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We have developed a cell library for superconducting large scale integrated (LSI) digital circuits based on Single Flux Quantum (SFQ) devices. The circuits were designed for a 6-kA/cm2 Nb/AlOx/Nb junction process with 10 mask levels including one ground plane and two Nb wiring layers. The initial design and optimization of the circuit parameters were achieved by means of the optimization functions in a circuit simulator, PSCAN2, to ensure that important circuit parameters, Josephson critical current ( IC ), bias current ( Ibias ), and inductance satisfied minimal margin requirements. Critical margins of IC and Ibias were then further improved manually. To compensate the scattering in the circuit parameters from fabrication by design as much as possible, the critical junction parameters were optimized to further centralize the IC . The library cells were laid out following our design rules determined by systematic experiments on process control monitors (PCM). Basic cells including Josephson transmission lines, splitters, confluence buffers, D flip-flops, T flip-flops, and XOR and NDRO gates were designed, fabricated, and successfully tested at low frequencies. Wide overlaps of the operating regions for the common bias voltages were confirmed.

We have developed a 6-kA/cm² superconducting integrated circuit fabrication process toward large-scale single flux quantum digital circuits. To evaluate the process reliability and controllability, a set of process control monitors (PCMs) for monitoring fabrication and design parameters was designed and tested. Fabrication parameters, for example, film thicknesses and etched depths of the layers, were measured by testing specific patterns during fabrication. Design parameters such as the critical current density J c, normal-state resistance R n and actual sizes of Josephson junctions, the sheet resistance of Mo resistor R sh, and the penetration depth of Nb films were evaluated by electrically characterizing PCM circuits after fabrication. We also monitored superconducting critical currents for vias and Nb lines, and insulation properties between the metal layers systematically.

We have developed a fabrication technology for superconducting integrated circuits with Nb-based Josephson junctions. The standard fabrication process with 10 mask levels includes 3 Nb superconducting layers and a Mo resistor layer. The influence of deposition parameters on film stress, electrical properties, and surface roughness were studied systematically. High quality Nb, Al, Mo, and SiO2 films were successfully deposited for the subsequent fabrication of circuits. The circuit fabrication started with the fabrication of Mo resistors with a target sheet resistance R sh of 2 Ω, followed by the deposition of Nb/Al-AlO x /Nb trilayer Josephson-junction. The target critical current density J c was set at 6 kA/cm2. Small-scale circuits such as our standard library cells have been successfully fabricated and tested, confirming the capability of our fabrication technology for superconducting integrated circuits.

An on-chip high-frequency testing method for rapid single flux quantum circuits is proposed. This method employs test vectors based on pseudo-random sequences created by an on-chip high-frequency clock generator (CG) and a linear feedback shift register with parallel outputs (LSPO). Downsamplers (DSMs) at the outputs of a circuit under test (CUT) decimate the CUT output data so that high-frequency testing can be performed at sufficiently low data rates between the CUT and room-temperature electronics. The proposed testing can continuously test the circuit at high frequency with low-cost instruments. All of the critical subsystems, including LSPO, CG, and DSM, were designed, fabricated, and successfully tested. We validated proper operation at up to 40 GHz, confirming the concept and execution.

We report a vector-scanning SQUID microscope for probing a vector magnetic field. We designed and fabricated a vector [three-dimensional (3-D)] SQUID sensor with the aid of Nb multilayer fabrication technology. Our vector SQUID sensor on a single chip is composed of a vector sensor with three orthogonal pick-up coils, which catch the X , Y, and Z magnetic field components. We improved both the sensitivity and the spatial resolution not only by reducing the inner diameter but also by increasing the number of pick-up-coil windings within the framework of the multilayer technique. Our SQUID sensors operated properly, and the characteristics showed a good agreement with our design parameters. Since the sensor is suitable for our SQUID microscope, we are constructing our scanning microscope platform by using a Gifford-McMahon cryocooler for allowing long-time measurements. We also succeeded in connecting our vector SQUID sensor to a commercial flux-locked loop readout circuit (Magnicon Inc., Model XXF-1) to generate the voltage outputs, which are proportional to the XYZ components of the external magnetic field. We also use an XYZ piezo-driven scanner for the precise control of the pick-up-coil positions over the 3-D range of 5 × 5 × 5 mm 3 with a step size of 10 nm at 4 K.

