Josephson Memories
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LETTER
Josephson Memories Hans Hilgenkamp 1 Received: 29 August 2020 / Accepted: 7 September 2020 # The Author(s) 2020
Abstract A brief overview will be provided on superconducting memory elements incorporating Josephson junctions, from the tunneling cryotrons in the 1960's to contemporary RSFQ devices, with an outlook to future developments such as Josephson junction-based neuromorphic circuitry. Keywords Josephson junctions . Superconducting memory . Superconducting electronics This manuscript is part of a Special Issue celebrating this years’ 80th birthday of Prof. Brian Josephson. I herewith like to express to him my sincere congratulations and best wishes! While we do not share personal memories, we do share our fascination for the beautiful phenomenon of superconductivity and the interest in superconductive tunneling. Researching and lecturing about Josephson junctions has always brought me great joy, for their clear-cut manifestation of profound quantum physics and for their mindboggling capabilities as central elements in highly sensitive or ultrafast electronic devices. As a tribute to Prof. Josephson I will reflect in the following on one particular device type of importance for applications in superconductive (quantum)-information technologies, namely, the Josephson memory element. First, it may be surprising to note that already in the years prior to Josephson’s discoveries [1], there was a major industrial effort going on, especially by institutions and companies in the USA, to build a superconducting computer. At some point in the late 1950s–early 1960s, superconductivity was even ahead of semiconductor technology in its level of integration, using such modern-sounding concepts as thin film-based integrated circuitry and electron beam lithography [2]. The basic element for superconducting processor and memory circuits was the “cryotron” [3]. This switching device utilized the principle that by applying a current through a control line, the superconductivity in the gate (a strip of superconducting material) could be switched on and off. Memory elements such as flip-flops were * Hans Hilgenkamp [email protected] 1
Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands
realized based on the rerouting of persistent superconducting currents along different paths using such cryotrons. While promising, an issue with the cryotrons was their limited switching speed, of typically several (tens of) nanoseconds. For the most popular “in-line” cryotron configuration this was found to be limited by the characteristic propagation speed for the phase boundary between the superconducting and normal states in the superconductor [4]. As semiconductor technology was advancing at a rapid speed, leading to faster devices at lower costs, and had the additional advantage of being a room temperature technology, the cryotron-based computing paradigm became largely abandoned in the early 1960s. Smaller research activities on superconducting electronics
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