We report on the fabrication and operation of all-NbN single flux quantum (SFQ) circuits with resistively shunted NbN/AlN/NbN tunnel junctions fabricated on silicon substrates. The critical current varied by about 5% in 400 NbN/AlN/NbN junction arrays, where the junction area was 88 µm2. Critical current densities of the NbN/AlN/NbN tunnel junctions showed exponential dependence on the deposition time of the AlN barrier. By using the 12-nm-thick Cu film as shunted resistors, non-hysteretic current-voltage characteristics were achieved. From dc-SQUID measurements, the sheet inductance of our NbN stripline was estimated to be around 1.2 pH at 4.2 K. We designed and fabricated circuits consisting of dc/SFQ converters, Josephson transmission lines, and T flip-flop-based SFQ/dc converters. The circuits demonstrated correct operation with a bias margin of more than 15% at 4.2 K.

This is a review of our high-Tc superconductor (HTS) sampler development. The design and experimental demonstration of a Josephson sampler circuit based on YBa2 Cu3Ox(YBCO)/PrBa2Cu3Ox/YBCO ramp-edge junctions is described. The sampler circuit contains five edge junctions with a stacked YBCO groundplane and is based on single-flux quantum (SFQ) operations. Computer simulation results show that the time resolution of the sampler circuit depends strongly on the IcRn product of the junction and can be reduced to a few picoseconds with realistic parameter values. The edge junctions were fabricated using an in-situ process in which a barrier and a counter-electrode layer are deposited immediately after the edge etching without breaking the vacuum. The in-situ process improved the critical current uniformity of the junctions to 1σ20% in twelve 4-µm-width junctions. An YBCO groundplane was placed on the junctions in a multilayer structure we call the HUG (HTS cricuit with an upper-layer groundplane) structure. The inductance of YBCO lines was reduced to 1 pH per square without junction-quality degradation in the HUG structure. SFQ current-pulse generation, SFQ storage, and SFQ readout in the circuit have been confirmed by function tests using 3-kHz pulse currents. The successful operation of the sampler circuit has been demonstrated by measuring a signal-current waveform at 50K.

In this review, we present recent advances relating to superconducting nanowire single-photon detectors (SSPDs or SNSPDs) and their broad range of applications. During a period exceeding ten years, the system performance of SSPDs has been drastically improved, and lately excellent detection efficiencies have been realized in practical systems for a wide range of target photon wavelengths. Owing to their advantages such as high system detection efficiency, low dark count rate, and excellent timing jitter, SSPDs have found application in various research fields such as quantum information, quantum optics, optical communication, and also in the life sciences. We summarize the photon detection principle and the current performance status of practical SSPD systems. In addition, we introduce application examples in which SSPDs have been applied.

To enable the use of passive transmission lines (PTLs) for the interconnection of single-flux-quantum (SFQ) circuits, we have implemented a driver and a receiver and have developed a method for designing SFQ circuits with passive interconnections. Basic components and properties of passive interconnections, such as the frequency characteristics of the driver and receiver, the PTL delay, and the crosstalk between PTLs, have been experimentally verified. Our developed components and design method have been applied to actual SFQ circuits, such as a 44 switch having block-to-block passive interconnections and a 22 switch having gate-to-gate passive interconnections. We have also shown the advantages of PTLs over Josephson transmission lines (JTLs). We also discuss the prospects of SFQ circuits having passive interconnections.

This paper describes the development of superconductor-insulator-superconductor (SIS) mixers for the Atacama Large Millimeter/submillimeter Array (ALMA) from the device point of view. During the construction phase of ALMA, the National Astronomical Observatory of Japan (NAOJ) successfully fabricated SIS mixers to meet the stringent ALMA noise temperature requirements of less than 230 K (5 times the quantum noise) for Band 10 (787-950 GHz) in collaboration with the National Institute of Information and Communications Technology. Band 10 covers the highest frequency band of ALMA and is recognized as the most difficult band in terms of superconducting technology. After the construction, the NAOJ began development studies for ALMA enhancement such as wideband and multibeam SIS mixers according to top-level science requirements, which are also presented.