We report on our progress in measuring the Boltzmann’s constant by Johnson noise thermometry (JNT) using an integrated quantum voltage noise source (IQVNS) that is fully implemented with superconducting integrated circuit technology. The IQVNS generates calculable pseudo white noise voltages to calibrate the JNT system. Thermal noise of a resistor placed at the temperature of the triple point of water was measured precisely by the IQVNS-based JNT. We accumulated more than 2000 data points to determine the Boltzmann’s constant. The experimentally obtained Boltzmann’s constant was $k_{\mathrm {exp}} = 1.3806493 \times 10^{-23}$ J/K with a relative statistical uncertainty of $8.6 \times 10^{-6}$ , which agreed well with the 2014 Committee on Data for Science and Technology value with a relative offset of $+ 0.6\times 10^{-6}$ .

We propose a prime factorizer operated in a framework of quantum annealing (QA). The idea is inverse operation of a multiplier implemented with QA-based Boolean logic circuits. We designed the QA machine on an application-specific-annealing-computing architecture which efficiently increases available hardware budgets at the cost of restricted functionality. The invertible operation of QA logic gates consisting of superconducting flux qubits was confirmed by circuit simulation with classical noise sources. The circuits were implemented and fabricated by using superconducting integrated circuit technologies with Nb/AlOx/Nb Josephson junctions. We also propose a 2.5Dimensional packaging scheme of a qubit-chip/interpose /package-substrate structure for realizing practically large-scale QA systems., Comment: 17 pages, 11 figures

We propose a novel system of a 3-D scanning superconducting quantum interference device (SQUID) microscope with high sensitivity for magnetic fields and high spatial resolution. Our 3-D scanning SQUID microscope consists of an XYZ piezodriven scanner for controlling the position of the pickup coils, three SQUID sensors, and the three-channel SQUID electronics at room temperature. Three SQUID sensors are fabricated in the same chip, and each SQUID has a pickup coil for XYZ directions, respectively. The SQUIDs used in our system are reproducibly fabricated using a Nb multilayer process of a fabrication center named CRAVITY. Pickup coils were built either in multiply winding numbers for enhancing sensitivity or in smaller inner diameters for improving spatial resolution. Three pickup coils with readout circuits are configured in orthogonal with each other and are acting as a vector sensor of the local magnetic field. At this moment, we succeeded to design some SQUIDs with 1-D and 3-D pickup coils. The critical current density J c of a Josephson junction (JJ) and the minimum critical current I√ of JJ are 320 A/cm 2 and 12.5 μA, respectively. The size of a chip is 2.9 mm × 2.9 mm, and the inner diameter of the pickup coil can be reduced down to 4 μm. The experimental I-V characteristic of fabricated SQUIDs was suitable for use in a scanning SQUID system. We have also fabricated a SQUID stage for installation in a cryostat for the experiment of 1-D SQUID sensors.

We designed and operated a 9-bit single-flux-quantum (SFQ) digital-to-analog converter (DAC). SFQ pulse-frequency modulation (PFM) was employed for generation of variable quantum output voltage, where a 9-bit variable pulse number multiplier and a 100-fold voltage multiplier were the key components. Test chips were fabricated using a Nb Josephson integration technology. Arbitrary voltage waveforms were synthesized with the maximum voltage of 2.54 mV. For ac voltage standard applications, relationships between the DAC resolution and the synthesized waveform frequency are discussed.

SFQ 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-μV PP bipolar triangle voltage waveform at the frequency of 35.7 Hz.

