A two-qubitentangling gate based on a two-spin gadget

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. We designed a two-qubit entangling gate using only flux bias control for flux qubits coupled with tunable couplers, based on a properly designed two-spin gadget which has small gaps during the evolution of energy levels. Starting from idle, by making an excursion to one small gap caused by anticrossing in our gadget, a CNOT-equivalent gate with a fidelity larger than 99.9% within 40ns can be realized. 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. Our two-qubit gate has implications in improving reverse annealing as well as realizing efficient two-qubit gates in flux qubit systems.