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Physical Review A

Entanglement, Bell violation, and phase decoherence of two atoms inside an optical cavity

Shangbin Li, Jing‐Bo Xu

Published Aug 23, 2005• 53 citations
Publisher

Abstract

The simple system of two atoms couple to single mode optical cavity with the phase decoherence is studied for investigating the entanglement and Bell violation between atoms and cavity or between two atoms. We show that in the resonance case (i) atom-field entanglement rapidly decays with phase decoherence and disappears in the stationary state, (ii) atom-atom entanglement is more robust against phase decoherence and survives in the stationary state. In the nonresonance case, the pairwise atom-atom and atom-field entanglement is sensitive to the detuning parameter and is not completely destroyed during evolution. On the other hand, violation of Bell-CHSH inequality is very fragile against the phase decoherence and finally disappears in the stationary state. The phenomenon that the more Bell violation, the less entanglement, or vice versa in such a realistic physical system, is revealed. This phenomenon maybe is the consequence of the choice of concurrence as the entanglement measure and the observables to build the Bell-CHSH inequality. The genuine three-partite entanglement is also analyzed by making use of the state preparation fidelity. It is shown that the genuine three-partite entanglement can appear in the evolution of the system even in the presence of the phase decoherence.

DOI
10.1103/physreva.72.022332

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The simple system of two atoms couple to single mode optical cavity with the phase decoherence is studied for investigating the entanglement and Bell violation between atoms and cavity or between two atoms. We show that in the resonance case (i) atom-field entanglement rapidly decays with phase decoherence and disappears in the stationary state, (ii) atom-atom entanglement is more robust against phase decoherence and survives in the stationary state. In the nonresonance case, the pairwise atom-atom and atom-field entanglement is sensitive to the detuning parameter and is not completely destroyed during evolution. On the other hand, violation of Bell-CHSH inequality is very fragile against the phase decoherence and finally disappears in the stationary state. The phenomenon that the more Bell violation, the less entanglement, or vice versa in such a realistic physical system, is revealed. This phenomenon maybe is the consequence of the choice of concurrence as the entanglement measure and the observables to build the Bell-CHSH inequality. The genuine three-partite entanglement is also analyzed by making use of the state preparation fidelity. It is shown that the genuine three-partite entanglement can appear in the evolution of the system even in the presence of the phase decoherence.
Considering the dipole-dipole coupling intensity between two atoms and the field in the Fock state, the entanglement dynamics between two atoms that are initially entangled in the system of two two-level atoms coupled to a single mode cavity in the presence of phase decoherence has been investigated. The two-atom entanglement appears with periodicity without considering phase decoherence, however, the phase decoherence causes the decay of entanglement between two atoms, with the increasing of the phase decoherence coefficient, the entanglement will quickly become a constant value, which is affected by the two-atom initial state. Meanwhile the two-atom quantum state will forever stay in the maximal entangled state when the initial state is proper even in the presence of phase decoherence. On the other hand, the Bell violation and the entanglement do not satisfy the monotonous relation, a large Bell violation implies the presence of a large amount of entanglement under certain conditions, while a large Bell violation corresponds to a little amount of entanglement in certain situations. However, the violation of Bell-CHSH inequality can reach the maximal value if two atoms are in the maximal entangled state, or vice versa.
We investigate the entanglement dynamics in a quantum system consisting of three two-level atoms resonantly coupled to a single mode cavity in the presence of phase decoherence. With the help of the dynamical algebraic method, we obtain an analytical solution of the present system. The influence of phase decoherence on the entanglement of the system is studied. We find that the entanglement between the cavity field and any atom will be destroyed completely by phase decoherence as the system evolves. However, there is long-lived entanglement (or stationary state entanglement) of arbitrary two atoms, that is, the entanglement of atoms is more robust against phase decoherence than that of the atom-field system.
Considering the interaction between two two-level atoms and a single-model field, we find an exact solution of the Milburn equation for the system. We investigate the time evolution of the entanglement between the two atoms and the maximal violation of Bell-CHSH inequality. The influence of dipole-dipole interaction and detuning between the caving and the atom on the entanglement and Bell violation is discussed. The effect of dipole-dipole interaction between two atoms on the entanglement and Bell inequality violation is obvious; with the detuning increasing, the entanglement between two atoms and Bell inequality violation becomes larger and we find that the stationary value of the entanglement between two atoms only depends on the difference between the dipole-dipole coupling intensity and the detuning. The Bell inequality violation and the entanglement does not satisfy the monotonic relation, a large Bell inequality violation may correspond to a small amount of entanglement.
We employ external optical white-noise fields as the actual driving fields of two atoms inside an optical cavity and investigate how controllable entanglement between two atoms arises in such a situation. Two different action configurations of noise are considered. If two atoms are simultaneously driven by two independent white-noise fields with the same intensity, the entanglement between them is suppressed and eventually completely destroyed by the noise. However, if only one atom is exposed in the white-noise fields, the steady state of the two atoms exhibits entanglement. A stochastic-resonance-like behaviour of steady state entanglement is also revealed. Finally, we examine the Bell violation between atoms and show that the steady state does not violate the Bell inequality even though it is inseparable. The stochastic-resonance-like behaviour cannot be observed in the Bell violation of two atoms during the evolution. The stronger the noise intensity, the more rapidly the Bell violation disappears.
Some investigations have been made on the evolution of the linear entropies of atom-field system, in which two identical two-level atoms simultaneously interact with a single-mode dissipative cavity field under large detuning. The effects of the initial atom states and the dissipative factors of the cavity on all linear entropies are also discussed. The results show that the influence of the cavity decay constant on different kinds of Bell states is completely different, and the state |Ψ〉12 is a robust entangled state against the decoherence while the state |Φ〉12 is a fragile entangled state.