AURIGA: the SQUID amplifier
The AURIGA dc-SQUID amplifier is a commercial device housed inside a niobium shield: this is inserted in a copper lead plated box (see fig. 3 in the AURIGA photo album). Below the niobium and the lead superconducting transition temperatures each screen provides for a suppression of external magnetic flux by a factor of 108. In order to minimize the electromagnetic interferences no thermometers are installed near the SQUID sensor; however the SQUID operation temperature is expected to be about 0.5 K. This is because the SQUID holder is thermally linked to the cold plate of the dilution refrigerator through a copper cantilever spring.
The in-situ measured value of the power spectral density of the SQUID noise corresponds to an energy sensitivity per unit bandwidth of about 4000 times the reduced Planck constant (=hbar). Here the energy sensitivity is expressed in terms of the numbers of times the sensor noise exceeds the quantum limit value hbar/2.
In principle with this amplifier sensitivity at a thermodynamic temperature of 100 mK the minimum detectable detector energy change (i.e. the energy resolution) should be less or equal to 0.4 mK: this is about twice the resolution obtained experimentally. This discrepancy is related to an excess of back-action noise whose origin is still not fully understood.
The SQUID input coil (see also fig. 2) is connected trough a superconducting feedthrough to the secondary coil of the matching transformer: this consists of a Nb-Ti wire three times wrapped around the primary coil. The latter is made of 1800 turns of NbTi wire distributed in 35 layers. The wires are packaged so to reduce the dielectric losses due to parasitic capacitance: these losses strongly harms the electrical quality factor.
In bench-tests the electrical quality factor was measured using low loss capacitors coupled to the AURIGA primary coil: these measurements led to an electrical quality factor of about 4x105 at the frequency of 1 kHz.