Discrepancies between decoherence and the Loschmidt echo

The Loschmidt echo and the purity are two quantities that can provide invaluable information about the evolution of a quantum system. While the Loschmidt echo characterizes instability and sensitivity to perturbations, purity measures the loss of coherence produced by an environment coupled to the system. For classically chaotic systems both quantities display a number of -- supposedly universal -- regimes that can lead on to think of them as equivalent quantities. We study the decay of the Loschmidt echo and the purity for systems with finite dimensional Hilbert space and present numerical evidence of some fundamental differences between them.Comment: 6 pages, 3 figures. Changed title. Added 1 figure. Published version

Matrix Product Density Operators: Renormalization Fixed Points and Boundary Theories

We consider the tensors generating matrix product states and density operators in a spin chain. For pure states, we revise the renormalization procedure introduced by F. Verstraete et al. in 2005 and characterize the tensors corresponding to the fixed points. We relate them to the states possessing zero correlation length, saturation of the area law, as well as to those which generate ground states of local and commuting Hamiltonians. For mixed states, we introduce the concept of renormalization fixed points and characterize the corresponding tensors. We also relate them to concepts like finite correlation length, saturation of the area law, as well as to those which generate Gibbs states of local and commuting Hamiltonians. One of the main result of this work is that the resulting fixed points can be associated to the boundary theories of two-dimensional topological states, through the bulk-boundary correspondence introduced by Cirac et al. in 2011.Comment: 63 pages, Annals of Physics (2016). Accepted versio

Nonsingular electrovacuum solutions with dynamically generated cosmological constant

We consider static spherically symmetric configurations in a Palatini extension of General Relativity including $R^2$ and Ricci-squared terms, which is known to replace the central singularity by a wormhole in the electrovacuum case. We modify the matter sector of the theory by adding to the usual Maxwell term a nonlinear electromagnetic extension which is known to implement a confinement mechanism in flat space. One feature of the resulting theory is that the non-linear electric field leads to a dynamically generated cosmological constant. We show that with this matter source the solutions of the model are asymptotically de Sitter and possess a wormhole topology. We discuss in some detail the conditions that guarantee the absence of singularities and of traversable wormholes.Comment: 7 double-column pages; v2: several changes in abstract and introductio

Matrix Product State Representations

This work gives a detailed investigation of matrix product state (MPS) representations for pure multipartite quantum states. We determine the freedom in representations with and without translation symmetry, derive respective canonical forms and provide efficient methods for obtaining them. Results on frustration free Hamiltonians and the generation of MPS are extended, and the use of the MPS-representation for classical simulations of quantum systems is discussed.Comment: Minor changes. To appear in QI

Dental Service Utilization and Neighborhood Characteristics in Young Adults in The United States: A Multilevel Approach

Objective: To investigate the association between neighborhood level factors and dental visits in young adults in the United States after adjusting for individual level factors. Methods: The National Longitudinal Study of Adolescent Health Wave 1 (1994-1995) to Wave III (2001-2002) was analyzed. The primary outcome of having had at least one dental visit in the previous 12 months was analyzed via a multilevel random-effects logistic model accounting for geographic clustering in Wave III and survey design clustering from Wave I. Neighborhood level covariates were defined at the census tract level. Results: Overall rate of dental visits was 57 percent, highest among 18-20 year olds (65 percent) and lowest in 23-26 year olds (52 percent). Increased proportion of African-Americans (≤5 percent to ≥20 percent) and Hispanics (≤5 percent to ≥20 percent) in a neighborhood corresponded with a decrease in dental visits (60 percent versus 52 percent) and (58 percent versus 51 percent), respectively. Neighborhoods with a high proportion of college-educated residents had a higher percentage of dental visits. Similar differences were found when comparing the lowest and highest tertiles defined by poverty level and unemployment with dental visits. Neighborhood education was significantly associated with dental service utilization after adjustment for individual level factors and dental utilization in adolescence (Waves I and II) in the random effects model. Conclusions: This study demonstrates that the education level of residents within a neighborhood was associated with dental service utilization in young adults in the United States

Unbounded violations of bipartite Bell Inequalities via Operator Space theory

In this work we show that bipartite quantum states with local Hilbert space dimension n can violate a Bell inequality by a factor of order $\sqrt{n}$ (up to a logarithmic factor) when observables with n possible outcomes are used. A central tool in the analysis is a close relation between this problem and operator space theory and, in particular, the very recent noncommutative $L_p$ embedding theory. As a consequence of this result, we obtain better Hilbert space dimension witnesses and quantum violations of Bell inequalities with better resistance to noise

Predictors of Dental Care Use: Findings From the National Longitudinal Study of Adolescent Health

Purpose To examine longitudinal trends and associated factors in dental service utilization by adolescents progressing to early adulthood in the United States. Methods The data source was the National Longitudinal Study of Adolescent Health from Waves I (1994–1995), II (1996), III (2001–2002), and IV (2007–2008). This was a retrospective, observational study of adolescents\u27 transition to early adulthood. We obtained descriptive statistics and performed logistic regression analyses to identify the effects of baseline and concurrent covariates on dental service utilization from adolescence to early adulthood over time. Results Dental service utilization within the prior 12 months peaked at age 16 (72%), gradually decreased until age 21 (57%), and remained flat thereafter. Whites and Asians had a 10–20 percentage points higher proportion of dental service utilization at most ages compared with Blacks and Hispanics. Dental service utilization at later follow-up visits was strongly associated with baseline utilization, with odds ratio = 10.7, 2.4, and 1.5 at the 1-, 7-, and 13-year follow-ups, respectively. These effects decreased when they were adjusted for current income, insurance, and education. Compared with Whites, Blacks were consistently less likely to report a dental examination. Conclusions Dental service utilization was highest in adolescence. Gender, education, health insurance, and income in young adulthood were significant predictors in reporting a dental examination. Blacks had lower odds of reporting a dental examination, either as adolescents or as young adults

What Do We Know about Recent Exchange Rate Models? In-Sample Fit and Out-of-Sample Performance Evaluated

Previous assessments of nominal exchange rate determination have focused upon a narrow set of models typically of the 1970’s vintage, including monetary and portfolio balance models. In this paper we re-assess the in-sample fit and out-of-sample prediction of a wider set of models that have been proposed in the last decade, namely interest rate parity, productivitybased models, and "behavioral equilibrium exchange rate" models. These models are compared against a benchmark model, the Dornbusch-Frankel sticky price monetary model. First, the parameter estimates of the models are compared against the theoretically predicted values. Second, we conduct an extensive out-of-sample forecasting exercise, using the last eight years of data to determine whether our in-sample conclusions hold up. We examine model performance at various forecast horizons (1 quarter, 4 quarters, 20 quarters) using differing metrics (mean squared error, direction of change), as well as the “consistency” test of Cheung and Chinn (1998). We find that no model fits the data particularly well, nor does any model consistently out-predict a random walk, even at long horizons. There is little correspondence between how well a model conforms to theoretical priors and how well the model performs in a prediction context. However, we do confirm previous findings that out-performance of a random walk is more likely at long horizons.exchange rates, monetary model, productivity, interest rate parity, behavioral equilibrium exchange rate model, forecasting performance