Evolution of HTS Josephson junctions and their application at ISTEC and SUSTERA
The evolution of HTS Josephson junction technology and its application during the last over twenty years is reviewed, mostly focusing on that at ISTEC and SUSTERA. The fabrication technology of oxide multilayer and ramp-edge Josephson junctions was much advanced during the decade from 1995 to 2005 to develop HTS single flux quantum (SFQ) devices. Although the development of SFQ devices resulted in only demonstration of small-scale circuits and a sampler system mainly due to a rather large critical current spread of HTS Josephson junctions, the developed multilayer and the junction technology was applied to fabrication of multilayer HTS SQUIDs with high resistance to external magnetic field. Using these HTS SQUIDs, a variety of systems, in particular, those for field use such as TEM systems for exploration or monitoring of natural resources and a nondestructive testing system for infrastructure have been developed and demonstrated. HTS SQUIDs can be now stably operated in various fields, for example, on the ground, in a borehole, and on an expressway. The application field of HTS SQUIDs is expected to further expand in the near future.
The History of SQUIDs Abstract
The SQUID in its various forms has been a mainstay in many application arenas for more than 50 years. This presentation will concentrate on the unique discovery and development of SQUIDs at the Ford Motor Company in the 1960’s from a first-person perspective.
The presentation will include:
- Discussion of critical prior developments that led to the first realization of macroscopic quantum interference in superconductors
- Experiments leading to the development of dc, rf, and resistive SQUIDs
- Experimental technologies available at that time (in contrast to today)
- Innovative improvement and expansion of SQUID technology since the 1960’s
Single Flux Quantum Logic for Digital Applications
It took about twenty years for superconducting single flux quantum (SFQ) digital electronics to progress from the invention, initial proof-principle experiments to the first application system of a practical significance. Rapid Single Flux quantum (RSFQ) logic was introduced in mid-80s as an alternative to then dominant superconducting latching logic and became the main digital and mixed-signal technology by mid-90s. In search for the practical applications, it went through multitudes of projects and attempts to solve real-world problems and find application niches to compete with omnipotent CMOS in heyday of Moore’s law. By mid-00s, this was successfully achieved for mixed-signal applications by riding on the superior RSFQ clock speed, quantum properties of superconducting Josephson circuits, and finding a solution for interfacing cryogenic low-power, fast RSFQ electronics with higher power, much slower room-temperature electronic environment. In recent years, CMOS started to lose its unquestionable application luster opening new opportunities for superconducting electronics. Achieving the highest energy-efficiency for high-end computing such as supercomputers and data centers became the priority. This triggered the development of several post-RSFQ logic families with significantly higher energy efficiency. The advent of quantum computing and quantum sensors opened a new application field in a classical infrastructure electronics capable of operating at cryogenic temperatures in a close proximity to quantum circuits. Here, the inherent strengths of SFQ logic including high-speed, low-power, and cryogenic operation offer a significant advantage over other technologies.