Optimum DC current resolution of a ferromagnetic-core flux transformer coupled SQUID instrument

The quantum Hall effect as an electrical resistance standard

“…A simple circuit analysis shows that the current resolution of the CCC is optimal when the inductance of the pick-up coil L pu equals the inductance of the input coil L of the SQUID. It can be shown [79,80] that under this condition the best current resolution or minimum noise current is given by…”

“…Another way to get close to the ideal situation is to introduce a ferromagnetic core around which the pick-up coil and the windings of the CCC are wound. The relative permeability of the core can be adjusted such that N s 1 is achieved with a reasonably sized sensor volume [79][80][81]. The drawback of this method, however, is an additional noise component from the ferromagnetic core.…”

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].…”