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Flux qubits interacting with a common Josephson resonator. The resonance frequency of the resonator can be tuned by changing the bias current . Each flux qubit with resonant frequency couples to the resonator via mutual inductance . We apply a microwave of frequency to make entanglement between the qubits and .
Schematic energy diagram for the two-photon Rabi oscillation in the two-qubit-resonator system. The Rabi oscillation causes coherent transition between and . The energy splitting between these two states corresponds to two times the microwave frequency . This is a nonlinear (second-order) transition. Energies of the intermediate states and are shifted approximately halfway between and by the interaction with the resonator, resulting in an enhancement of the nonlinear (second-order) transition.
Time evolution caused by microwave irradiation where there is no decoherence in the system. The initial state is . Rabi oscillation makes a superposition of and , which is an entangled state. (a) Populations of the two states. (b) Concurrence of the entangled state.
Time evolution caused by microwave irradiation with relaxation. Linear loss in the resonator is provided. There is no direct decoherence to qubits. (a) Populations in the two states. (b) Concurrence of entanglement. Additional loss in the resonator significantly pollutes or reduces the entanglement of the two qubits.
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