We investigate the freezing and sudden transition in the dynamical behavior of quantum and classical correlations in a system composed of two identical non-interacting qubits locally subjected to their own non-equilibrium environments. In contrast to the equilibrium case, one can observe striking results when a bipartite quantum system couples with the non-equilibrium dephasing environment with non-stationary and non-Markovian features. Remarkably, the finite time interval in which the quantum correlation remains impervious to decoherence can be further prolonged as the environment deviates from equilibrium. This reveals that the non-equilibrium parameter provides an alternative tool to efficiently control the appearance of a sudden transition in the decay rates of correlations and their immunity towards the decoherence. Furthermore, for certain initial states, the appearance of another time-interval over which quantum correlation remains constant and the revival of classical correlation not only depends on the non-Markovianity but also on the non-equilibrium parameter.

Protection of the quantum coherence and discord in realistic quantum systems interacting with the environment of depolarizing noise is an important subject in quantum information processing. Weak measurement and measurement reversal can effectively suppress the amplitude damping-class decoherence. In this paper, we examine the effect of this protocol in the protection of quantum coherence and discord subjected to depolarizing noise environments. Our scheme consists of a prior weak measurement on each qubit before interacting with noisy channels followed by post measurement reversal. It is found that quantum coherence and discord can be enhanced to an optimal value by performing weak measurements and adjusting measurement parameters on each qubit. In addition, the maximal value of the quantum coherence and discord is found to be independent of the initial-state parameters.

We investigate the time evolution of quantum correlations of a hybrid qubit-qutrit system under the classical Ornstein-Uhlenbeck (OU) noise. Here we consider two different one-parameter families of qubit-qutrit states which independently interact with the non-Markovian reservoirs. A comparison with the Markovian dynamics reveals that for the same set of initial condition parameters, the non-Markovian behavior of the environment plays an important role in the enhancement of the survival time of quantum correlations. In addition, it is observed that the non-Markovian strength has a positive impact on the correlations time. For the initial separable states it is found that there is a finite time interval in which the geometric quantum discord is frozen despite the presence of a noisy environment and that interval can be further prolonged by using the non-Markovian property. Moreover, its decay can be significantly delayed.

It is well known that externally generated resonant magnetic perturbations (RMPs) can form islands in the plasma edge. In turn, large overlapping islands generate stochastic fields, which are believed to play a role in the avoidance and suppression of edge localized modes (ELMs) at DIII-D. However, large coalescing islands can also generate, in the middle of these stochastic regions, KAM surfaces effectively acting as "barriers" against field-line dispersion and, indirectly, particle diffusion. It was predicted in [H. Ali and A. Punjabi, Plasma Phys. Control. Fusion 49 (2007), 1565-1582] that such magnetic barriers can form in piecewise analytic DIII-D plasma equilibria. In the present work, the formation of magnetic barriers at DIII-D is corroborated by field-line tracing calculations using experimentally constrained EFIT [L. Lao, et al.,, Nucl. Fusion 25, 1611] DIII-D equilibria perturbed to include the vacuum field from the internal coils utilized in the experiments. According to these calculations, the occurrence and location of magnetic barriers depends on the edge safety factor q 95 . It was thus suggested that magnetic barriers might contribute to narrowing the edge stochastic layer and play an 2 indirect role in the RMPs failing to control ELMs for certain values of q 95 . The analysis of DIII-D discharges where q 95 was varied, however, does not show anti-correlation between barrier formation and ELM suppression. IntroductionChaotic fields [1] and magnetic barriers emerging from chaos [2] recently received a great deal of attention in magnetic confinement research for their capability to, respectively, suppress edge localized modes (ELMs) in tokamaks [3,4] and improve confinement in reversed field pinches [5]. The DIII-D tokamak creates stochastic fields with resonant magnetic perturbations (RMPs), typically of toroidal mode number n=3, by means of control coils external or internal to the vessel, called respectively C-coils and I-coils. These stochastic fields reduce the local density gradient, and are experimentally observed to suppress ELMs [1, 3, 4]. This and other means of avoiding or suppressing ELMs are very important for ITER, where ELMs could significantly shorten the lifetime of the divertor and first wall [6]. One of the limitations of RMPs, however, is that experimentally they only suppress ELMs for certain values of the edge safety factor q 95 .Recent evidence suggests that this is connected with a modulation of thermal transport and of the width of the stochastic layer [7], but the fundamental reason of the q 95 dependence remains still unclear.Here it is proposed that the modulation of the width of the outermost stochastic layer might be due to the formation and disappearance of magnetic barriers. Kolmogorov-Arnold-Moser (KAM) surfaces [8][9][10][11] can form in the middle of chaotic regions. In the context of magnetic confinement, KAM surfaces locally inhibit the dispersion of magnetic field-lines and are therefore also known as magnetic barriers. They were predicted theoretically [12] ...

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