Shinichi Yorozu

A single flux quantum standard logic cell library

To expand designable circuit scale, we have developed a new cell-based circuit design for single flux quantum (SFQ) circuit. We call it CONNECT cell library. The CONNECT cell library has over 100 cells at present. Each CONNECT cell consists of a Verilog digital behavior model, circuit information, and a physical layout. All circuit parameter values have been optimized for obtaining the widest margins and minimizing interactions between cells. At the layout level, we have defined a minimum standard cell size and made cell height and width a multiple of the size. Using this cell library, we can easily expand circuit scale without the time-consuming dynamic simulations of whole circuits. For estimation of the reliability of the library, we designed and fabricated test circuits using CONNECT cells. We demonstrated experimentally correct operations, which means the CONNECT cell library is sufficiently reliable.

Demonstration of a single-flux-quantum microprocessor using passive transmission lines

We have successfully demonstrated an 8-bit microprocessor using passive transmission lines based on single-flux-quantum LSI technology. In the microprocessor designed here, called CORE1/spl alpha/6, a simple bit-serial architecture with seven instructions was employed. In the CORE1/spl alpha/6, the floor plan was designed with consideration toward integration of a memory, and superconductive passive transmission lines (PTLs) were used to connect circuit blocks. Utilization of PTLs helped us reduce the propagation delay in long interconnections. The design flexibility of the floor plan was enhanced and the performance of the microprocessor was improved by 20% compared with our previous design. The CORE1/spl alpha/6 was composed of 6319 Josephson junctions and 15 PTLs with power consumption of 2.1 mW. We have confirmed the complete operations of the CORE1/spl alpha/6 by on-chip high-speed tests. The maximum clock frequencies for bit operation and instruction execution have been found to be 18 GHz and 1.2 GHz, respectively, where the performance corresponds to 240 million instructions per second (MIPS).

The effects of DC bias current in large-scale SFQ circuits

The supply of bias current is one of the key problems to be overcome for fully operational large-scale SFQ circuits. Large currents cause various undesirable effects that degrade circuit operation. The magnetic field induced by a large DC bias current affects the operation of several SFQ cells, even with a microstrip-line structure applied for the bias lines. In this paper, experimental results using SQUIDs show that bias-line shielding is the most effective way to reduce the effect of the DC bias current. The operating margins of SFQ cells were also found to be sensitive to the current in the ground plane. The location of a ground bonding should be close to the point of current injection if we wish to avoid undesirable diffusion of current through the ground plane. We confirmed the effectiveness of bias-line shielding and of the location of ground bonding with a circuit composed of about 500 junctions.

Demonstration of chip-to-chip transmission of single-flux-quantum pulses at throughputs beyond 100 Gbps

We report a demonstration of single-flux-quantum (SFQ) pulse transmission between superconductor chips at throughputs beyond 100 Gbps. A fabrication process with a high junction critical current density of 10kA∕cm2 was used to increase the operation speed of a pulse driver and receiver. The chips were flip-chip bonded on a passive microstrip carrier using small solder bumps with diameters of 30μm. With experiments based on a ring-shaped circuit, chip-to-chip SFQ pulse transmission has been demonstrated up to 117 Gbps with an error rate of less than 10−15. The power dissipated by the driver and receiver was only 0.24μW at 117 Gbps.

Thermoelectric Generation Based on Spin Seebeck Effects

The spin Seebeck effect (SSE) refers to the generation of a spin current as a result of a temperature gradient in magnetic materials including insulators. The SSE is applicable to thermoelectric generation because the thermally generated spin current can be converted into a charge current via spin-orbit interaction in conductive materials adjacent to the magnets. The insulator-based SSE device exhibits unconventional characteristics potentially useful for thermoelectric applications, such as simple structure, device-design flexibility, and convenient scaling capability. In this article, we review recent studies on the SSE from the viewpoint of thermoelectric applications. Firstly, we introduce the thermoelectric generation process and measurement configuration of the SSE, followed by showing fundamental characteristics of the SSE device. Secondly, a theory of the thermoelectric conversion efficiency of the SSE device is presented, which clarifies the difference between the SSE and conventional thermoelectric effects and the efficiency limit of the SSE device. Finally, we show preliminary demonstrations of the SSE in various device structures for future thermoelectric applications and discuss prospects of the SSE-based thermoelectric technologies.