We propose a prime factoring machine operated in a frame work of quantum annealing (QA). The idea is inverse operation of a quantum-mechanically reversible multiplier implemented with QA-based Boolean logic circuits. We designed the QA machine on an application-specific-annealing-computing architecture which efficiently increases available hardware budgets at the cost of restricted functionality. The circuits are to be implemented and fabricated by using superconducting integrated circuit technology. We propose a three-dimensional packaging scheme of a qubit-chip / interposer / package-substrate structure for realizing practically-large scale QA systems., Comment: 3 pages, 6 figures, to appear in IEEE Xplore Conference Proceedings of the 16th International Superconductive Electronics Conference (ISEC 2017)

We have fabricated two- and three-dimensional nanostructured superconductors, and observed vortex penetrations by magneto-optical imaging. In the case of two-dimensional superconducting networks with square holes on a square lattice, anomalous diagonal penetrations are widely observed. Two kinds of diagonal vortex penetrations at high and low temperatures have been interpreted as originating from the repulsive interaction of vortices and sharp fan-shaped vortex penetration from the corners of the square holes, respectively. In the case of three-dimensional stack of superconducting strip arrays with double and triple layers, vortex avalanches have been observed in a wide temperature and dimension ranges due to enhanced demagnetization effect. While spotlike avalanches are observed when the overlap of strips is small, anomalous linear avalanches traversing many strips in different layers are observed when the overlap is large. In triple-layer strip arrays, in addition to the spotlike and linear avalanches, vortex penetrations along the line of strips are also observed. Origins of the anomalous diagonal penetration and vortex avalanches are discussed.

A digital-to-analogue converter (DAC) consisting of single-flux-quantum (SFQ) circuitry is known to generate accurate analogue voltages defined by the Josephson relationship. We have been developing SFQ-DACs of the pulse-frequency modulation (PFM) type. Toward voltage standard applications of SFQ-DACs, we have set the target values for the voltage amplitude and resolution at 20 mV pp and 10 bits, respectively. So far, we have reported a 5-bit, 10-μV pp quasi-sine voltage waveform generator comprising a PFM-type SFQ-DAC integrated with an on-chip digital code generator. Its small peak-to-peak voltage amplitude was due to the lack of an on-chip voltage multiplier (VM). In this paper, we present a 6-bit, 0.25-mV pp quasi-sine voltage waveform generator integrated with a 10-fold VM. The resolution is improved by introducing efficient logic sequences into the SFQ-DAC.

We mounted a vector-scanning superconducting quantum interference device (SQUID) sensor on a commercial SQUID microscope and successfully improved the sensitivity and spatial resolution of the sensor. Our proposed vector 3D SQUID sensor used multilayered Niobium (Nb)-based technology; we realized three SQUID sensors in the structure of a vector pickup coil system on a single chip. The vector pickup coil system was built with three pickup coils that were orthogonal to one another to obtain the alt;iagt;Xalt;/iagt;, alt;iagt;Yalt;/iagt;, and alt;iagt;Zalt;/iagt; components of a magnetic field vector. To improve both sensitivity and spatial resolution, we attempted to reduce the inner diameter by increasing the number of windings of the pickup coils using the multilayered Nb process. The design value for the dc SQUID sensors was either the critical current density alt;iagt;Jalt;/iagt;alt;subagt;calt;/subagt; = 320 A/cmalt;supagt;2alt;/supagt; for two Josephson junctions (JJs) or the critical current alt;iagt;Ialt;/iagt;alt;subagt;calt;/subagt; = 12.8 ?A for the 2 ?m × 2 ?m JJs. To measure the current?voltage (alt;iagt;I?Valt;/iagt;) and voltage?flux (alt;iagt;Valt;/iagt;??) characteristics of a sensor, we constructed a homemade measurement system. The fundamental characteristics of our SQUID sensors were in good agreement with the design parameters. We mounted our vector SQUID sensor on a commercial scanning SQUID microscope (SQM-2000, Seiko Instrument Inc.) with a single flux-locked loop (FLL) channel to test one of the three channels. We repeated the measurements three times to obtain the alt;iagt;Xalt;/iagt;, alt;iagt;Yalt;/iagt;, and alt;iagt;Zalt;/iagt; componential images of the magnetic field from a single vortex, and we synthesized the 3D mapping of the magnetic field vectors from a single vortex. We proved that our sensor mounted on a SQUID microscope was suitable for measuring the alt;iagt;Xalt;/iagt;, alt;iagt;Yalt;/iagt;, and alt;iagt;Zalt;/iagt; components of a vector magnetic field.

