  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.