We developed a Nb 4-layer process for fabricating superconducting integrated circuits that involves using caldera planarization to increase the flexibility and reliability of the fabrication process. We call this process the planarized high-speed standard process (PHSTP). Planarization enables us to flexibly adjust most of the Nb and SiO2 film thicknesses; we can select reduced film thicknesses to obtain larger mutual coupling depending on the application. It also reduces the risk of intra-layer shorts due to etching residues at the step-edge regions. We describe the detailed process flows of the planarization for the Josephson junction layer and the evaluation of devices fabricated with PHSTP. The results indicated no short defects or degradation in junction characteristics and good agreement between designed and measured inductances and resistances. We also developed single-flux-quantum (SFQ) and adiabatic quantum-flux-parametron (AQFP) logic cell libraries and tested circuits fabricated with PHSTP. We found that the designed circuits operated correctly. The SFQ shift-registers fabricated using PHSTP showed a high yield. Numerical simulation results indicate that the AQFP gates with increased mutual coupling by the planarized layer structure increase the maximum interconnect length between gates.

Superconducting nanowire single-photon detectors(SSPDs or SNSPDs) can detect single photons in a wide spectrum range from ultraviolet to mid-infrared wavelengths. We developed SSPDs for the light wavelength of 900-1100 nm, where it is difficult to achieve high detection efficiency by either Si or InGaAs avalanche photodiodes. We designed and fabricated the SSPD with non-periodic dielectric multilayers (DMLs) composed of SiO2 and TiO2 to enhance the optical absorptance in the wavelength range of 900-1100 nm. We measured the detection efficiency (DE) in the wavelength range of 800-1360 nm using a supercontinuum light source and found that the wavelength dependence of DE was in good agreement with the simulated spectrum of the optical absorptance of the nanowire device on the designed DML. The highest system DE was 81.0% for the wavelength of 980 nm.

We report the energy-efficient optical input interface using NbN superconducting nanowire-based optical-to-electrical (SN-OE) converters for a single-flux-quantum (SFQ) data processing system. The SN-OE converters with small active areas ranging from 1$, imes,$1 to 10$, imes,$10,$mu$m$^2$ were fabricated to improve the recovery time by reducing the kinetic inductance of the nanowire. The SN-OE with the smallest area of 1$, imes,$1 $mu$m$^2$ showed the recovery time of around 0.3 ns, while its detection efficiency for a single photon was reduced below 0.1% due to insufficient coupling efficiency with a single-mode optical fiber. However, the optical power dependence of the error rate of this device showed that the required optical power to achieve the error rate below $10^{-12}$ at 10 GHz operation is as large as 70 $mu$W, which is still one order of magnitude lower than semiconductor photo diodes. We also demonstrated the operation of the SN-OE converters combined with the SFQ readout circuit and confirmed the operating speed up to 77~MHz.

The adiabatic quantum flux parametron (AQFP) is an energy-efficient, high-speed superconducting logic device. To observe the tiny output currents from the AQFP in experiments, high-speed voltage drivers are indispensable. In the present study, we develop a compact voltage driver for AQFP logic based on a Josephson latching driver (JLD), which has been used as a high-speed driver for rapid single-flux-quantum (RSFQ) logic. In the JLD-based voltage driver, the signal currents of AQFP gates are converted into gap-voltage-level signals via an AQFP/RSFQ interface and a four-junction logic gate. Furthermore, this voltage driver includes only 15 Josephson junctions, which is much fewer than in the case for the previously designed driver based on dc superconducting quantum interference devices (60 junctions). In measurement, we successfully operate the JLD-based voltage driver up to 4 GHz. We also evaluate the bit error rate (BER) of the driver and find that the BER is 7.92×10-10 and 2.67×10-3 at 1GHz and 4GHz, respectively.