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/cm 2 Nb process. We confirm that the experimental results demonstrate the 1000-fold operation up to 13.2 GHz input SFQ pulse repetition frequency.

We have performed measurement of the Boltzmann constant k by Johnson noise thermometry (JNT) using an integrated quantum voltage noise source (IQVNS). IQVNS implemented on a single chip generates calculable pseudo white noise voltages. We have accumulated numerous data of the measurements at the temperature of the triple point of water. At the moment the obtained values of k shows significant difference from the 2014 CODATA value of the order of a few parts in 105.

For establishing a new generation of ac voltage standards, digital-to-analog converters (DACs) based on rapid single flux quantum (RSFQ) technology, which enable synthesis of arbitrary waveforms with fundamental accuracy, are developed. An RSFQ-DAC consists of three main subsystems: a pulse-number multiplier (PNM), a pulse distributor (PD) and voltage multipliers (VMs). Conventional RSFQ-DACs generate arbitrary voltages by switching a binary array of the VM cells. In this paper, we propose a Variable-PNM for a new RSFQ-DAC based on frequency modulation. The output voltage is determined by the multiplication factor m of the Variable-PNM that consists of a ring oscillator, an n -bit counter and a decoder. We designed a 2-bit Variable-PNM using the CONNECT cell library. The circuit was fabricated using the ISTEC Nb standard process (STP2). We have confirmed the correct functionalities for 2-, 4-, 6- and 8-fold multiplication in functional testing at low speed.

A cryogenic current comparator (CCC) is an instrument of great importance in electrical metrology, which provides a ratio of two currents with ultimate accuracy based on superconductivity. It is used for dc resistance calibration in many national metrology institutes. Conventional CCCs consist of multi-turn coils of wire windings, a multi-layered shield of superconductor foils and a separate SQUID sensor. This implementation results in a bulky device too massive to cool with a mechanical cryocooler, making a system inconvenient. A new implementation of CCC, an integrated CCC (ICCC) consisting of thin-film spiral coils, a thin-film superconducting shield and a SQUID sensor integrated on a single chip, enables a user-friendly system operated with a compact cryocooler. Prototype ICCC chips were designed and fabricated by using a superconducting Nb integrated circuit technology with chemical-mechanical polishing. The basic operation of the prototype ICCCs was confirmed by monitoring periodic flux-voltage characteristics of the SQUIDs.

The design and operation of a 9 bit superconductive single-flux-quantum (SFQ) digital-to-analogue converter (DAC) are presented. The DAC comprises a 9 bit variable SFQ pulse number multiplier and a 100-fold voltage multiplier. An SFQ pulse-frequency modulation technique is employed for coding the analogue output voltage. The DAC is fabricated using an Nb Josephson integration technology. Its output voltage waveform, the amplitude of which is up to 2.54 mV pp, is controlled by digital input signals from a room-temperature data generator.

