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Research in Optical Sciences

Dissipation-boosted entanglement in coupled harmonic oscillators

Chaitanya S. Joshi, Michael J. W. Hall, M. Jonson, Patrik Öhberg, Erika Andersson

Published Jan 1, 2012• 0 citations
Publisher

Abstract

We show that entanglement in initially classical states of coupled harmonic oscillators, caused by squeezing, is enhanced by dissipation. The enhancement vanishes if the oscillator baths are identical, suggesting that “heat flow” may be necessary.
DOI
10.1364/qim.2012.qt3a.2

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We theoretically investigate a system of two coupled bosonic modes subject to both dissipation and external driving. We show that in the steady state the degree of entanglement between the coupled bosonic modes can be enhanced by dissipation. The non-monotonic dependence of entanglement on the decay rates is observed when the bosonic modes are asymmetrically coupled to their local baths. This counterintuitive result opens a new way to better understand the interplay between noise and coherence in continuous variable systems driven away from equilibrium.
The entanglement between a Pauli-like two-level system and a quantum harmonic oscillator enhanced by an interaction between them and a δδδ-pulse sequence is studied, with the decoherence due to their coupling with a Markovian bath. Without the Markovian bath, the entanglement is enhanced to maximum possible values. With the Markovian bath, the entanglement is enhanced up to some time and then dissipated, with the system in thermal equilibrium with the Markovian bath after a very long time. The time for achieving the maximum entanglement shows discontinuous jumps over the parameters of decoherence.
We investigate the dynamical enhancement of entanglement by means of local squeezing of initial separable Gaussian states in two coupled harmonic oscillator systems. Interestingly, the maximum generation of entanglement between the two modes is not a monotonically increasing function of the squeezing parameter. Instead, there exists a critical degree of squeezing r c for the maximum entanglement to be enhanced. For the coupled oscillator systems considered, we have derived the analytical expressions of r c .
We study a quantum harmonic oscillator linearly coupled through the position operator to a first bath and through the momentum operator to a second bath yielding an Ohmic–Drude dissipation. We analyse the oscillator's fluctuations as a function of the ratio between the strength of the two couplings, focusing in particular on the situation in which the two dissipative interactions are comparable. Analytic formulas are derived in the relevant regimes corresponding to the low temperature limit and when the Drude high frequency cutoff is much larger than all other frequencies. At low temperature, each bath operates to suppress the oscillator's ground state quantum fluctuations or appearing in the corresponding interaction. When one of the two dissipative interactions dominates over the other, the fluctuations for the coupling operator are squeezed. When the two interactions are comparable, the two baths enter in competition as the two conjugate operators do not commute yielding quantum frustration . In this regime, remarkably, the fluctuations of both two quadratures can be enhanced by increasing the dissipative coupling.
We investigate the quantum-classical border, the entanglement and decoherence of an analytically solvable model, comprising a first subsystem (a harmonic oscillator) coupled to a driven and damped second subsystem (another harmonic oscillator). We choose initial states whose dynamics is confined to a couple of two-level systems, and show that the maximum value of entanglement between the two subsystems, as measured by concurrence, depends on the dissipation rate to the coupling-constant ratio and the initial state. While in a related model the entropy of the first subsystem (a two-level system) never grows appreciably (for large dissipation rates), in our model it reaches a maximum before decreasing. Although both models predict small values of entanglement and dissipation, for fixed times of the order of the inverse of the coupling constant and large dissipation rates, these quantities decrease faster, as a function of the ratio of the dissipation rate to the coupling constant, in our model.