Quantum key distribution over 120 km using ultrahigh purity single-photon source and superconducting single-photon detectors

Advances in single-photon sources (SPSs) and single-photon detectors (SPDs) promise unique applications in the field of quantum information technology. In this paper, we report long-distance quantum key distribution (QKD) by using state-of-the-art devices: a quantum-dot SPS (QD SPS) emitting a photon in the telecom band of 1.5 μm and a superconducting nanowire SPD (SNSPD). At the distance of 100 km, we obtained the maximal secure key rate of 27.6 bps without using decoy states, which is at least threefold larger than the rate obtained in the previously reported 50-km-long QKD experiment. We also succeeded in transmitting secure keys at the rate of 0.307 bps over 120 km. This is the longest QKD distance yet reported by using known true SPSs. The ultralow multiphoton emissions of our SPS and ultralow dark count of the SNSPD contributed to this result. The experimental results demonstrate the potential applicability of QD SPSs to practical telecom QKD networks.

Transmission Experiment of Quantum Keys over 50 km Using High-Performance Quantum-Dot Single-Photon Source at 1.5 µm Wavelength

We have developed a high-performance single-photon source (SPS) operating at 1.5 µm wavelength. The source is an InAs/InP quantum dot with a horn-shaped nanostructure. A resonant excitation to the p-shell state helps achieve a single-photon efficiency of 5.8% after coupling into a single-mode fiber with a second-order correlation value of g(2)(0)~0.055. The performance of the source has been assessed by integrating it into a conventional quantum key distribution system. We have successfully transmitted secure keys over a 50 km commercial fiber, exceeding the previously reported range for an SPS operating below 1.3 µm.

Vacuum Rabi splitting with a single quantum dot embedded in a H1 photonic crystal nanocavity

We report here the first observation of vacuum Rabi splitting in a single quantum dot (QD) embedded in a H1 photonic crystal nanocavity by photoluminescence measurement. The QD emission was tuned into a cavity mode by controlling the temperature. At the resonance condition, clear anticrossing with a Rabi splitting of ∼124 μeV was observed, where the cavity mode possesses the smallest mode volume V∼0.43(λ/n)3 among strongly coupled QD-cavity systems reported to date.

A design approach to passive interconnects for single flux quantum logic circuits

We developed a design approach for interface circuits to connect Single Flux Quantum (SFQ) cells by using passive transmission lines (PTLs). In the approach, an interface circuit between a PTL and JTL is optimized to obtain a standard interface circuit, and then, modifications are made to previously designed SFQ cells and the standard interface circuit to connect the SFQ cells by using PTLs. The key point is the use of approximately the same interface circuit with every SFQ cell to maintain the matching condition between the interface circuit and the PTLs. Based on this approach, we designed an interface circuit and a test circuit composed of two D-flip-flops connected using 2-mm-long PTLs via the interface circuits. The impedance of the PTL was 2 /spl Omega/. We achieved high-speed operation of the test circuit up to 35 GHz with a bias margin of -15/+30%.

Timing jitter measurement of single-flux-quantum pulse in Josephson transmission line

Timing jitter is an important factor in determining the limit of speed and scale of single-flux-quantum circuits. We estimated the timing jitter by a simple method using 2-bit shift registers. It was estimated by observing the transition between 1- and 2-mode operations while changing the bias current to the clock line. A sharp transition was observed in the shift register with a smaller number of Josephson junctions in the clock and data lines, suggesting the existence of a finite timing jitter. The timing jitter per Josephson junction was estimated to be 0.09 ps at a designed bias current. The temperature dependence supported that the timing jitter comes from thermal fluctuation.