We have demonstrated that a superconductor-insulator-superconductor (SIS) mixer is applicable for a power detector to evaluate the sideband noise radiated from a submillimeter-wave local oscillator (LO) under development. A simple analysis shows SIS mixers provide better resolution of LO noise than Schottky barrier diode mixers when the available LO power is less than ~ 0.4 mW. Systematic measurement of dependence of receiver noise temperature T RX on components and operational conditions of a photonic LO (Ph-LO) is carried out, which provides a guide to their optimization, e.g., source-laser power, photodiode drive current, use of an optical filter, and selection of an optical amplifier. Noise temperature of the Ph-LO estimated from experimental results ranges from ~ 1 times 103 K to ~ 5 times 104 K as a function of operational conditions and parameters.

We observed constant voltage steps for an overdamped Josephson array driven by current pulse trains generated by triggering a photo detector located in a cryostat with a mode locked fiber laser. We measured the voltages of the Josephson array as a function of an optical power. Quantized voltage steps were verified at multiples of about 10 μV which corresponds to the repetition frequency of 50 MHz and the number of junctions of 100. A quantitative analysis was done on the current pulse shape which generated by the MSM detector.

Rapid single flux quantum digital-to-analog (D/A) converters which synthesize arbitrary waveforms with metrological accuracy are under development. We propose a 1-bit RSFQ D/A converter which is expected to operate at higher sampling frequencies than the multi-bit converter and is suitable for multi-chip operation to achieve the output voltages exceeding 100 mV. Calculations of the noise power and the attenuation of the signal suggested that the rms error in a 10 kHz sine wave synthesized by the 1-bit converter with a third-order low-pass filter can be smaller than 10 −7 at the sampling frequency of 100 MHz. A prototype 1-bit D/A converter was fabricated and the generation of dc voltages was confirmed as expected.

We developed a multi-chip module (MCM) system of superconducting quantum bits with single-flux quantum (SFQ) interface circuits. The MCM system consists of chips with quantum bits and chips with SFQ circuits mounted on a base chip by flip-chip bonding (FCB) with superconducting solder. We report on a reliability test of the MCM system. The test was done using a resistance measurement of an MCM with 30 mum diameter bumps. Tested samples showed good durability against thermal cycles between room temperature and 4.2 K. At 4.2 K, the resistance measured through more than two hundred bumps in a series was less than 1 mOmega. Along with achieving a high reliability, reducing thermal interference from SFQ circuits to quantum bits is important. We must understand the thermal conductance between and within the chips to design a system with minimum interference. An MCM on-chip thermometer which is based on the Johnson noise thermometry, has been designed and fabricated to achieve this desired level. The preliminary results of the thermal measurement are reported.

A SIS mixer with RF bandwidth of 1 octave is designed, fabricated, and characterized. The mixer chip consists of 8 Nb/AlOx/Nb junctions connected in parallel through Nb/SiO2/Nb microstrip lines (MSL), a twin-slot antenna, and a 3-stage MSL transformer. Experimentally, the 2.5 dB RF band is found to be 230-440 GHz, which agrees quantitatively with theoretical calculation. Receiver noise temperature Trx is measured as 260-680 K in the band. After correction for IF-amplifler noise and beam-splitter reflectivity, the noise is 170-320 K, i.e., 13-20 times of the quantum noise. The sensitivity flatness in the band can be improved to 1.9 dB by further optimization of the transformer.

A 10-kA/cm2 Nb/AlOchi/Nb junction technology for rapid single flux quantum (RSFQ) integrated circuits has been developed. Modifying our standard 1.6-kA/cm2 cell library, we have implemented elementary cells used to build RSFQ digital-to-analog converters for ac voltage standard applications. The 10-kA/cm2 library cells have been fabricated by utilizing existing process tools. Correct operations of the circuits have been confirmed at low speed. The maximum operating frequencies evaluated by an average-voltage measurement technique were consistent with the scaling rules of RSFQ circuits.

