Considering two atomic qubits initially in Bell states, we send one qubit into a vacuum cavity with two-photon resonance and leave the other one outside. Using quantum information entropy squeezing theory, the time evolutions of the entropy squeezing factor of the atomic qubit inside the cavity are discussed for two cases, i.e., before and after rotation and measurement of the atomic qubit outside the cavity. It is shown that the atomic qubit inside the cavity has no entropy squeezing phenomenon and is always in a decoherent state before the operating atomic qubit outside the cavity. However, the periodical entropy squeezing phenomenon emerges and the optimal entropy squeezing state can be prepared for the atomic qubit inside the cavity by adjusting the rotation angle, choosing the interaction time between the atomic qubit and the cavity, controlling the probability amplitudes of subsystem states. Its physical essence is cutting the entanglement between the atomic qubit and its environment, causing the atomic qubit inside the cavity to change from the initial decoherent state into maximum coherent superposition state, which is a possible way of recovering the coherence of a single atomic qubit in the noise environment.
By solving the Milburn equation, we investigate the thermal entanglement properties of a two-qubit Heisenberg XY chain in the presence of intrinsic decoherence. The controls of nonuniform magnetic field, the initial state of two qubits, the relative phases and the amplitudes of the polarized qubits on thermal entanglement are studied. The results show that for a particular initial state, the thermal entanglement can be increased by the external magnetic field. The time behavior of the entanglement exhibits a strong dependence on the initial state of two qubits, and it can be manipulated by changing the relative phase and the amplitudes of the polarized qubits. It is also notable that stable entanglement, which is dependent on initial state of the qubit, occurs even in the presence of decoherence. The magnetic field may have a constructive effect on the stable entanglement for a certain initial state, and the Bell-diagonal state turns out to be a dark state of the system in the absence of the magnetic field.
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