An Ultra High Sensitive Current Sensor based on Superconducting Quantum Interference Device

The design of the reported current sensor is based on a suitable superconducting intermediary (matching) magnetic flux transformer magnetically coupled to a niobium based dc-SQUID (Superconducting Quantum Interference Device). The 60 square niobium turns (20 μm width) signal coil is tightly coupled to the matching flux transformer consisting of a square single turn primary coil connected in series with a multiturn secondary coil. The obtained signal current to magnetic flux transfer factor (current sensitivity) is equal to 62 nA/Φ0 measured by using a current sensing noise thermometer technique. The sensor has been characterized in liquid helium by using a direct coupling low noise readout electronic and the flux locked loop configuration. Despite the circuit complexity, the sensor has exhibited a smooth and free resonance voltage-flux characteristic ensuring a stable working operation. Considering a SQUID magnetic flux noise √SΦ =1.8 μΦ0/√Hz at T=4.2 K, a current noise as low as 110 fA/√Hz has been obtained. Due to his high performance such sensor can be employed in all application requiring an extremely current sensitivity, like the readout of the gravitational wave detectors and the current sensing noise thermometry.