A generalized multi-polaron expansion for the spin-boson model: Environmental entanglement and the biased two-state system

We develop a systematic variational coherent state expansion for the many-body ground state of the spin-boson model, in which a quantum two-level system is coupled to a continuum of harmonic oscillators. Energetic constraints at the heart of this technique are rationalized in terms of polarons (displacements of the bath states in agreement with classical expectations) and antipolarons (counter-displacements due to quantum tunneling effects). We present a comprehensive study of the ground state two-level system population and coherence as a function of tunneling amplitude, dissipation strength, and bias (akin to asymmetry of the double well potential defining the two-state system). The entanglement among the different environmental modes is investigated by looking at spectroscopic signatures of the bipartite entanglement entropy between a given environmental mode and all the other modes. We observe a drastic change in behavior of this entropy for increasing dissipation, indicative of the entangled nature of the environmental states. In addition, the entropy spreads over a large energy range at strong dissipation, a testimony to the wide entanglement window characterizing the underlying Kondo state. Finally, comparisons to accurate numerical renormalization group calculations and to the exact Bethe Ansatz solution of the model demonstrate the rapid convergence of our variationally-optimized multi-polaron expansion, suggesting that it should also be a useful tool for dissipative models of greater complexity, as relevant for numerous systems of interest in quantum physics and chemistry.Comment: 17 pages, 14 figure

Coherent and incoherent dynamics in excitonic energy transfer: correlated fluctuations and off-resonance effects

We study the nature of the energy transfer process within a pair of coupled two-level systems (donor and acceptor) subject to interactions with the surrounding environment. Going beyond a standard weak-coupling approach, we derive a master equation within the polaron representation that allows for investigation of both weak and strong system-bath couplings, as well as reliable interpolation between these two limits. With this theory, we are then able to explore both coherent and incoherent regimes of energy transfer within the donor-acceptor pair. We elucidate how the degree of correlation in the donor and acceptor fluctuations, the donor-acceptor energy mismatch, and the range of the environment frequency distribution impact upon the energy transfer dynamics. In the resonant case (no energy mismatch) we describe in detail how a crossover from coherent to incoherent transfer dynamics occurs with increasing temperature [A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)], and we also explore how fluctuation correlations are able to protect coherence in the energy transfer process. We show that a strict crossover criterion is harder to define when off-resonance, though we find qualitatively similar population dynamics to the resonant case with increasing temperature, while the amplitude of coherent population oscillations also becomes suppressed with growing site energy mismatch.Comment: 14 pages, 7 figures, builds upon PRL 103, 146404 (2009) (arXiv:0906.0592). Comments welcome. V2 - Section IV shortened to improve presentation, references updated, new Imperial College affiliation added for A. Nazir. Published versio

Stabilizing Spin Coherence Through Environmental Entanglement in Strongly Dissipative Quantum Systems

The key feature of a quantum spin coupled to a harmonic bath---a model dissipative quantum system---is competition between oscillator potential energy and spin tunneling rate. We show that these opposing tendencies cause environmental entanglement through superpositions of adiabatic and antiadiabatic oscillator states, which then stabilizes the spin coherence against strong dissipation. This insight motivates a fast-converging variational coherent-state expansion for the many-body ground state of the spin-boson model, which we substantiate via numerical quantum tomography.Comment: 5 pages, 3 figures, supplementary file attached. This article supersedes arXiv:1301.743

Flood vulnerability and resilience: Exploring the factors that influence flooding in Sarawak

Flooding is the most common natural disaster in Malaysia, resulting in chaos and disruptions to social and economic activities, damage to roads and railway tracks, vehicles, affecting the level of properties, loss of life, and rise in vulnerability. In addition to natural causes, floods are mainly attributed to continuous heavy rainfall, rapid development, unplanned urbanization, poor drainage system, and environmental degradation. This annual occurrence of floods has given a big impact on the lives of humans and other living beings. Due to the negative impact of floods, we need to pay serious attention and take alternative ways to reduce this disaster. This study aimed to investigate the vulnerability factors and understand the concept of vulnerability and resilience to manage floods in Sarawak. The researcher primarily focused on reviewing the printed and documented material of disasters in order to find the vulnerability factors. Based on the findings, it can be concluded that nature, human, and governance factors, contributed mostly to floods. The present study shows that vulnerability reduction along with emergency preparedness can reduce flood damages and fatalities. The innovative technologies must have to be considered to reduce vulnerability and build resilience for sustainable disaster risk reduction