We report the experimental results on a Josephson junction array (JJA) driven by current pulse trains generated by triggering a photo detector located in a cryostat with an optical comb. The key components in the experimental system are a mode locked fiber laser, a metal-semiconductor-metal (MSM) photoconductive switch, and an overdamped JJA. Optical pulse trains generated at room temperature are transmitted via an optical fiber to the MSM-photo detector installed at 4.2 K, and then converted into electric current pulse trains to drive the overdamped JJA. Quantized voltage steps have been verified at multiples of 10 muV, which agrees with the repetition rate of the pulse laser and the number of Josephson junctions in the JJA.

In this paper, quantized voltage steps were observed by driving an overdamped Josephson junction array (JJA) with current pulse trains. The current pulse trains were generated by triggering a metal-semiconductor-metal photodetector installed close to JJA at 4.2 K with a mode-locked fiber laser. The intervals of the constant voltage steps were 10 muV, which agrees with the repetition frequency of the light source (49.7 MHz) and the number of Josephson junctions (100) in the JJA. This is the first step toward establishing an ac voltage standard at the National Institute of Advanced Industrial Science and Technology

We designed a vector sensor for sensing magnetic fields by using a 3-dimensional (3D) scanning superconducting quantum interference device (SQUID) microscope. A vector senor consists of three SQUID sensors, which are fabricated on the same chip and each SQUID sensor has a pick-up coils for X, Y, Z directions. Three coils with readout circuits are configured in orthogonal with each other to measure the magnetic field vector. We succeeded in fabricating some SQUIDs sensors with three pick-up coils. The critical current density Jc of a Josephson junction (JJ) is 320 A/cm² and the minimum critical current Ic of JJ becomes 12.5 µA. We have also designed a prototype model of the instrument. We use an XYZ piezo-driven scanner for controlling the position of the pick-up coils in the range of 5 mm x 5 mm at low temperatures. Two heaters are installed for controlling the temperatures of a sensor and a sample. The room-temperature electronics can control three-channel SQUIDs simultaneously, and has the bandwidth of 6 MHz.

We present fabrication and operation of a 1024-stage voltage multiplier (VM) that is the key subsystem of a rapid single flux quantum digital-to-analog converter for ac voltage standard applications. Our fabrication process has been improved to reduce inter- and intralayer short probabilities, which was a key to developing large-scale circuits. A 1024-stage VM including over 5000 junctions was fabricated using the improved process. Correct operation was confirmed at the input frequencies up to 34 GHz which corresponded to the maximum output voltage of 72 mV.

We propose a simple method of measuring bit error rates (BERs) for rapid single flux quantum (RSFQ) circuits operating at high frequencies. An error detector based on a SQUID current comparator, which operates without a timing clock, is employed to simplify the test system and measurements. We successfully investigated high-frequency operation of RSFQ cells that were utilized to implement our RSFQ digital-to-analogue converter for metrology applications. Sufficiently low BERs were demonstrated in the circuit operation at several tens of gigahertz.

The pulse-driven Josephson junction array is a promising candidate for an accurate waveform synthesizer. The accuracy of the synthesized waveform is chiefly limited by the in-band frequency components of the driving current because they couple to the output signal through parasitic circuit elements and are difficult to filter out. In this study, we propose a more accurate waveform synthesis method, in which the array is driven by current pulses which have the same waveform as the impulse response of the ideal bandpass filter. Simulations have shown that the current amplitude of the constant-voltage step of a junction driven by the filtered pulses exceeds 70% of the Josephson critical current in a wide range of output signals. Synthesis of a sine wave using this method was also demonstrated by simulation.

A digital-to-analog (D/A) converter based on rapid single flux quantum (RSFQ) circuits is a potential device for establishing a new generation of ac voltage standards. We are developing RSFQ D/A converters that can generate an ac waveform with an accurately defined rms value. Our goal in the near future is the synthesis of an ac waveform with a 100-mV peak voltage that will allow accurate characterization of the thermal voltage converters currently used for setting ac–dc transfer standards. We have designed, fabricated and successfully tested key subsystems of the RSFQ D/A converter including a 10-stage pulse-number multiplier, a 1-bit slice of a pulse distributor and a 9.5-bit voltage multiplier.

