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arXiv:0911.1223 · Journal of Modern Optics

Quantum entanglement in dense multiqubit systems

Mihai Macovei, Jörg Evers, Christoph H. Keitel

Published Apr 6, 2010• 8 citations
arXivHTMLPDF

Abstract

The pairwise entanglement of an arbitrary atomic pair randomly extracted from a laser-driven dense multiqubit sample in the presence of quantum dissipation due to spontaneous emission is considered. The dipole–dipole interaction between the particles shifts the laser-qubit resonance frequency and consequently modifies the quantum entanglement. By means of an appropriate tuning of the laser frequency, one can optimize the entanglement in this system. For large ensembles, the maximum entanglement occurs near the laser parameters where the steady-state sample exhibits phase transition phenomena.

DOI
10.1080/09500341003654443

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Engineering a stationary entanglement between atoms or ions placed at small distances is a challenging problem in quantum information science. In this paper, the stationary and dynamics of entanglement in a system of dipole-coupled qubits interacting with a single-mode optical cavity in the strong coupling regime are theoretically investigated. We find that the entanglement in the steady state can be induced and tuned in nonresonance cases by considering the dipole-dipole interaction (DDI) between the qubits. We also point out that the novel measure used in this study can quantify the net multiqubit entanglement in the system. By increasing the DDI intensity, the behavior of the system depends on the number of qubits. Interestingly, with only two qubits, the amounts of steady-state entanglement enhances as the DDI intensity increases. For a system with more than two qubits, we find that DDI in the weak range of intensity plays a constructive role in the entanglement between qubits. However, it could destroy the stationary entanglement as the interaction between qubits intensifies. The entanglement of a system consisting of more than two qubits tends to disappear for an ensemble of qubits with smaller interatomic distances that lead to stronger dipolar interaction between nonadjacent qubits. Finally, we study the atomic population transfer as well as the transmitted spectrum.
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Recent advances in generating well controlled dense arrangements of individual atoms in free space have generated interest in understanding how the extended nature of these systems influences superradiance phenomena. Here, we provide an in-depth analysis on how space-dependent light shifts and decay rates induced by dipole-dipole interactions modify the steady-state properties of coherently driven arrays of quantum emitters. We characterize the steady-state phase diagram, with particular focus on the radiative properties in the steady state. Interestingly, we find that diverging from the well-established Dicke paradigm of equal all-to-all interactions significantly modifies the emission properties. In particular, the prominent quadratic scaling of the radiated light intensity with particle number in the steady state—a hallmark of steady-state Dicke superradiance—is entirely suppressed, resulting in only linear scaling with particle number. We show that this breakdown of steady-state superradiance occurs due to the emergence of additional dissipation channels that populate not only superradiant states but also subradiant ones. The additional contribution of subradiant dark states in the dynamics leads to a divergence in the time scales needed to achieve steady states. Building on this, we further show that measurements taken at finite times for extended atom ensembles reveal properties closely mirroring the idealized Dicke scenario. Published by the American Physical Society 2024