  An improved tunable coupling element for building networks of coupled
rf-SQUID flux qubits has been experimentally demonstrated. This new form of
coupler, based upon the compound Josephson junction rf-SQUID, provides a sign
and magnitude tunable mutual inductance between qubits with minimal nonlinear
crosstalk from the coupler tuning parameter into the qubits. Quantitative
agreement is shown between an effective one-dimensional model of the coupler's
potential and measurements of the coupler persistent current and
susceptibility.
