  We experimentally confirm the functionality of a coupling element for
flux-based superconducting qubits, with a coupling strength $J$ whose sign and
magnitude can be tuned {\it in situ}. To measure the effective $J$, the
groundstate of a coupled two-qubit system has been mapped as a function of the
local magnetic fields applied to each qubit. The state of the system is
determined by directly reading out the individual qubits while tunneling is
suppressed. These measurements demonstrate that $J$ can be tuned from
antiferromagnetic through zero to ferromagnetic.
