Controllable Flux Coupling for the Integration of Flux Qubits
  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. 
C. Cosmelli, M. G. Castellano, F. Chiarello, R. Leoni, D. Simeone, G.   Torrioli, P. Carelli