The voltage multiplier (VM) is a principal component in high-precision digital-to-analog (D/A) converters based on rapid single flux quantum (RSFQ) circuits. Experimental confirmation of correct operation of the VM is important to guarantee the accuracy of the D/A converter. In this study, a circuit for direct comparison of the output voltages of VMs was designed and demonstrated. An on-chip SQUID sensor was utilized to detect the VM errors. Bit error rates of a single-stage VM were successfully measured down to 10/sup -12/ in the constant-voltage regions. A method of detecting errors during transients was also proposed and carried out.

A low-noise rf amplifier based on a dc SQUID (SQA) is developed and tested within frequency range 3.0-4.5 GHz. The 4-channel chips are designed using proven balanced output configuration; they employ special damping filters. Each channel is tested separately demonstrating the following parameters at 3.95 GHz central frequency: gain (9.5 /spl plusmn/ 1.0) dB, 3-dB bandwidth 300 MHz and noise temperature (1.0 /spl plusmn/ 0.5) K. The 1-dB gain compression estimate is 50 K*GHz for bias voltage 80 /spl mu/V and characteristic voltage 285 /spl mu/V.

A pulse-number multiplier (PNM) is a principal subsystem of our rapid single flux quantum digital-to-analog converter for metrology applications. The PNM circuit generates a high-speed pulse train from an input of a precise low-frequency clock signal. We designed, fabricated, and successfully tested a PNM test circuit. The PNM test circuit was implemented by using our standard cell library on 1.6 kA/cm/sup 2/ Nb trilayer technology. Fully functional operation was confirmed by low-speed testing. Experimental bias margins were wider than /spl plusmn/20%, and these results agreed with a simulation. We also tested the circuit by average-voltage measurements for estimating the frequencies of output pulse trains. Results of average-voltage measurements implied that the output frequency of the PNM circuit was tunable from 5 GHz to 17 GHz. The bit error rate (BER) of the PNM circuit was measured, and extrapolation of the error-function fits suggested a sufficiently small BER at the optimum bias point.

Rapid single flux quantum (RSFQ) digital-to-analog (D/A) converters that synthesize arbitrary waveforms with metrological accuracy are under development. In the D/A converter, a magnetically coupled voltage multiplier (VM) is utilized as a precise frequency-to-voltage converter that generates a voltage proportional to the input frequency. To increase the output voltage of the VM, parameters that determine the cycle time of the VM cell were optimized. Experimental results obtained with 16-stage VMs showed that the maximum operating frequency of the new VM was 50 GHz, which is twice that of the previous type.

We present the development of a rapid single flux quantum (RSFQ) standard cell library for middle-scale applications. The library includes elementary cells frequently used to build large circuit blocks. The standard cells were designed for a simple process with 8 mask levels. The circuits were optimized and laid out to minimize effects of high parasitic inductance associated with a single ground plane layer. Circuit parameters on the wafer were determined by electrical measurements on parametric test circuits, which gave feedback on the design and process. All cells were individually fabricated and tested, and the design and layout were confirmed.

We investigated the effects of magnetic fields induced by bias currents on rapid single flux quantum (RSFQ) circuits. We measured the mutual inductance between a signal line and a bias line in the circuit. The mutual inductance values obtained were in the range of 5–50 fH. We simulated operation of a 2-stage shift register under the magnetic fields induced by an additional control current. The calculated operating regions were significantly reduced by the magnetic fields induced by the control currents on the order of 10 mA. The experimental results obtained for the shift register agreed with the simulation. These results suggest that it is important to take the effects of current-induced magnetic fields into account in the design of large-scale RSFQ circuits.

We present design and experimental results of a voltage multiplier (VM) that is a main component of a Josephson D/A converter based on single flux quantum pulses. We have analyzed the VM operation through detailed simulations and have re-designed the circuit. The amplitude of the constant-voltage step for a 64-stage VM with the new design is 80 /spl mu/A at 10 GHz of an input frequency, which is sufficiently large for D/A converter applications.

