Our group has developed terahertz(THz)-waves emitting devices utilizing single crystals of high temperature superconductor Bi2Sr2CaCu2O8+δ (Bi2212). The working principle of the device is based on the AC Josephson effect which is originated in the intrinsic Josephson junctions (IJJs) constructed in Bi2212 single crystals. In principle, based on the superconducting gap of the compound and the AC Josephson effect, the emission frequency range from 0.1 to 15 THz can be generated by simply adjusting bias voltages to the IJJs. In order to improve the device performances, we have performed continuous improvement to the device structures. In this paper, we present our recent approaches to high performance Bi2212 THz-waves emitters. Firstly, approaches to the reduction of self Joule heating of the devices is described. In virtue of improved device structures using Bi2212 crystal chips, the device characteristics, such as the radiation frequency and the output power, become better than previous structures. Secondly, developments of THz-waves emitting devices using IJJs-mesas coupled with external structures are explained. The results clearly indicate that the external structures are very useful not only to obtain desired radiation frequencies higher than 1 THz but also to control radiation frequency characteristics. Finally, approaches to further understanding of the spontaneous synchronization of IJJs is presented. The device characteristics obtained through the approaches would play important roles in future developments of THz-waves emitting devices by use of Bi2212 single crystals.

Studies on intrinsic Josephson junctions (IJJs) of cuprate superconductors are reviewed. A system consisting of a few IJJs provides phenomena to test the Josephson phase dynamics and its interaction between adjacent IJJs within a nanometer scale, which is unique to cuprate superconductors. Quasiparticle density of states, which provides direct information on the Cooper-pair formation, is also revealed in the system. In contrast, Josephson plasma emission, which is an electromagnetic wave radiation in the sub-terahertz frequency range from an IJJ stack, arises from the synchronous phase dynamics of hundreds of IJJs coupled globally. This review summarizes a wide range of physical phenomena in IJJ systems having capacitive and inductive couplings with different nanometer and micrometer length scales, respectively.

Direct-current superconducting quantum interference device (dc-SQUID) based on intrinsic Josephson junction (IJJ) has been fabricated using Bi2Sr2CaCu2O8+δ (Bi-2212) films grown on MgO substrates with surface steps. The superconducting loop parallel to the film surface across the step edge contains two IJJ stacks along the edge. The number of crystallographically stacked IJJ for each SQUIDs were 40, 18 and 3. Those IJJ SQUIDs except for one with 40 stacked IJJs revealed clear periodic modulation of the critical current for the flux quanta through the loops. It is anticipated that phase locking of IJJ has an effect on the modulation depth of the IJJ dc-SQUID.

Intrinsic Josephson junctions (IJJs) in the high-Tc cuprate superconductors have several fascinating properties, which are superior to the usual Josephson junctions obtained from conventional superconductors with low Tc, as follows; (1) a very thin thickness of the superconducting layers, (2) a strong interaction between junctions since neighboring junctions are closely connected in an atomic scale, (3) a clean interface between the superconducting and insulating layers, realized in a single crystal with few disorders. These unique properties of IJJs can enlarge the applicable areas of the superconducting qubits, not only the increase of qubit-operation temperature but the novel application of qubits including the macroscopic quantum states with internal degree of freedom. I present a comprehensive review of the phase dynamics in current-biased IJJs and argue the challenges of superconducting qubits utilizing IJJs.

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.

This review provides a current overview of the fabrication processes for superconducting digital circuits at CRAVITY (clean room for analog and digital superconductivity) at the National Institute of Advanced Industrial Science and Technology (AIST), Japan. CRAVITY routinely fabricates superconducting digital circuits using three types of fabrication processes and supplies several thousand chips to its collaborators each year. Researchers at CRAVITY have focused on improving the controllability and uniformity of device parameters and the reliability, which means reducing defects. These three aspects are important for the correct operation of large-scale digital circuits. The current technologies used at CRAVITY permit ±10% controllability over the critical current density (Jc) of Josephson junctions (JJs) with respect to the design values, while the critical current (Ic) uniformity is within 1σ=2% for JJs with areas exceeding 1.0 µm2 and the defect density is on the order of one defect for every 100,000 JJs.

