Superconducting switches for various electrical circuit applications have been studied, and several different devices have been developed. Some superconducting switches, such as the cryotron developed by Dudley Buck in the 1950's and the superconducting flux-flow transistor (SFFT) developed in more recent years, rely on an externally-applied magnetic field to quench superconductivity between current-carrying terminals of the device. (See e.g., U.S. Pat. No. 2,832,897; G. K. Hohenwarter, “Superconducting High TC Thin Film Vortex-Flow Transistor,” Sponsored Research Report (1991) available at www.dtic.mil/dtic/tr/fulltext/u2/a235025.pdf.) Because these devices switched a large region of the device between a “superconducting” state and a “normal” state, the devices tend to exhibit slow switching speeds, e.g., on the order of 10's of microseconds.
In the late 1960's, alternative approaches to fabricating superconducting switches incorporated Josephson junctions into a multi-layer integrated device. (See e.g., J. Matisoo, “The Tunneling Cryotron—A Superconductive Logic Element Based on Electron Tunneling,” Proc. IEEE, Vol. 55, No. 2, February 1967, p. 172.) The Josephson junction devices include a tunnel junction at a gate region of the device. Tunneling current through the junction can be switched between a superconducting state and a normal tunneling state by the application of a current pulse that exceeds a critical current level at the Josephson junction. Because the junction volume is small, the device can be switched quickly, e.g., at switching times approaching 10 picoseconds. However, Josephson-junction-based devices have a number of limitations including: (1) sensitivity to magnetic fields, (2) limited gain, (3) inability to drive large impedances, and (4) difficulty in manufacturing control of the junction critical current (which depends sensitively on sub-Angstrom-scale thickness variation of the tunneling barrier). Additionally, fabrication of the device requires multi-layer and multi-material processing, as well as precise control of the tunneling barrier's thickness.
Recently, superconducting sensors and amplifiers have been developed for applications such as single-photon detection. (See, O. Quaranta et al., “Superconductive Three-Terminal Amplifier/Discriminator,” IEEE Trans. Appl. Supercond., Vol. 19, No. 3 (2009) p. 367.) An example of superconducting pulse amplifier 100 described in this work is depicted in FIG. 1. The device comprises an input terminal 102 and two main terminals 104, 106. Current ICin flowing into a main input terminal 104 may flow across parallel channels 110, 112 to a main output terminal 106. There may be more than two parallel channels. The input terminal 102 connects to one of the parallel channels at a T-shaped junction 120. In operation, the current in each of the parallel channels is biased close to a “critical current” value Icrit for each channel. The critical current value is a value of current above which superconductivity in the channel cannot be supported.
In operation, receipt of a signal pulse at the input terminal 102 with a peak current value Ip adds current to the current IC1 flowing in the first channel 110. Since the first channel 110 is biased near its critical current level, the resulting current in the first channel then exceeds the critical current for the first channel: (Ip+IC1)>Icrit1. Because of the excess current, superconductivity is no longer supported by the first channel, so that current from that channel is diverted to the second channel 112, where the critical current in that channel is also exceeded. As a result, superconductivity is quenched in the device's channels and a voltage across the main terminals 104, 106 increases from a zero value. Depending upon the device's internal resistances and external resistances connected to the device 100, the input pulse signal can produce an amplified pulse signal as measured across the main terminals 104, 106.