Characteristics of Switchable Superconducting Flux Transformer with DC Superconducting Quantum Interference Device

We have investigated the flux transfer characteristics of a switchable flux transformer comprising a superconducting loop and a DC superconducting quantum interference device (DC-SQUID). This system can be used to couple multiple flux qubits with a controllable coupling strength. Its characteristics were measured using a flux input coil and a DC-SQUID for readout coupled to the transformer loop in a dilution refrigerator. The observed characteristics are consistent with the calculation results. We have demonstrated the reversal of the slope of the characteristics and the complete switching off of the transformer, which are useful features for its application as a controllable coupler for flux qubits.