We present a technology based on Nb/NbxSi1-x/Nb junctions, with barriers near the metal-insulator transition, for applications in superconducting electronics (SCE) as an alternative to Nb/AlOx/Nb tunnel junctions. Josephson junctions with co-sputtered amorphous Nb-Si barriers can be made with a wide variety of electrical properties: critical current density (Jc), capacitance (C), and normal resistance (Rn) can be reliably selected within wide ranges by choosing both the barrier thickness and Nb concentration. Nonhysteretic Nb/NbxSi1-x/Nb junctions with IcRn products greater than 1 mV, where Ic is the critical current, and Jc values near 100 kA/cm2 have been fabricated and are promising for superconductive digital electronics. These barriers have thicknesses of several nanometers; this improves fabrication reproducibility and junction uniformity, both of which are necessary for complex digital circuits. Recent improvements to our deposition system have allowed us to obtain better uniformity across the wafer.

We have developed the fabrication process, the circuit design technology, and the cryopackaging technology for high-Tc single flux quantum (SFQ) devices with the aim of application to an analog-to-digital (A/D) converter circuit for future wireless communication and a sampler system for high-speed measurements. Reproducibility of fabricating ramp-edge Josephson junctions with IcRn products above 1 mV at 40 K and small Ic spreads on a superconducting groundplane was much improved by employing smooth multilayer structures and optimizing the junction fabrication process. The separated base-electrode layout (SBL) method that suppresses the Jc spread for interface-modified junctions in circuits was developed. This method enabled low-frequency logic operations of various elementary SFQ circuits with relatively wide bias current margins and operation of a toggle-flip-flop (T-FF) above 200 GHz at 40 K. Operation of a 1:2 demultiplexer, one of main elements of a hybrid-type Σ-Δ A/D converter circuit, was also demonstrated. We developed a sampler system in which a sampler circuit with a potential bandwidth over 100 GHz was cooled by a compact stirling cooler, and waveform observation experiments confirmed the actual system bandwidth well over 50 GHz.

High-end telecommunication systems in the larger nationwide networks of the next decade will require routers having a packet switching throughput capacity of over 10 Tbps. In such future high-end routers, the packet switch, which is the biggest bottleneck of the router, will need higher processing speeds than semiconductor devices. We propose a high-end router system architecture using single flux quantum (SFQ) technology. This system consists of semiconductor line card units and an SFQ switch card unit. The features of this switch card architecture are (1) using internal speedup architecture to reduce effective loads in the network, (2) using a packet switch scheduler to attain non-blocking characteristics. This architecture can expand the switching capacity to a level greater than tens of Tbps scale, keeping with non-blocking characteristics.

We developed a compact rf receiver subsystem using a high-Tc superconducting sharp skirt band-pass filter with a center frequency tuning function. A 24-pole hairpin-type 2 GHz microstrip-line filter was fabricated with YBa2Cu3Oy thin films deposited on a LaAlO3 substrate. Attenuation characteristics were more than 30 dB at 1 MHz apart from both the lower and the higher pass-band edges. For center frequency tuning, a 1-mm-thick dielectric sapphire plate was stacked on the filter, and the filtering characteristics were tuned by moving the plate using a piezoelectric bending actuator. The range of the center frequency modulation was more than 12 MHz with no degradation of the low-loss and sharp-skirt characteristics.

Within the next few decades, high-end telecommunication systems on the larger nationwide network will require a switching capacity of over 5 Tbps. Advanced optical transmission technologies, such as wavelength division multiplexing (WDM) will support optical-fiber data transmission at such speeds. However, semiconductors may not be capable of high-throughput data switching because of the limitations by power consumption and operating speed, and pin count. Superconducting single flux quantum (SFQ) technology is a promising approach for overcoming these problems. This paper proposed an optical-electrical-SFQ hybrid switching system and a novel switch architecture. This architecture uses time-shifted internal speedup, shuffle and grouping exchange and a Batcher-Banyan switch. Our proposed switch consists of an interface circuit with small buffers, a Batcher sorter, a time-shift-speedup buffer (TSSB), a Banyan switch, and a slowdown buffer. Simulations showed good scalability up to 100 Tbps, which no router could ever offer such features.

A 90 GHz band planar-type superconducting mixer using Ba1-xKxBiO3 (BKBO) bicrystal junctions was fabricated on a MgO bicrystal substrate. The mixer is integrated with microwave circuits and two junctions, but we could not operate the mixer in image rejection mode because of process damage to the junction properties. However we confirmed the mixing operation; the intermediate frequency (IF) signal was observed up to 17K (LO87 GHz, RF92 GHz).

High temperature superconductors are eminently suitable for use in high frequency devices because of their large energy gap. We fabricated weak link Josephson junctions connected in series. The junctions were constructed of EuBa2Cu3O7-x (EBCO) superconducting thin films on bicrystal MgO substrates. We measured their microwave broadband detection (video detection) characteristics. The responsivity (Sr) of the junctions depended on the bias current and their normal state resistance. The array junctions were effective in increasing normal state resistance. We obtained a maximum Sr of 22.6 [V/W].

A new circuit design of Josephson ternary δ-gate composed of Josephson junction devices is presented. Mathematical theory for synthesizing, analyzing, and realizing any given function in ternary system using Josephson ternary δ-gate is introduced. The Josephson ternary δ-gate is realized using SQUID technique. Circuit simulation results using J-SPICE demonstrated the feasibility and the reliability operations of Josephson ternary δ-gate with very high performances for both speed and power consumption (max. propagation delay time44 ps and max. power consumption2.6µW). The Josephson ternary δ-gate forms a complete set (completeness) with the ternary constants (1, 0, 1). The number of SQUIDs that are needed to perform the operation of δ-gate is 6. Different design with less than 6 SQUIDs is not possible because it can not perform the operation of δ-gate. The advantages of Josephson ternary δ-gate compared with different Josephson logic circuits are as follows: The δ-gate has the property that a simple realization to any given ternary logic function as the building blocks can be achieved. The δ-gate has simple construction with small number of SQUIDs. The δ-gate can realize a large number of ternary functions with small number of input/output pins. The performances of δ-gate is very high, very low power consumption and ultra high speed switching operation.