  The faster speed and operational convenience of two-qubit gate with flux bias
control makes it an important candidate for future large-scale quantum
computers based on high coherence flux qubits. Based on a properly designed
two-spin gadget which has small gaps during the evolution of energy levels, we
build a CNOT-equivalent gate which can reach a fidelity larger than 99.9%
within 40ns. Moreover, we also use the Schrieffer-Wolff Transformation to
translate the spin model Ising coefficients schedule to circuit model flux bias
schedule for realistic flux qubit circuits coupled by a tunable rf-squid. The
two-qubit entangling gate scheme introduced here is suitable for realizing
efficient two-qubit gates in the large scale flux qubit systems dominated by
inductive couplings. Comparing with the current gate-based quantum computation
systems dominated by capacitive couplings, it can resolve the conflict between
the speed and a high coherence.
