The cryotron tube utilizes a brilliant concept which was somehow forgotten in the past thirty years, well, maybe due to understandable reasons. I have been lately reading about RSFQ circuits and by accident I ran across a year 1956 paper by D. A. Buck entitled "The Cryotron - A Superconductive Computer Component". What an esoteric idea one would say, however I find it fascinating and decided to share this rare paper.
This appears to be one of the first publications elaborating more on the practical use of Cryotron tubes. The fundamental concept standing behind the operation of the Cryotron tube is explained by the Meissner effect. In a nutshell in both type I and II superconductors the strength of an externally applied magnetic field to the superconductor changes its critical temperature.
This is the effect utilized in Cryotron tubes, by applying an external magnetic field by the means of pushing current through a simple coil wrapped around the superconductor we can change its resistive state - either superconductive or not. The diagram shown in Buck's paper summarizes the basic operating regions of this element. The unique here is the achievable switch speed. According to Buck's paper and various online sources, switching speeds in the order of pico and femto seconds can be easily achieved.
Buck's paper focuses on digital circuit design with Cryotrons. In a similar fashion as the flip-flop (above) shown in his paper he has managed to build a full arithmetic unit based on Cryotron logic circuits. Further reading in the full paper.