  Galvanic coupling of small-area (three-junction) flux qubits, using shared
large Josephson junctions, has been shown to yield appreciable interaction
strengths in a flexible design, which does not compromise the junctions'
intrinsic good coherence properties. For an introduction, I recapitulate an
elementary derivation of the coupling strength, which is subsequently
generalized to the case of tunable coupling for a current-biased shared
junction. While the ability to vary coupling constants by, say, 20% would be
useful in experiments, sign-tunability (implying switchability) is highly
preferable for several quantum-computing paradigms. This note sketches two
ideas: a "crossbar" design with competing ferro- and antiferromagnetic
current-biased tunable couplings, and a "mediated" one involving an extra loop
between the qubits. The latter is a variation on proposals for tunable
capacitive coupling of charge qubits, and tunable inductive coupling of
large-area flux qubits.
