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IEEE Transactions on Applied Superconductivity

Investigation Into the Individual Configuration of Superconducting Phase Shift Elements Made of Ferromagnetic Patterns for Reconfigurable Circuits

Shozo TANIGUCHI, Hiroshi T. ITO, Kouta Ishikawa, Sota Kurokawa, Masamitsu Tanaka, Hiroyuki Akaike, Akira Fujimaki

Published Dec 20, 2016• 5 citations
Publisher

Abstract

Reconfigurable superconducting phase shift elements (PSEs) provide high design flexibility in rapid single flux quantum (RSFQ) circuits. Small ferromagnetic patterns were placed on superconducting loops in an RSFQ circuit. The patterns serve as reconfigurable PSEs providing the direction and magnitude of magnetization can be controlled for each pattern individually. A two-dimensional wire mesh architecture was employed to allow for individual configuration and to reduce the number of wires required. Although opposing magnetic fields may occasionally be generated during half-selection using this architecture, experimentally obtained magnetization curves revealed that the orientation of the magnetic field was maintained. An RSFQ circuit, using a reconfigurable PSE, has been successfully demonstrated, potentially leading to large-scale reconfigurable RSFQ circuits

DOI
10.1109/tasc.2016.2642054

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Reconfigurable superconducting phase shift elements (PSEs) provide high design flexibility in rapid single flux quantum (RSFQ) circuits. Small ferromagnetic patterns were placed on superconducting loops in an RSFQ circuit. The patterns serve as reconfigurable PSEs providing the direction and magnitude of magnetization can be controlled for each pattern individually. A two-dimensional wire mesh architecture was employed to allow for individual configuration and to reduce the number of wires required. Although opposing magnetic fields may occasionally be generated during half-selection using this architecture, experimentally obtained magnetization curves revealed that the orientation of the magnetic field was maintained. An RSFQ circuit, using a reconfigurable PSE, has been successfully demonstrated, potentially leading to large-scale reconfigurable RSFQ circuits
Functional switching has been demonstrated based on quantum flux parametron (QFP) circuits with ferromagnetic patterns toward reconfigurable circuits. We form small ferromagnetic patterns of a PdNi alloy on a superconducting loop without degradation of superconductivity. Direction and strength of magnetization of ferromagnetic patterns are controlled by direction and magnitude of an applied magnetic field in a cooling process around the Currie points of the alloy. The ferromagnetic patterns induce a circulating current serving as one of the input currents in a three-inputs QFP majority gate. We obtained functional switching between the AND to the OR operation depending on the direction and strength of magnetization of the ferromagnetic patterns experimentally. The obtained operation margin agreed with a numerically obtained margin, though the magnitude of magnetization at the switching between two operation modes was different. This technique add new functionality and flexibility to the QFP-based integrated circuits.
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The rapid single-flux quantum electronics (RSFQ) is a superconducting, naturally digital circuit family which is currently close to being commercially applied. RSFQ is outstanding because of its very low switching energy resulting in very low power consumption. This advantage causes, however, a significant influence of thermal noise. For industrial applications, a certain noise immunity is required which is still a challenge, especially for circuits of higher complexity. Integrating phase-shifting elements is a new concept for further improvements concerning stability against the influence of thermal noise. We have already shown that the implementation of phase-shifting elements significantly reduces the influence of thermal noise on circuit behavior by experimentally analyzing the bit-error rate (Mielke et al 2009 IEEE Trans. Appl. Supercond. 19 621–5). Concepts which are easily implementable in standard niobium technology are especially promising. The π-phaseshifter consists of a superconducting loop which is able to store a single flux quantum. The loop current related to the stored flux creates a well-defined phase shift. To achieve the correct functionality of complex circuits it is essential to store exactly one flux quantum in each π-phaseshifter during the cooling down of the chip. Thus, for studying the feasibility of this new approach, the initialization reliability of the π-phaseshifter needs to be verified. We present an experimental investigation of this reliability to obtain a general assessment for the application of the π-phaseshifter in niobium technology. Furthermore, we compare the configuration shielded by a solid ground plane with a configuration with a ground-plane hole below the π-phaseshifter. Justified by the experimental results we suggest programmable RSFQ circuits based on π-phaseshifters. The characteristics of these devices can be influenced by a controlled initialization of the π-phaseshifter. The fabrication was performed by FLUXONICS Foundry.
We present a concept of an advanced rapid single-flux-quantum (RSFQ) logic circuit family using the combination of 0-shifted and π-shifted Josephson junctions. A π-shift in the current-phase relationship can be obtained in several types of Josephson junctions, such as Josephson junctions containing a ferromagnet barrier layer, depending on its thickness and temperature. We use a superconducting quantum interference devices composed of a pair of 0- and π-shifted Josephson junctions (0-π SQUIDs) as a basic circuit element. Unlike the conventional RSFQ logic, bistability is obtained by spontaneous circular currents without using a large superconductor loop, and the state can be flipped by smaller driving currents. These features lead to energy- and/or space-efficient logic gates. In this paper, we show several example circuits where we represent signals by flips of the states of a 0-π SQUID. We obtained successful operation of the circuits from numerical simulation.
The reduction of the critical current density in rapid single-flux quantum (RSFQ) circuits enables new application fields, like quantum computing and photonic detector readout. The low current density fabrication process creates new design challenges, such as lower stability against thermal fluctuations, violation of the lumped elements condition for microstrip inductances and increased sensitivity to the technological spread. To overcome these issues, we suggest a passive phase shifter as a promising alternative technique for superconductive phase dropping in the RSFQ electronics. Here, we study experimentally their applicability in high-speed RSFQ digital circuits. Conclusions are drawn about the impact of the passive phase shifters on the complexity, the speed and the bit error rate of the investigated RSFQ circuits. We demonstrate the successful operation of different circuits with implemented passive phase shifters at low and high speeds.
A vital precondition for the realization of rapid single-flux quantum (RSFQ) digital circuits with reduced critical currents of the Josephson junctions is the implementation of an efficient technique for superconductive phase dropping. In this paper, we present a novel phase shifting element consisting of a miniature superconductive ring located over a ground plane hole. Contrary to the solutions reported up to now, this topology can be simply integrated within complex digital RSFQ circuits realized with conventional fabrication technology.