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SQUID based current noise measurement

Published on 13 December 2018

At LaTEQS, we have built an experimental set-up to measure current fluctuations in multi-terminal nanostructures. It uses one or several SQUIDs (Superconducting QUantum Interference Devices) as very sensitive current detectors at low frequency (<10 kHz). We use commercial SQUIDs made of two Josephson Junctions in a superconducting loop. In such a device, the flux threading the loop is quantified and a conteracting current is naturally created to keep the flux equal to nΦ0. Under appropriate current bias conditions, the voltage across the SQUID oscillate as a function of the flux.

In our set-up, the current and its fluctuations to be measured are converted into a flux at the SQUID by a superconducting winding coupled to the SQUID. The SQUID are mounted in two dilution fridges operating at a base temperature of 30 mK with a cooling power of few µW at 100 mK. The SQUID are located at the 4K flange of the dilution insert and the spectral density of noise is obtained after fast Fourier transform with a digital low frequency dual spectrum analyzer.

The noise sensitivity is better than few 10-24 A2/Hz wich corresponds to the thermal noise of a 1 Ω resistor at 100 mK. The system has therefore a voltage noise of few 10-24 V2/Hz too.