  We have realized controllable coupling between two three-junction flux qubits
by inserting an additional coupler loop between them, containing three
Josephson junctions. Two of these are shared with the qubit loops, providing
strong qubit--coupler interaction. The third junction gives the coupler a
nontrivial current--flux relation; its derivative (i.e., the susceptibility)
determines the coupling strength J, which thus is tunable in situ via the
coupler's flux bias. In the qubit regime, J was varied from ~45
(antiferromagnetic) to ~ -55 mK (ferromagnetic); in particular, J vanishes for
an intermediate coupler bias. Measurements on a second sample illuminate the
relation between two-qubit tunable coupling and three-qubit behavior.
