Impedance Matching a Josephson Galvanometer by Means of a Superconducting Transformer

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Abstract
The concept of an effective noise temperature is developed and applied to superconducting galvanometers. It is shown that even when the device is being used at a temperature above its noise temperature, Johnson noise in a source resistance may be observed only if this resistance is below a certain value, defined as the critical resistance. A detailed design is given for a superconducting transformer which matches the galvanometer to higher resistance and thereby appreciably increases the critical resistance. The transformer may be used at zero frequency. A practical transformer is described which has been used with the Josephson device known as the Slug. The transformer increased the critical resistance from 7×10−8 to 2×10−2 Ω and could be readily adapted to match to resistances up to 1 Ω.

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