We designed and fabricated rapid single-fluxquantum (RSFQ) circuits using a new 20-kA/cm 2 process based on conventional Nb/AlO x /Nb Josephson junction (JJ) technology. Circular JJs were fabricated to improve their uniformity. They had sufficiently low spread of the critical currents for large-scale RSFQ circuits. We selected the circuit design parameters carefully, including the bias voltage and McCumber-Stewart parameter, balancing the energy consumption, switching speeds, margins, and integration density. We demonstrated several logic gates and shift registers at a clock frequency of 154 GHz with the designed bias voltage of 2.5 mV, and demonstrated energy-efficient shift registers using the low-voltage RSFQ (LV-RSFQ) technique at 83 GHz.Index Terms-Energy efficient, low power, low-voltage, rapid single-flux-quantum logic, shift register, superconducting integrated circuits.

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SUMMARYOne of the fundamental problems in many-pixel detectors implemented in cryogenics environments is the number of bias and read-out wires. If one targets a megapixel range detector, number of wires should be significantly reduced. One possibility is that the detectors are serially connected and biased by using only one line and read-out is accomplished by on-chip circuitry. In addition to the number of pixels, the detectors should have fast response times, low dead times, high sensitivities, low inter-pixel crosstalk and ability to respond to simultaneous irradiations to individual pixels for practical purposes. We have developed an equivalent circuit model for a serially connected superconducting strip line detector (SSLD) array together with the read-out electronics. In the model we take into account the capacitive effects due to the ground plane under the detector, effects of the shunt resistors fabricated under the SSLD layer, low pass filters placed between the individual pixels that enable individual operation of each pixel and series resistors that prevents the DC bias current flowing to the read-out electronics as well as adjust the time constants of the inductive SSLD loop. We explain the results of investigation of the following parameters: Crosstalk between the neighbor pixels, response to simultaneous irradiation, dead times, L/R time constants, low pass filters, and integration with the SFQ front-end circuit. Based on the simulation results, we show that SSLDs are promising devices for detecting a wide range of incident radiation such as neurons, X-rays and THz waves in many-pixel configurations.

We report successful demonstration of individual readout of the response to an optical signal in serially-connected superconducting strip line detectors (SSLDs). Based on the numerical analysis, placement of low-pass filters between adjacent SSLDs and use of a current-source drive enable seriallyconnected SSLDs to work as independent multiple SSLDs. This leads to reduction of the number of cables from room temperature electronics. We irradiate a laser just on a certain SSLD (SSLD#1) with On/Off modulation. Synchronized voltage output is obtained at only corresponding SQUID. Then we move the position of a laser spot on the neighbor SSLD (SSLD#2). The voltage disappears at the SQUID connecting to SSLD#1, and the voltage is developed only at the SQUID connecting to SSLD#2. We have also given a demonstration of readout using the singleflux-quantum circuits connected to the SQUIDs. We confirmed from those experimental results that our driving method for multiple SSLDs is effective not only to enhance detecting area but also to obtain position sensitivity.