Realizable spin models and entanglement dynamics in superconducting flux qubit systems

Abstract

Realizable spin models are investigated in a two superconducting flux qubit system. It is shown that a specific adjustment of system parameters in the two flux qubit system makes it possible to realize an artificial two-spin system that cannot be found naturally. For the artificial two-spin systems, time evolution of a prepared quantum state is discussed to quantify quantum entanglement dynamics. The concurrence and fidelity as a function of time are shown to reveal a characteristic entanglement dynamics of the artificial spin systems. It is found that the unentangled input state can evolute to be a maximally entangled output state periodically due to the exchange interactions induced by two-qubit flipping tunneling processes while single-qubit flipping tunneling processes plays a role of magnetic fields for the artificial spins. [6,7,8]. Moreover, such realizations of artificial spin systems make possible to observe entangled states of two qubits [1,2,4,5]. Indeed, for the time evolution of states in the experiments of charge [1] and phase qubits [4], a partial entanglement has been observed. An experiment of a capacitively coupled two phase qubits [5] shows that higher fidelity for the entanglement exhibits in an excited level. The higher fidelity is caused by two-qubit tunneling processes [9] between two qubit states, i.e., flipping both qubits. Such a two-qubit tunneling processes contributes exchange interactions between the two artificial spins.