Controllable Coupling of Superconducting Flux Qubits

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 $\sim 45$ (antiferromagnetic) to $\sim -55\mathrm{mK}$ (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.