for realizing ac voltage standards consisting of SFQ-based digital-to-analog converters (DACs). Among several key components in SFQ-based DACs, a double-flux-quantum amplifier (DFQA) is a potential candidate to amplify the output voltage. In this paper, we present a redesigned DFQA without flux bias lines. We found a parameter set that provided wide bias margins and zero-flux-bias operation. Improved bias regions were numerically demonstrated, whereas a ±20-fold DFQA excluding flux biases was fabricated using Nb integration technology and tested in a liquid helium bath.
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.
Rapid-single-flux-quantum digital-to-analogue converters (RSFQ-DACs) are now under development for ac voltage standard applications. The voltage multiplier (VM), which precisely amplifies the input voltage, is a key component for RSFQ-DACs. Because the amount of bias current for a conventional VM increases in proportion to its multiplication factor, we have been looking for a VM device which is operated on a different principle. In this paper, we report our design of a 1000-fold VM comprising double flux quantum amplifiers (DFQAs) of which the amount of bias current is independent of its multiplication factor. Test circuits were fabricated using a 2.5 kA/cm2 Nb process. We confirm that the experimental results demonstrate the 1000-fold operation up to 13.2 GHz input SFQ pulse repetition frequency.
Single-flux-quantum (SFQ) circuits can generate highly accurate average voltage. We have developed SFQ digital-to-analog converters (DACs) for application of new ac voltage standards. In this paper, we present a bipolar voltage amplifier (BVA) based on a double flux quantum amplifier (DFQA) for generation of intrinsic bipolar voltage in an SFQ-DAC. The BVA realizes the identical multiplication factors for the positive and negative output, whereas the conventional DFQA has different multiplication factors in the bipolar output. The BVA can be used for the output stage of the SFQ- DAC. We have designed a 19-fold BVA and confirmed that the 19-fold BVA operated for input frequency of up to 31.2 GHz in positive voltage output and 33.6 GHz in negative voltage output.
A Double-Flux-Quantum (DFQ) amplifier can multiply its input voltage with quantum accuracy using DFQ generation at under-damped Josephson junctions. We conducted research on digital-to-analogue converters that were implemented using DFQ amplifiers for metrological applications. Conventional DFQ amplifiers were usually operated using two flux-biasing lines. In this study, we redesigned the circuit layout to realize operation using a single flux-biasing line. We designed a new layout of a 20-fold DFQ amplifier and fabricated it using a 2.5-kA/cm 2 Nb/AlO x /Nb process. The measurement was performed in liquid He. The Single-Flux-Quantum (SFQ) pulse train was fed using an over-biasing method, and the input-output characteristics are acquired using an oscilloscope. When the magnetic flux bias current I fb1 was not supplied, the maximum input voltage V inmax was 68 μV, which was 4.2 times larger than the value of 17 μV that was observed in our previous study. V inmax was enhanced to 120 μV when we supplied an I fb1 of 1.00 mA.