We present a digital double relaxation oscillation SQUID (DROS) with a digital flux-locked-loop (FLL) circuit consisting of an up/down counter and a digital-to-analog (D/A) converter. The up/down counter was designed using 4 jucntion logic (4JL) gates operated with a 2-phase power system. The D/A converter was designed using an R-2R ladder-type D/A converter. We simulated the dynamic behavior of the digital DROS with a digital FLL circuit combined with the 5-bit ripple up/down counter and the D/A converter. Simulation results show correct flux-locked behavior and a high slew rate of 107Φ0/s for the digital DROS.

The superconducting bridges with constrictions as small as 0.5 µm were fabricated by high-quality Ba2YCu3Ox thin films on silicon with buffer layers. The observed I-V characteristics of this bridge could be successfully analyzed by the modified form of Aslamazov and Larkin theory which shows that our microbridge junction is a homogeneous wide superconducting bridge and the characteristics of this bridge are not dependent on the defects such as grain boundaries of the film.

We have designed a 44 Banyan switch using SFQ logic circuits. The switch is composed of three parts; one is an input buffer, the second is a contention solver which checks packet contention in a distribution network, and the third is a packet distribution network which distributes contention-free packets to their destination address. The packet distribution network is composed of Batcher-Banyan switch with the input buffer. The contention solver decides to send a data packet to the distribution network, using only internal routing tags which are added to packets in the switch. As the circuit is composed of two parts, the contention solver and the packet distribution network, the transfer rate is raised because it doesn't need to wait any more while a data packet passes through the distribution network. Simulation results using JSIM show that the switch circuit can operate at a clock frequency of 40 GHz.

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.

We propose a universal NAND logic gate based on single flux quantum (SFQ) logic. The NAND gate enables the construction of any logic circuits. In the proposed gate, three superconducting loops share two Josephson junctions (JJs). The critical currents of the JJs were designed to allow each of any two loops to trap an SFQ at the same time. We simulated dynamic operation of this NAND gate. The results show that the NAND gate can operate with a delay time of 45 ps, and the power consumption of this circuit is close to 0.06 µW/gate.

A normal-distribution-function-shaped superconducting tunnel junction (NDF-STJ) which consists of Nb/Al-AlOx/Al/Nb has been fabricated as an X-ray detector. Current - voltage characteristics were measured at 0.4 K using three kinds of STJs, which have the dispersion parameters σ of 0.25, 0.45 and 0.75. These STJs showed very low subgap leakage current of about 5 nA. By irradiating with 5.9 keV X-rays, we obtained the spectrum of these NDF-STJs. They showed good energy resolution with small magnetic fields of below 3 mT, which is about one-tenth of those for conventional-shaped STJs.

A lumped element kinetic inductance detector (LeKID) relying on a superconducting resonator is a promising candidate for sensing high energy particles such as neutrinos, X-rays, gamma-rays, alpha particles, and the particles found in the dark matter owing to its large-format capability and high sensitivity. To develop a high energy camera, we formulated design rules based on the experimental results from niobium (Nb)-based LeKIDs at 1 K irradiated with alpha-particles of 5.49 MeV. We defined the design rules using the electromagnetic simulations for minimizing the crosstalk. The neighboring pixels were fixed at 150 µm with a frequency separation of 250 MHz from each other to reduce the crosstalk signal as low as the amplifier-limited noise level. We examined the characteristics of the Nb-based resonators, where the signal decay time was controlled in the range of 0.5-50 µs by changing the designed quality factor of the detectors. The amplifier noise was observed to restrict the performance of our device, as expected. We improved the energy resolution by reducing the filling factor of inductor lines. The best energy resolution of 26 for the alpha particle of 5.49 MeV was observed in our device.