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arXiv:1112.4275 · Physical Review A

Correlations in optically controlled quantum emitters

Cristian E. Susa, John Henry Reina

Published Feb 8, 2012• 20 citations
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Abstract

We address the problem of optically controlling and quantifying the dissipative dynamics of quantum and classical correlations in a setup of individual quantum emitters under external laser excitation. We show that both types of correlations, the former measured by the quantum discord, are present in the system's evolution even though the emitters may exhibit an early stage disentanglement. In the absence of external laser pumping, we demonstrate analytically, for a set of suitable initial states, that there is an entropy bound for which quantum discord and entanglement of the emitters are always greater than classical correlations, thus disproving an early conjecture that classical correlations are greater than quantum correlations. Furthermore, we show that quantum correlations can also be greater than classical correlations when the system is driven by a laser field. For scenarios where the emitters' quantum correlations are below their classical counterparts, an optimization of the evolution of the quantum correlations can be carried out by appropriately tailoring the amplitude of the laser field and the emitters' dipole-dipole interaction. We stress the importance of using the entanglement of formation, rather than the concurrence, as the entanglement measure, since the latter can grow beyond the total correlations and thus give incorrect results on the actual system's degree of entanglement.

DOI
10.1103/physreva.85.022111

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We address the problem of optically controlling and quantifying the dissipative dynamics of quantum and classical correlations in a setup of individual quantum emitters under external laser excitation. We show that both types of correlations, the former measured by the quantum discord, are present in the system's evolution even though the emitters may exhibit an early stage disentanglement. In the absence of external laser pumping, we demonstrate analytically, for a set of suitable initial states, that there is an entropy bound for which quantum discord and entanglement of the emitters are always greater than classical correlations, thus disproving an early conjecture that classical correlations are greater than quantum correlations. Furthermore, we show that quantum correlations can also be greater than classical correlations when the system is driven by a laser field. For scenarios where the emitters' quantum correlations are below their classical counterparts, an optimization of the evolution of the quantum correlations can be carried out by appropriately tailoring the amplitude of the laser field and the emitters' dipole-dipole interaction. We stress the importance of using the entanglement of formation, rather than the concurrence, as the entanglement measure, since the latter can grow beyond the total correlations and thus give incorrect results on the actual system's degree of entanglement.
In this paper, we have investigated the dynamics of the quantum correlation between two qubits of a system composed of an arbitrary number of qubits dissipating into a common environment. Our results imply that, when the initial state of the system contains one excitation, the quantum correlation of the quantum discord and the entanglement of formation can be generated between two qubits which have no interaction with each other but are interacting with a common environment. From our analysis, we find that the dynamics of the quantum discord and the entanglement of formation depends not only on the scaled time but also on the size of the system. The larger the size of the system is, the less the quantum discord and the entanglement of formation is generated. By using a comparison between the quantum discord and the entanglement of formation, we find that the quantum discord is more robust to the size of the system than the entanglement of formation is, in the sense that the former decreases more slowly with an increase of the size of the system than the latter does. In addition, the case of the initial state with two excitations is discussed.
Abstract. We report on quantum, classical, and total correlations in a set of distant quantum emitters coupled via their interaction with the plasmon modes of a one-dimensional waveguide driven by an external laser field. The coupling of the emitters with the plasmonic modes and its influence on the collective decay rate suggest that entanglement does not play a significant role in the qubit dynamics. Rather, discord is the quantity that matters and should be harnessed as a resource. PACS numbers: 03.65.Ud, 03.67.Lx, 42.79.Gn 1.
How strongly can one system be correlated with another? In the classical world, this basic question concerning correlative capacity has a very satisfying answer: The ``effective size'' of the marginal system, as quantified by the Shannon entropy, sets a tight upper bound to the correlations, as quantified by the mutual information. Although in the quantum world bipartite correlations, like their classical counterparts, are also well quantified by mutual information, the similarity ends here: The correlations in a bipartite quantum system can be twice as large as the marginal entropy. In the paradigm of quantum discord, the correlations are split into classical and quantum components, and it was conjectured that both the classical and quantum correlations are (like the classical mutual information) bounded above by each subsystem's entropy. In this work, by exploiting the interplay between entanglement of formation, mutual information, and quantum discord, we disprove that conjecture. We further indicate a scheme to restore harmony between quantum and classical correlative capacities. The results illustrate dramatically the asymmetric nature of quantum discord and highlight some subtle and unusual features of quantum correlations.
We have studied the dynamics of quantum correlations such as entanglement, Bell nonlocality and quantum discord between identical and unidentical atoms interacting with a single-mode cavity field and subject to cavity decay. The effect of single-atom detuning, cavity decay rate and initial preparation of the atoms on the corresponding correlation measures have been investigated. It is found that even under strong dissipation, time evolution can create high quantum discord while entanglement and Bell nonlocality stay zero for an initially separable state. Quantum discord increases while entanglement decreases in a certain time period under dissipation for the initial state that both atoms are in the excited state if the qubits are identical. For some types of initial states, cavity decay is shown to drive the system to a stationary state with high entanglement and quantum discord.
We investigate analytically the dynamics of classical and quantum correlations between two strongly driven atoms, each of which is trapped inside a dissipative cavity. It is found that there exists a finite time interval during which the quantum discord initially prepared in the X -type states is not destroyed by the decay of the cavities. The sudden transition between classical correlation and quantum discord is sensitive to the initial-state parameter, the cavity decay rate, and the cavity mode-driving field detuning. Interestingly, we show that the transition time can be prolonged significantly by increasing the degree of the detuning.