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Journal of Optics B Quantum and Semiclassical Optics

The effect of finite bandwidth squeezed light on entanglement creation in the Dicke model

Azeddine Messikh, Mohamed Ridza Wahiddin, Chin Hee Pah, Z. Ficek

Published Apr 27, 2004• 4 citations
Publisher

Abstract

We analyse the relation between local two-atom and total multi-atom entanglements in the Dicke system composed of a large number of atoms. We use concurrence as a measure of entanglement between two atoms in the multi-atom system, and the spin squeezing parameter as a measure of entanglement in the whole n -atom system. In addition, the influence of the squeezing phase and bandwidth on entanglement in the steady-state Dicke system is discussed. It is shown that the introduction of a squeezed field leads to a significant enhancement of entanglement between two atoms, and the entanglement increases with increasing degree of squeezing and bandwidth of the incident squeezed field. In the presence of a coherent field the entanglement exhibits a strong dependence on the relative phase between the squeezed and coherent fields, that can jump quite rapidly from unentangled to strongly entangled values when the phase changes from zero to π. We find that the jump of the degree of entanglement is due to a flip of the spin squeezing from one quadrature component of the atomic spin to the other component when the phase changes from zero to π. We also analyse the dependence of the entanglement on the number of atoms and find that, despite the reduction in the degree of entanglement between two atoms, a large entanglement is present in the whole n -atom system and the degree of entanglement increases as the number of atoms increases.

DOI
10.1088/1464-4266/6/7/005

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We analyse the relation between local two-atom and total multi-atom entanglements in the Dicke system composed of a large number of atoms. We use concurrence as a measure of entanglement between two atoms in the multi-atom system, and the spin squeezing parameter as a measure of entanglement in the whole n -atom system. In addition, the influence of the squeezing phase and bandwidth on entanglement in the steady-state Dicke system is discussed. It is shown that the introduction of a squeezed field leads to a significant enhancement of entanglement between two atoms, and the entanglement increases with increasing degree of squeezing and bandwidth of the incident squeezed field. In the presence of a coherent field the entanglement exhibits a strong dependence on the relative phase between the squeezed and coherent fields, that can jump quite rapidly from unentangled to strongly entangled values when the phase changes from zero to π. We find that the jump of the degree of entanglement is due to a flip of the spin squeezing from one quadrature component of the atomic spin to the other component when the phase changes from zero to π. We also analyse the dependence of the entanglement on the number of atoms and find that, despite the reduction in the degree of entanglement between two atoms, a large entanglement is present in the whole n -atom system and the degree of entanglement increases as the number of atoms increases.
The quantum entanglement of two entangled atoms interacting with the squeezed coherent state is studied by means of full quantum theory. The influences of coherent amplitude factor, squeezing factor, ratio of two coupling coefficients on the entanglement property are discussed. The results show that two atoms entanglement evolution property is opposed to atom-field entanglement evolution property when the coherent amplitude factor was zero or small. The interaction of the atoms and field weakens the entanglement degree of atom-atom. With increasing coherent amplitude factor or decreasing squeezing factor of light, the time of two atoms staying in the steady entangled state becomes longer. The increase of the coherent amplitude factor or the decrease of squeezing factor of light lead to the increase of atom-atom entanglement degree while the decrease of atoms-field entanglement degree. Moreover, the two-atom quantum state stays in the maximum entangled state when the atomic dipole-dipole coupling coefficient is large enough.
We analyze the relation between the entanglement and spin-squeezing parameter in the two-atom Dicke model and identify the source of the discrepancy recently reported by Banerjee and Zhou et al that one can observe entanglement without spin squeezing. Our calculations demonstrate that there are two criteria for entanglement, one associated with the two-photon coherences that create two-photon entangled states, and the other associated with populations of the collective states. We find that the spin-squeezing parameter correctly predicts entanglement in the two-atom Dicke system only if it is associated with two-photon entangled states, but fails to predict entanglement when it is associated with the entangled symmetric state. This explicitly identifies the source of the discrepancy and explains why the system can be entangled without spin-squeezing. We illustrate these findings in three examples of the interaction of the system with thermal, classical squeezed vacuum and quantum squeezed vacuum fields.
We analyze the relation between the entanglement and spin-squeezing parameter in the two-atom Dicke model and identify the source of the discrepancy recently reported by Banerjee and Zhou et al that one can observe entanglement without spin squeezing. Our calculations demonstrate that there are two criteria for entanglement, one associated with the two-photon coherences that create two-photon entangled states, and the other associated with populations of the collective states. We find that the spin-squeezing parameter correctly predicts entanglement in the two-atom Dicke system only if it is associated with two-photon entangled states, but fails to predict entanglement when it is associated with the entangled symmetric state. This explicitly identifies the source of the discrepancy and explains why the system can be entangled without spin-squeezing. We illustrate these findings in three examples of the interaction of the system with thermal, classical squeezed vacuum and quantum squeezed vacuum fields.
We calculate the concurrence and spin squeezing parameter of three atoms induced by a coherent field. It shows that when the mean numbers of photon of the field is very small, concurrence exhibits a certain kind of approximative periodic vibration. With the increase of the mean numbers of photon, its periodicity will be interrupted. As for the relationship between the concurrence and spin squeezing, numeric result shows that under Dick model, spin squeezing serves as a sufficient but not necessary condition for concurrence.
The field entropy evolution in a system which is composed of two moving entangled atoms interacting with the squeezed coherent state is studied by means of the full quantum theory.The influences of the coherent amplitude factor,the squeezing factor,the atomic motion and the field-model structure parameter on the field entropy are discussed.The result shows that the atomic motion leads to the periodic evolution of the field entropy.An increase of coherent amplitude factor weaken the degree of entanglement between the atoms and the light field.However,an increase of squeezing factor of light strengthen the degree of entanglement between the atoms and the light field.With increasing atomic speed or field-model structure parameter,the evolution period of the field entropy becomes shorter,and the degree of entanglement between field and moving atoms becomes smaller.
We consider a system of finite number of particles collectively interacting with a single-mode coherent field inside a cavity. Depending on the strength of the initial field compared to the number of atoms, we consider three regimes of weak-, intermediate-, and strong-field interaction. The dynamics of multiparticle entanglement detected by quantum Fisher information and spin squeezing are studied in each regime. It is seen that in the weak-field regime, spin squeezing and quantum Fisher information coincide. However, by increasing the initial field population toward the strong-field regime, quantum Fisher information is more effective in detecting entanglement compared to spin squeezing. In addition, in the two-atom system, we also study concurrence. In this case, the quantum Fisher information as a function of time is in good agreement with concurrence in predicting entanglement peaks.