The cryotron as an ultra-low-noise amplifier elimination of excessive cryotron noise

A study of the noise characteristics of the cryotron when used as a linear amplifier is reported. A large contribution to the cryotron amplifier noise is traced to bubbling of the liquid helium which is in physical contact with the cryotron. The noise results from the bubbling of the liquid because of certain steps or discontinuities which occur in the resistance variation of tin films in their transition region. The bubbling at the surface of the film causes it to switch between the steps or discontinuities in a random fashion, resulting in the above mentioned noise. The noise has been eliminated by isolating the thin films and cryotrons from physical contact with the liquid helium. This isolation has been accomplished by enclosing the substrates, on which the films and cryotrons are deposited, in tightly sealed aluminum foil bags. Finally, the steps or discontinuities can be eliminated in very thin films by using a magnetic field perpendicular to the film to control the transition. This offers a second possible method for the elimination of the noise.