Strategic optimization framework considering unobservability in multi-voltage active distribution networks

An increase in the share of weather-dependent generation at low voltage levels necessitates incorporating the low-voltage network in optimizing a distribution network. Optimization in a multi-voltage network requires significant computation time and effort due to many nodes operating at different voltage levels. This research proposes a decomposition and strategic optimization method to reduce the computation requirements for such large multi-voltage distribution networks. The proposed algorithm reduces the space complexity and the computation time required for solving the optimization routines of these multi-voltage distribution networks. A virtual transformer model incorporates tap-changer as a continuous variable in the semidefinite programming power flow optimization model. The zero-duality gap condition for multiple virtual transformers is proven empirically. Compared to a centralized optimization using the same power flow model, the proposed framework reduced the computation time by 96%.</p

Comparison of different cephalometric variables in patients with and without history of snoring

Background: Obstructive sleep apnea syndrome has a high prevalence among adults. Cephalometric variables can be a valuable method for evaluating patients with this syndrome. The objective of this study is to evaluate and compare the craniofacial variables in subjects with and without snoring by cephalometric analysis.Methods: 40 patients were included in the study who were divided into two groups; group 1 of 20 snoring patients and group 2 control group of 20 non-snoring patients and were evaluated and compared for various cephalometric variables.Results: The distance from the hyoid bone to the mandibular plane (MP-H) was the only variable that showed a statistically significant correlation between two groups.Conclusions: Cephalometric variables are useful tools for evaluating the patients with snoring who are predisposed to obstructive sleep apnoea. The distance from the hyoid bone to the mandibular plane showed a statistically significant correlation between two groups

Molecular basis of arsenite (As+3)-induced acute cytotoxicity in human cervical epithelial carcinoma cells

Background: Rapid industrialization is discharging toxic heavy metals into the environment, disturbing human health in many ways and causing various neurologic, cardiovascular, and dermatologic abnormalities and certain types of cancer. The presence of arsenic in drinking water from different urban and rural areas of the major cities of Pakistan, for example, Lahore, Faisalabad, and Kasur, was found to be beyond the permissible limit of 10 parts per billion set by the World Health Organization. Therefore the present study was initiated to examine the effects of arsenite (As+3) on DNA biosynthesis and cell death.Methods: After performing cytotoxic assays on a human epithelial carcinoma cell line, expression analysis was done by quantitative polymerase chain reaction, western blotting, and flow cytometry.Results: We show that As+3 ions have a dose- and time-dependent cytotoxic effect through the activation of the caspase-dependent apoptotic pathway. In contrast to previous research, the present study was designed to explore the early cytotoxic effects produced in human cells during exposure to heavy dosage of As+3 (7.5 μg/ml). Even treatment for 1 h significantly increased the mRNA levels of p21 and p27 and caspases 3, 7, and 9. It was interesting that there was no change in the expression levels of p53, which plays an important role in G2/M phase cell cycle arrest.Conclusion: Our results indicate that sudden exposure of cells to arsenite (As+3) resulted in cytotoxicity and mitochondrial-mediated apoptosis resulting from up-regulation of caspases.Keywords: apoptosis; epithelial carcinoma; cytotoxicity; arsenite; caspases; Pakistan Responsible Editor: Amin Bredan, VIB Inflammation Research Center &amp; Ghent University, Belgium

Separation-dependent localization in a two-impurity spin-boson model

Using a variational approach we investigate the delocalized to localized crossover in the ground state of an Ohmic two-impurity spin-boson model, describing two otherwise non-interacting spins coupled to a common bosonic environment. We show that a competition between an environment-induced Ising spin interaction and externally applied fields leads to variations in the system-bath coupling strength, $\alpha_c$, at which the delocalized-localized crossover occurs. Specifically, the crossover regime lies between $\alpha_c=0.5$ and $\alpha_c=1$ depending upon the spin separation and the strength of the transverse tunneling field. This is in contrast to the analogous single spin case, for which the crossover occurs (in the scaling limit) at fixed $\alpha_c\approx1$. We also discuss links between the two-impurity spin-boson model and a dissipative two-spin transverse Ising model, showing that the latter possesses the same qualitative features as the Ising strength is varied. Finally, we show that signatures of the crossover may be observed in single impurity observables, as well as in the behaviour of the system-environment entanglement.Comment: 12 pages, 9 figures. Published version. Expanded discussion of the distance dependence between the impurities, and added a related figur