We have investigated the influence of the ferromagnet magnetization on the transport properties of intrinsic Josephson junctions in Co/Au/BSCCO/Au/Co hybrid structure under applied magnetic fields. The current-voltage characteristic at 77K in a zero-field showed the multiple quasiparticle branches with hysteresis similar to that of conventional intrinsic Josephson junctions. On the other hand, it was observed that the critical current shows a clear asymmetric field dependence with respect to the direction of the field sweep, resulting in hysteretic behavior. By comparing the field dependence of critical current with magnetization curve of the sample, we found that the critical current is strongly suppressed in the antiparallel configuration of the relative magnetization orientation of two Co layers due to the accumulation of spin-polarized quasiparticles in intrinsic Josephson junctions. The observed suppression of the critical current is as large as more than 20%.

The physics and applications of superconducting phase shifts and their control in superconducting systems are reviewed herein. The operation principle of almost all superconducting devices is related to the superconducting phase, and an efficient control of the phase is crucial for improving the performance and scalability. Furthermore, employing new methods to shift or control the phase may lead to the development of novel superconducting device applications, such as cryogenic memory and quantum computing devices. Recently, as a result of the progress in nanofabrication techniques, superconducting phase shifts utilizing π states have been realized. In this review, following a discussion of the basic physics of phase propagation and shifts in hybrid superconducting structures, interesting phenomena and device applications in phase-shifted superconducting systems are presented. In addition, various possibilities for developing electrically and magnetically controllable 0 and π junctions are presented; these possibilities are expected to be useful for future devices.

We present a concept of an advanced rapid single-flux-quantum (RSFQ) logic circuit family using the combination of 0-shifted and π-shifted Josephson junctions. A π-shift in the current-phase relationship can be obtained in several types of Josephson junctions, such as Josephson junctions containing a ferromagnet barrier layer, depending on its thickness and temperature. We use a superconducting quantum interference devices composed of a pair of 0- and π-shifted Josephson junctions (0-π SQUIDs) as a basic circuit element. Unlike the conventional RSFQ logic, bistability is obtained by spontaneous circular currents without using a large superconductor loop, and the state can be flipped by smaller driving currents. These features lead to energy- and/or space-efficient logic gates. In this paper, we show several example circuits where we represent signals by flips of the states of a 0-π SQUID. We obtained successful operation of the circuits from numerical simulation.

CMOS microprocessors are limited in their capacity for clock speed improvement because of increasing computing power, i.e., they face a power-wall problem. Single-flux-quantum (SFQ) circuits offer a solution with their ultra-fast-speed and ultra-low-power natures. This paper introduces our contributions towards ultra-high-speed cryogenic SFQ computing. The first step is to design SFQ microprocessors. From qualitatively and quantitatively evaluating past-designed SFQ microprocessors, we have found that revisiting the architecture of SFQ microprocessors and on-chip caches is the first critical challenge. On the basis of cross-layer discussions and analysis, we came to the conclusion that a bit-parallel gate-level pipeline architecture is the best solution for SFQ designs. This paper summarizes our current research results targeting SFQ microprocessors and on-chip cache architectures.

We present our design and operation of a 6-bit quasi-triangle voltage waveform generator comprising three circuit blocks; an improved variable Pulse Number Multiplier (variable-PNM), a Code Generator (CG), and a Double-Flux-Quantum Amplifier (DFQA). They are integrated into a single chip using a niobium Josephson junction technology. While the multiplication factor of our previous m-bit variable-PNM was limited between 2m-1 and 2m, that of the improved one is extended between 1 and 2m. Correct operations of the 6-bit variable-PNM are confirmed in low-speed testing with respect to the codes from the CG, whereas generation of a 6-bit, 0.20mVpp quasi-triangle voltage waveform is demonstrated with the 10-fold DFQA in high-speed testing.

We describe the recent progress on a Nb nine-layer fabrication process for large-scale single flux quantum (SFQ) circuits. A device fabricated in this process is composed of an active layer including Josephson junctions (JJ) at the top, passive transmission line (PTL) layers in the middle, and a DC power layer at the bottom. We describe the process conditions and the fabrication equipment. We use both diagnostic chips and shift register (SR) chips to improve the fabrication process. The diagnostic chip was designed to evaluate the characteristics of basic elements such as junctions, contacts, resisters, and wiring, in addition to their defect evaluations. The SR chip was designed to evaluate defects depending on the size of the SFQ circuits. The results of a long-term evaluation of the diagnostic and SR chips showed that there was fairly good correlation between the defects of the diagnostic chips and yields of the SRs. We could obtain a yield of 100% for SRs including 70,000JJs. These results show that considerable progress has been made in reducing the number of defects and improving reliability.

