  Superconducting flux qubits are a promising candidate for solid-state quantum
computation. One of the reasons is that implementing a controlled coupling
between the qubits appears to be relatively easy, if one uses tunable Josephson
junctions. We evaluate possible coupling strengths and show, how much extra
decoherence is induced by the subgap conductance of a tunable junction. In the
light of these results, we evaluate several options of using intrinsically
shunted junctions and show that based on available technology, Josephson field
effect transistors and high-Tc junctions used as pi-shifters would be a good
option, whereas the use of magnetic junctions as pi-shifters severely limits
quantum coherence.
