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Superconducting qubit circuits require high quality low critical current density Josephson tunnel junctions. We have developed high quality epitaxial NbN/AlN/NbN tunnel junctions with critical current density, Jc, ranging from as low as a few A/cm^2 to a few tens of kA/cm^2. In this work, we measured current–voltage characteristics for junctions with a Jc\sim 32\ \rm A/cm^2 in the temperature range from 17 mK to above 6.7 K. All the junctions are in good quality and have a high gap voltage (>5.5 mV). We found that the junctions' sub-gap resistances were temperature dependent above 2.5 K and saturated at temperatures below 2.5 K. A junction sub-gap leakage current Ir of about 23 pA was measured. The voltage noise power spectral density for 3 junctions with different sizes has been measured at the base temperature of 17 mK with the junctions biased above the gap voltage. The spectral densities clearly show 1/f behavior at low frequency and are proportional to the junction's linear dimension. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

Abstract: We have designed, fabricated and tested a time-to-digital converter (TDC) using SFQ logic circuits. The proposed TDC consists of two sets of ring oscillators and binary counters, and a coincidence detector (CD), which detects the coincidence of the arrival of two SFQ pulses from two ring oscillators. The advantage of the proposed TDC is its simple circuit structure with wide measurement range in addition to the high resolution and the high sensitivity of the SFQ TDC compared to semiconductor TDCs. The time resolution of the proposed TDC is limited by the resolution of the CD. In order to improve the resolution, we have developed a dynamic AND (DAND) gate, which can detect two simultaneous SFQ signal inputs with high accuracy. It was demonstrated that the time resolution of the TDC using the DAND gate is improved to be ±4ps. [Copyright &y& Elsevier]

We have designed and implemented an SFQ programmable clock generator (PCG), which can generate the variable number of SFQ pulses according to its internal state. The PCG is composed of an SFQ ring oscillator, a control circuit which counts up the number of SFQ pulses and stops the operation of the ring oscillator, and a decoder which defines the initial state of the control circuit. The PCG can generate the variable number of SFQ pulses ranging from 2 to 2N, where N is the number of T flip-flops in the control circuit. The oscillation frequency of the PCG is designed to be ranging from 6.2 to 18.8 GHz. In this study, we have implemented a PCG generating SFQ pulses ranging from 2 to 24 using a cell-based design methodology and confirmed its correct functionality. [Copyright &y& Elsevier]