Entanglement between charge qubit and quantized field

We investigate time evolution of the entanglement between charge qubit initially in mixed states and quantized field. By calculating the concurrence lower bound, we study the effect of initial mixedness λ and detuning Δ on the entanglement of the system. In the case of weak field, the degree of entanglement for charge qubit being initially in excited state is much larger than that for charge qubit being initially in ground state, and detuning Δ can efficiently reduce the entanglement. But in the case of strong field, the time evolution of the entanglement is about the same for charge qubit being initially in excited state and in ground state, the more mixed the initial state of charge qubit is prepared, the weaker the entanglement is, and Δ has less effect on the entanglement than in the case of weak field.