Characterization of SQUID-Based Null Detector for a Quantum Metrology Triangle Experiment

We present characterization of a null detector designed for an experiment aimed to close the quantum metrological triangle (QMT). The device is a SQUID amplifier equipped with a large winding-ratio planar input transformer improving the current resolution. We demonstrate equivalent input current noise down to 14 fA/Hz^1/2. Furthermore, operation in a realistic environment of a dry dilution refrigerator hosting the QMT experiment is studied. We discuss particular challenges set by such system as the cryogenics introduces relatively large magnetic interference. We present data from measurements simulating the final QMT experiment and introduce the requirements for the averaging and bias reversal.