Masaaki Maezawa

Overdamped Josephson junctions with Nb/AlOx/Al/AlOx/Nb structure for integrated circuit application

We present characteristics of overdamped Josephson junctions consisting of Nb/AlOx/Al/ AlOx/Nb structures. The junctions were fabricated using a well-developed Nb/AlOx/Nb-junction technology and showed well-defined Josephson characteristics at 4.2 K. The characteristic voltage Vc [the product of the critical current Ic and the effective normal resistance Rn(eff)] of junctions, which determines high-frequency performance of the junction, was in the range of 0.1–0.5 mV, and the critical current density Jc in the range of 102–104 A/cm2. Maximum-to-minimum variations in Ic over a wafer were ±4% for junctions with Vc=0.15 mV and ±13% for junctions with Vc=0.5 mV.

Specific capacitance of Nb/AlOx/Nb Josephson junctions with critical current densities in the range of 0.1–18 kA/cm2

The specific capacitance Cs of Nb/AlOx/Nb Josephson tunnel junction has been measured by means of the superconducting quantum interference device (SQUID) resonance technique. We have investigated the junctions with critical current densities Jc in the range of 0.1–18 kA/cm2 and found that 1/Cs linearly depends on a logarithm of Jc. This suggests that the barrier thickness is moderately uniform in the junction area and increases continuously during growth. The results also show that increasing Jc reduces time constants of the junction.

RSFQ-based D/A converter for AC voltage standard

Digital to analog converters based on the Josephson effect are promising for voltage standards, because they produce voltage steps with ultimate precision and stability. In this paper, we describe a project to develop a Josephson D/A converter designed for synthesizing a sinusoidal waveform with metrological accuracy. The D/A converter is based on RSFQ (Rapid Single Flux Quantum) logic circuits, and consists of a frequency multiplier (FM), a pulse distributor (PD), and a number of voltage multipliers (VMs). Each VM circuit, corresponding to the n-th bit digital code, multiplies the number of SFQ pulses by a factor of 2/sup n/. By gating the input SFQ pulses from the FM to the VMs using the PD circuits, a programmable output voltage is obtained. Possible sources of uncertainties in the measurement of the rms value of the synthesized sine wave are discussed.

A 0.2–0.5THz single-band heterodyne receiver based on a photonic local oscillator and a superconductor-insulator-superconductor mixer

We have demonstrated that a superconductor-insulator-superconductor (SIS) mixer pumped by a photonic local oscillator (LO) covers the whole frequency range of 0.2–0.5THz. In the bandwidth of 74% of the center frequency, this single-band receiver exhibits noise temperature of TRX⩽20hf∕kB, where h is Planck’s constant, f is the frequency, and kB is Boltzmann’s constant. Resultant TRX is almost equal to TRX of the identical SIS mixer pumped by three conventional frequency-multiplier-based LOs which share the 0.2–0.5THz band. This technique will contribute to simple, wide-band, and low-noise heterodyne receivers in the terahertz region.

Chip-to-chip communication using a single flux quantum pulse

Chip-to-chip propagation of a single flux quantum (SFQ) pulse has been demonstrated using an active multichip module (MCM) technique in which active transmitters and receivers are integrated on an MCM substrate as well as on a chip. A chip-to-chip SFQ transfer circuit has been designed and implemented by a standard Nb-trilayer process and solder-bumped flip-chip technology. The correct operation of the circuit has been confirmed by low-frequency testing. Experimental margins for the global factor for bias currents have been as large as /spl plusmn/33%.

An interface circuit for a Josephson-CMOS hybrid digital system

For broadband data communication between Josephson and CMOS digital systems, amplification of small Josephson-output signals and synchronization between the systems are important issues. We present an interface circuit for a Josephson-CMOS hybrid digital system. The interface circuit consists of a parallel-in-parallel-out (PIPO) circuit and built-in Josephson-MOS amplifiers. The PIPO circuit, implemented based on 4JL latching logic technology, performs synchronized data transfer between the Josephson and CMOS circuits. The Josephson-MOS amplifiers consists of stacked Josephson junctions (Suzuki stacks) and MOS inverters which are monolithically integrated on a chip. The circuits have been designed, fabricated and tested. We have successfully confirmed correct operation of the circuits.

Operation of a Josephson arbitrary waveform synthesizer with optical data input

In order to establish a quantum ac voltage standard at NMIJ/AIST, we have developed a Josephson arbitrary waveform synthesizer based on overdamped Josephson junction arrays (JJAs) using optoelectronics and a liquid helium free compact cryocooler. We have succeeded in the generation of sine waves at 60 Hz and 152.6 kHz. To verify the performance of the system, two different types of JJAs with different characteristic frequencies have been developed. An application to a quantized voltage noise source for Johnson noise thermometry is also discussed, with experimental results.

Flux avalanches in Nb superconducting shifted strip arrays

Flux penetrations into three-dimensional Nb superconducting strip arrays, where two layers of strip arrays are stacked by shifting a half period, are studied using a magneto-optical imaging method. Flux avalanches are observed when the overlap between the top and bottom layers is large even if the width of each strip is well below the threshold value. In addition, anomalous linear avalanches perpendicular to the strip are observed in the shifted strip array when the overlap is very large and the thickness of the superconductor is greater than the penetration depth. We discuss possible origins for the flux avalanches, including linear ones, by considering flux penetration calculated by the Campbell method assuming the Bean model.

An Octave Bandwidth SIS Mixer for Accurate and Compact Terahertz Spectrometers

A SIS mixer with RF bandwidth of 1 octave is designed, fabricated, and characterized. The mixer chip consists of 8 Nb/AlOx/Nb junctions connected in parallel through Nb/SiO2/Nb microstrip lines (MSL), a twin-slot antenna, and a 3-stage MSL transformer. Experimentally, the 2.5 dB RF band is found to be 230-440 GHz, which agrees quantitatively with theoretical calculation. Receiver noise temperature Trx is measured as 260-680 K in the band. After correction for IF-amplifler noise and beam-splitter reflectivity, the noise is 170-320 K, i.e., 13-20 times of the quantum noise. The sensitivity flatness in the band can be improved to 1.9 dB by further optimization of the transformer.

5-Bit Quasi-Sinusoidal Voltage Waveform Synthesized Using Single-Flux-Quantum Pulse-Frequency Modulation

Synthesis of sinusoidal voltage waveform is a unique application in single-flux-quantum (SFQ) digital electronics. In this paper, the authors present a waveform synthesizer based on SFQ pulse-frequency modulation. It comprises a variable SFQ pulse-number-multiplier (PNM) and a code generator (CG) integrated on the same chip. The output voltage is determined by the multiplication factor in the variable-PNM, which is controlled by the code from the CG instead of room-temperature electronics. The variable-PNM realizes 5-bit resolution with the multiplication factors between 33 and 64. The CG generates the 5-bit codes for 16-step quasi-sinusoidal waveform. The whole circuits including I/O elements are designed using an SFQ digital cell library, and fabricated using a niobium integration process. The modulation of the multiplication factor for the 16-step quasi-sinusoidal waveform is confirmed by counting SFQ pulses in the low-speed testing, whereas the voltage waveform of 11 μV peak-to-valley is demonstrated using a high-speed SFQ input of 156.25 MHz.