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To meet the specification of the qubits in our laboratory, a 0.4 kA·cm−2 superconducting rapid single flux quantum (RSFQ) circuit was designed and successfully fabricated with an improved Nb-based self-aligned lift-off process. This circuit consists of a single-flux-quantum (SFQ) pulse generator, a Josephson transmission line (JTL) and a T-flip flop (TFF), and it acts as a frequency divider. The values of the inductors in this circuit were extracted using InductEX and the basic function of this circuit was confirmed using the simulation software WRspice before fabrication. After fabrication, the basic parameters of this circuit were measured at ~ 2.5 K in a Janis He-3 cryostat. This work laid the theoretical and experimental basis for the future research on the RSFQ-qubit control circuits. [ABSTRACT FROM AUTHOR]

We have developed a sub-μm Nb/Al-AlNx /Nb Josephson junction and integrated circuit fabrication process using deep-UV lithography and inductively coupled plasma etch tools. The baseline process consists of 11 masking steps including ground plane, PdAu resistor, Nb/A1-AlNx/Nb trilayer, and two additional Nb wiring layers. The AINx tunnel barriers are grown with plasma nitridation. These junctions exhibit low subgap leakage even at current densities exceeding 100 kA/cm2. The critical current spread of a series array of 50-kA/cm2, 0.6 μm diameter junctions is under 3%. For very high current density applications, these junctions are a good candidate to replace Nb/Al-A1Ox /Nb junctions particularly in future generations of very high speed, rapid single flux quantum logic circuits. In this paper we discuss our baseline fabrication process and device characterization including junction capacitance extraction from direct measurements of the Josephson plasma frequency. [ABSTRACT FROM AUTHOR]

A 10-kA/cm2 Nb/A1Ox,/Nb junction technology for rapid single flux quantum (RSFQ) integrated circuits has been developed. Modifying our standard 1.6-kA/cm2 cell library, we have implemented elementary cells used to build RSFQ digital-to-analog converters for ac voltage standard applications. The 10-kA/cm2 library cells have been fabricated by utilizing existing process tools. Correct operations of the circuits have been confirmed at low speed. The maximum operating frequencies evaluated by an average-voltage measurement technique were consistent with the scaling rules of RSFQ circuits. [ABSTRACT FROM AUTHOR]

One of the remaining challenges in the application of superconducting electronics is the interfacing between superconducting and semiconducting environments. The voltage and speed mismatch between RSFQ pulses and semiconducting read-out electronics makes it necessary to amplify as well as stretch the RSFQ pulses. Moreover, circuits based on HTS (High Temperature Superconductor) technology are very attractive since they can operate under considerably relaxed cooling effort, which is one of the main problems with LTS (Low Temperature Superconductor) circuits. Within the European project SuperADC, a HTS second order sigma delta modulator and a pulse stretcher, used as an interface between the modulator and the first semi-conducting amplifier stage, have been designed at Twente University and will be presented here. [ABSTRACT FROM AUTHOR]

The rapid single flux quantum (RSFQ) mixed-signal technology offers a way to enhance performance and reduce complexity and cost of SQUID systems. Ultra-sensitive analog SQUID front ends can be combined with low-power and ultra-fast RSFQ digital circuits on a single chip to form mixed-signal circuits combining the sensitivity and noise level of an “analog SQUID” and the increased slew-rate, dynamic range, linearity, and simple multi-channel readout electronics of digital RSFQ circuits. We have developed several RSFQ-based circuits to perform sensing, digitization, and timing of various weak signals including outputs of detector arrays. The key enabling factor for the implementation of all these SQUID-RSFQ circuits is the availability of a high-yield, low-noise fabrication process. [Copyright &y& Elsevier]