We outline a number of high temperature superconducting Josephson junction-based devices including superconducting quantum interference devices (SQUIDs) developed for a wide range of applications including geophysical exploration, magnetic anomaly detection, terahertz (THz) imaging and microwave communications. All these devices are based on our patented technology for fabricating YBCO step-edge junction on MgO substrates. A key feature to the successful application of devices based on this technology is good stability, long term reliability, low noise and inherent flexibility of locating junctions anywhere on a substrate.

Superconducting integrated circuits should be operated at low temperature below a half of their critical temperatures. Thermal heat from a bias resistor could rise the temperature in Josephson junctions, and would reduce their critical currents. In this study, we estimate the temperature in a Josephson junction heated by a bias resistor at the bath temperature of 4.2 K, and introduce a parameter β that connects the thermal heat from a bias resistor and the temperature elevation of a Josephson junction. By using β, the temperature in the Josephson junction can be estimated as functions of the current through the resistor.

Recent developments of electronic devices containing Josephson junctions (JJ) with high-Tc superconductors (HTS) are reported. In particular, the fabrication process and the properties of superconducting quantum interference devices (SQUIDs) with a multilayer structure and ramp-edge-type JJs are described. The JJs were fabricated by recrystallization of an artificially deposited Cu-poor precursory layer. The formation mechanism of the junction barrier is discussed. We have fabricated various types of gradiometers and magnetometers. They have been actually utilized for several application systems, such as a non-destructive evaluation (NDE) system for deep-lying defects in a metallic plate and a reel-to-reel testing system for striated HTS-coated conductors.

The present status of superconducting terahertz emitter using the intrinsic Josephson junctions in high-Tc superconductor Bi2Sr2CaCu2O8+δ is reviewed. Fabrication methods of the emitting device, electrical and optical characteristics of them, synchronizing operation of two emitters and an example of applications to the terahertz imaging will be discussed. After the description of fabrication techniques by an Argon ion milling with photolithography or metal masks and by a focused ion beam, optical properties of radiation spectra, the line width, polarization and the spatial distribution of emission are presented with some discussion on the operation mechanism. For electrical properties, reversible and irreversible operations at high and low electrical currents, respectively, and electrical modulation of the radiation intensity for terahertz imaging are presented.

We describe the fabrication processes and electrical characteristics of two types of NbN junctions. One is a self-shunted NbN/NbNx/AlN/NbN Josephson junction, which is expected to improve the density of integrated circuits; the other is an underdamped NbN/AlNx/NbN tunnel junction with radical-nitride AlNx barriers, which has highly controllable junction characteristics. In the former, the junction characteristics were changed from underdamped to overdamped by varying the thickness of the NbNx layer. Overdamped junctions with a 6-nm-thick NbNx film exhibited a characteristic voltage of Vc = 0.8 mV and a critical current density of Jc = 22 A/cm2 at 4.2 K. In the junctions with radical-nitride AlNx barriers, Jc could be controlled in the range 0.01-3 kA/cm2 by varying the process conditions, and good uniformity of the junction characteristics was obtained.

AC-waveform synthesis with quantum-mechanical accuracy has been attracting many researchers, especially metrologists in national metrology institutes, not only for its scientific interest but its potential benefit to industries. We describe the current status at National Metrology Institute of Japan of development of a Josephson arbitrary waveform synthesizer based on programmable and pulse-driven Josephson junction arrays.

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.

A physical random number generator, which generates truly random number trains by using the randomness of physical phenomena, is widely used in the field of cryptographic applications. We have developed an ultra high-speed superconductive physical random number generator that can generate random numbers at a frequency of more than 10 GHz by utilizing the high-speed operation and high-sensitivity of superconductive integrated circuits. In this study, we have statistically evaluated the quality of the random number trains generated by the superconductive physical random number generator. The performances of the statistical tests were based on a test method provided by National Institute of Standards and Technology (NIST). These statistical tests comprised several fundamental tests that were performed to evaluate the random number trains for their utilization in practical cryptographic applications. We have generated 230 random number trains consisting of 20,000-bits by using the superconductive physical random number generator fabricated by the SRL 2.5 kA/cm2 Nb standard process. The generated random number trains passed all the fundamental statistical tests. This result indicates that the superconductive random number generator can be sufficiently utilized in practical applications.