5,449 research outputs found
Decoherence in a dynamical quantum phase transition
Motivated by the similarity between adiabatic quantum algorithms and quantum
phase transitions, we study the impact of decoherence on the sweep through a
second-order quantum phase transition for the prototypical example of the Ising
chain in a transverse field and compare it to the adiabatic version of Grovers
search algorithm, which displays a first order quantum phase transition. For
site-independent and site-dependent coupling strengths as well as different
operator couplings, the results show that (in contrast to first-order
transitions) the impact of decoherence caused by a weak coupling to a rather
general environment increases with system size (i.e., number of spins/qubits).
This might limit the scalability of the corresponding adiabatic quantum
algorithm.Comment: 14 pages, 9 figure
Entanglement correlations, Bell inequalities, and the concurrence
Probabilities of measurement outcomes of two-particle entangled states give a
physically transparent interpretation of the concurrence and of the
I-concurrence as entanglement measures. The (I)-concurrence can thus be
measured experimentally. The tight connection between these measures and Bell
inequalities is highlighted.Comment: Updated version of quant-ph/0110139, with extension to mixed states.
To appear in Phys. Lett.
Integration of microRNA changes in vivo identifies novel molecular features of muscle insulin resistance in type 2 diabetes
Skeletal muscle insulin resistance (IR) is considered a critical component of type II diabetes, yet to date IR has evaded characterization at the global gene expression level in humans. MicroRNAs (miRNAs) are considered fine-scale rheostats of protein-coding gene product abundance. The relative importance and mode of action of miRNAs in human complex diseases remains to be fully elucidated. We produce a global map of coding and non-coding RNAs in human muscle IR with the aim of identifying novel disease biomarkers. We profiled >47,000 mRNA sequences and >500 human miRNAs using gene-chips and 118 subjects (n = 71 patients versus n = 47 controls). A tissue-specific gene-ranking system was developed to stratify thousands of miRNA target-genes, removing false positives, yielding a weighted inhibitor score, which integrated the net impact of both up- and down-regulated miRNAs. Both informatic and protein detection validation was used to verify the predictions of in vivo changes. The muscle mRNA transcriptome is invariant with respect to insulin or glucose homeostasis. In contrast, a third of miRNAs detected in muscle were altered in disease (n = 62), many changing prior to the onset of clinical diabetes. The novel ranking metric identified six canonical pathways with proven links to metabolic disease while the control data demonstrated no enrichment. The Benjamini-Hochberg adjusted Gene Ontology profile of the highest ranked targets was metabolic (P < 7.4 × 10-8), post-translational modification (P < 9.7 × 10-5) and developmental (P < 1.3 × 10-6) processes. Protein profiling of six development-related genes validated the predictions. Brain-derived neurotrophic factor protein was detectable only in muscle satellite cells and was increased in diabetes patients compared with controls, consistent with the observation that global miRNA changes were opposite from those found during myogenic differentiation. We provide evidence that IR in humans may be related to coordinated changes in multiple microRNAs, which act to target relevant signaling pathways. It would appear that miRNAs can produce marked changes in target protein abundance in vivo by working in a combinatorial manner. Thus, miRNA detection represents a new molecular biomarker strategy for insulin resistance, where micrograms of patient material is needed to monitor efficacy during drug or life-style interventions
Experimental and First-Principles Spectroscopy of Cu₂SrSnS₄ and Cu₂BaSnS₄ Photoabsorbers
Cu2BaSnS4 (CBTS) and Cu2SrSnS4 (CSTS) semiconductors have been recently proposed as potential wide band gap photovoltaic absorbers. Although several measurements indicate that they are less affected by band tailing than their parent compound Cu2ZnSnS4, their photovoltaic efficiencies are still low. To identify possible issues, we characterize CBTS and CSTS in parallel by a variety of spectroscopic methods complemented by first-principles calculations. Two main problems are identified in both materials. The first is the existence of deep defect transitions in low-temperature photoluminescence, pointing to a high density of bulk recombination centers. The second is their low electron affinity, which emphasizes the need for an alternative heterojunction partner and electron contact. We also find a tendency for downward band bending at the surface of both materials. In CBTS, this effect is sufficiently large to cause carrier-type inversion, which may enhance carrier separation and mitigate interface recombination. Optical absorption at room temperature is exciton-enhanced in both CBTS and CSTS. Deconvolution of excitonic effects yields band gaps that are about 100 meV higher than previous estimates based on Tauc plots. Although the two investigated materials are remarkably similar in an idealized, defect-free picture, the present work points to CBTS as a more promising absorber than CSTS for tandem photovoltaics
Degree of entanglement for two qubits
In this paper, we present a measure to quantify the degree of entanglement
for two qubits in a pure state.Comment: 5 page
The Lyot Project Direct Imaging Survey of Substellar Companions: Statistical Analysis and Information from Nondetections
The Lyot project used an optimized Lyot coronagraph with Extreme Adaptive
Optics at the 3.63m Advanced Electro-Optical System telescope (AEOS) to observe
86 stars from 2004 to 2007. In this paper we give an overview of the survey
results and a statistical analysis of the observed nondetections around 58 of
our targets to place constraints on the population of substellar companions to
nearby stars. The observations did not detect any companion in the substellar
regime. Since null results can be as important as detections, we analyzed each
observation to determine the characteristics of the companions that can be
ruled out. For this purpose we use a Monte Carlo approach to produce artificial
companions, and determine their detectability by comparison with the
sensitivity curve for each star. All the non-detection results are combined
using a Bayesian approach and we provide upper limits on the population of
giant exoplanets and brown dwarfs for this sample of stars. Our nondetections
confirm the rarity of brown dwarfs around solar-like stars and we constrain the
frequency of massive substellar companions (M>40Mjup) at orbital separation
between and 10 and 50 AU to be <20%.Comment: 32 pages, 11 figures, 2 tables. Published in the Astrophysical
Journa
Analytic properties of the Landau gauge gluon and quark propagators
We explore the analytic structure of the gluon and quark propagators of
Landau gauge QCD from numerical solutions of the coupled system of renormalized
Dyson--Schwinger equations and from fits to lattice data. We find sizable
negative norm contributions in the transverse gluon propagator indicating the
absence of the transverse gluon from the physical spectrum. A simple analytic
structure for the gluon propagator is proposed. For the quark propagator we
find evidence for a mass-like singularity on the real timelike momentum axis,
with a mass of 350 to 500 MeV. Within the employed Green's functions approach
we identify a crucial term in the quark-gluon vertex that leads to a positive
definite Schwinger function for the quark propagator.Comment: 42 pages, 16 figures, revtex; version to be published in Phys Rev
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