Impedance Matching a Josephson Galvanometer by Means of a Superconducting Transformer

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.

Superconducting Quantum Interference Devices for Low Frequency Measurements

“…We might expect to be able to increase Li by at least an order of magnitude without significantly reducing a, so that an upper limit on R 1 of at least tens of ohms for a 1 K noise temperature seems perfectly feasible. If necessary, the upper limit on R 1 would be further extended by means of an additional superconducting transformer between the voltmeter circuit and the SQUID, as described by Clarke, et al [89], and Davidson et al [52].…”

Stabilizing a SQUID current amplifier in high magnetic fields

“…In such setups, noise level below 1 pA ( √ Hz) −1 can be achieved [2]. Furthermore, when impedance matching devices are used, such as transformers [3][4][5][6][7][8][9][10][11] or cryogenic current comparators [12][13][14][15][16][17][18][19], noise levels in the fA ( √ Hz) −1 range are accessible. Due to their exquisite sensitivity, such SQUID-based current sensors are often used for Johnson noise thermometry [20][21][22][23][24][25][26] or as null detectors in high precision bridge measurements in quantum metrology [12][13][14][15][16][17][18][19].…”