A demultiplexer (DMUX) is a key subsystem of rapid single flux quantum (RSFQ) circuits and systems in practical applications. High-speed data from RSFQ circuits should be converted into sufficiently low-speed ones for transmission to and processing by room temperature electronics. We designed, fabricated and successfully tested an RSFQ DMUX based on the synchronous shift-and-dump architecture whose advantages are modularity and compactness. A 1-to-8 DMUX was implemented using our standard cell library on 1.6 kA cm−2 Nb trilayer technology. Fully functional operation was confirmed by low speed testing. Experimental bias margins were as large as ±16%. Results of average voltage measurements implied that the DMUX was operated at data rates up to 25 Gb s−1.

We are developing a digital-to-analog (D/A) converter based on rapid single flux quantum (RSFQ) logic circuits. Since the RSFQ D/A converter operates according to the Josephson frequency–voltage relationship, arbitrary waveforms can be synthesized with an accuracy equivalent to that of the Josephson dc voltage standard. We have examined the characteristics of magnetically coupled voltage multipliers (VMs). The VM is a key component in the RSFQ D/A converter for use in metrological applications.

A conventional voltage multiplier cell in a rapid-single-flux-quantum (RSFQ) digital-to-analog converter is composed of an output dc-superconducting quantum interference device (dc-SQUID) magnetically coupled to an input Josephson transmission line. The authors previously demonstrated the operation of a voltage doubler cell that doubled the input voltage by introducing the second-mode operation. In this paper, we propose another type of a voltage multiplier cell, a voltage quadrupler, where the output dc-SQUID is replaced with two dc-SQUIDs sharing one coupling inductor. The proposed cell quadruples the input voltage when each SQUID is operated in the second-mode. A single voltage quadrupler has been designed and fabricated using the ISTEC standard process (STP2). Although operation margins for the bias current and the input SFQ repetition frequency are smaller than the values expected from simulation, its fundamental operation has been confirmed in experiment.

We are developing an analog-to-digital converter (ADC) based on rapid single-flux quantum technology for radio-astronomic spectroscopy applications where very wide bandwidth is required and relatively small resolution is sufficient. The ADC consists of periodic-flash comparators, a demultiplexer and output drivers; and is sufficiently simple to be implemented on a single chip using a conventional 2?3 ?m Nb trilayer technology. A 2-bit ADC has been designed for operation at 16 GHz of the sampling frequency. All the subsystems of the ADC have been fabricated using standard 1.6 kA cm?2 Nb/AlOx/Nb technology and have been successfully tested.

We report experimental results on chip-to-chip transfer of single-flux-quantum (SFQ) pulses using an active multichip module (MCM). Josephson transmitters and receivers are integrated on both a chip and an MCM substrate. An MCM consisting of a 4.8-mm chip and an 8.2-mm MCM substrate was fabricated using a 1.6-kA/cm/sup 2/ Nb-trilayer process and a solder-bumped flip-chip bonding technology. The correct operation of the circuit was confirmed by low-speed testing. Bit-error rate (BER) of the circuit was measured down to 10/sup -5/, and extrapolation of the error-function fits suggested a very small BER, lower than 10/sup -200/, at the optimum bias point. Experimental margins on the bias voltage were as large as /spl plusmn/34%.

We present two versions of self-timed pipelined parallel carry-look-ahead adders. The adders are designed based on delay-insensitive (DI) rapid single-flux-quantum (RSFQ) primitives. Basic binary gates employ dual-rail encoded data, which include timing information in themselves. One version uses wave pipelining and the other delay-insensitive pipelining with a request-acknowledge data transfer protocol. We show simulation results of 4 to 32-bit adders and their sensitivity to delay variations. Two design schemes are compared in terms of area, speed, robustness, interface and design process for large systems.