  We show a novel method for controlling the coupling of flux-based qubits by
means of a superconducting transformer with variable flux transfer function.
The device is realized by inserting a small hysteretic dc SQUID with unshunted
junctions, working as a Josephson junction with flux controlled critical
current, in parallel to a superconducting transformer; by varying the magnetic
flux coupled to the dc-SQUID, the transfer function for the flux coupled to the
transformer can be varied. Measurements carried out on a prototype at 4.2 K
show a reduction factor of about 30 between the 'on' and the 'off' states. We
discuss the system characteristics and the experimental results.
