1,704 research outputs found
Quantum Optics with Surface Plasmons
We describe a technique that enables strong, coherent coupling between
individual optical emitters and guided plasmon excitations in conducting
nano-structures at optical frequencies. We show that under realistic
conditions, optical emission can be almost entirely directed into the plasmon
modes. As an example, we describe an application of this technique involving
efficient generation of single photons on demand, in which the plasmon is
efficiently out-coupled to a dielectric waveguide.Comment: 11 pages, 3 figure
Phase transition in the modified fiber bundle model
We extend the standard fiber bundle model (FBM) with the local load sharing
in such a way that the conservation of the total load is relaxed when an
isolated fiber is broken. In this modified FBM in one dimension (1D), it is
revealed that the model exhibits a well-defined phase transition at a finite
nonzero value of the load, which is in contrast to the standard 1D FBM. The
modified FBM defined in the Watts-Strogatz network is also investigated, and
found is the existences of two distinct transitions: one discontinuous and the
other continuous. The effects of the long-range shortcuts are also discussed.Comment: 7 pages, to appear in Europhys. Let
Optical quenching and recovery of photoconductivity in single-crystal diamond
We study the photocurrent induced by pulsed-light illumination (pulse
duration is several nanoseconds) of single-crystal diamond containing nitrogen
impurities. Application of additional continuous-wave light of the same
wavelength quenches pulsed photocurrent. Characterization of the optically
quenched photocurrent and its recovery is important for the development of
diamond based electronics and sensing
Three level atom optics via the tunneling interaction
Three level atom optics (TLAO) is introduced as a simple, efficient and
robust method to coherently manipulate and transport neutral atoms. The
tunneling interaction among three trapped states allows to realize the spatial
analog of the stimulated Raman adiabatic passage (STIRAP), coherent population
trapping (CPT), and electromagnetically induced transparency (EIT) techniques.
We investigate a particular implementation in optical microtrap arrays and show
that under realistic parameters the coherent manipulation and transfer of
neutral atoms among dipole traps could be realized in the millisecond range.Comment: 5 pages, 6 figure
Hydrodynamic dispersion within porous biofilms
Many microorganisms live within surface-associated consortia, termed biofilms, that can form intricate porous structures interspersed with a network of fluid channels. In such systems, transport phenomena, including flow and advection, regulate various aspects of cell behavior by controlling nutrient supply, evacuation of waste products, and permeation of antimicrobial agents. This study presents multiscale analysis of solute transport in these porous biofilms. We start our analysis with a channel-scale description of mass transport and use the method of volume averaging to derive a set of homogenized equations at the biofilm-scale in the case where the width of the channels is significantly smaller than the thickness of the biofilm. We show that solute transport may be described via two coupled partial differential equations or telegrapher's equations for the averaged concentrations. These models are particularly relevant for chemicals, such as some antimicrobial agents, that penetrate cell clusters very slowly. In most cases, especially for nutrients, solute penetration is faster, and transport can be described via an advection-dispersion equation. In this simpler case, the effective diffusion is characterized by a second-order tensor whose components depend on (1) the topology of the channels' network; (2) the solute's diffusion coefficients in the fluid and the cell clusters; (3) hydrodynamic dispersion effects; and (4) an additional dispersion term intrinsic to the two-phase configuration. Although solute transport in biofilms is commonly thought to be diffusion dominated, this analysis shows that hydrodynamic dispersion effects may significantly contribute to transport
Coherence of Nitrogen-Vacancy Electronic Spin Ensembles in Diamond
We present an experimental and theoretical study of electronic spin
decoherence in ensembles of nitrogen-vacancy (NV) color centers in bulk
high-purity diamond at room temperature. Under appropriate conditions, we find
ensemble NV spin coherence times (T_2) comparable to that of single NVs, with
T_2 > 600 microseconds for a sample with natural abundance of 13C and
paramagnetic impurity density ~10^15 cm^(-3). We also observe a sharp decrease
of the coherence time with misalignment of the static magnetic field relative
to the NV electronic spin axis, consistent with theoretical modeling of NV
coupling to a 13C nuclear spin bath. The long coherence times and increased
signal-to-noise provided by room-temperature NV ensembles will aid many
applications of NV centers in precision magnetometry and quantum information.Comment: 5 pages, 3 figures; v2 minor correction
Optical control and entanglement of atomic Schroedinger fields
We develop a fully quantized model of a Bose-Einstein condensate driven by a
far off-resonant pump laser which interacts with a single mode of an optical
ring cavity. In the linear regime, the cavity mode exhibits spontaneous
exponential gain correlated with the appearance of two atomic field side-modes.
These side-modes and the cavity field are generated in a highly entangled
state, characterized by thermal intensity fluctuations in the individual modes,
but with two-mode correlation functions which violate certain classical
inequalities. By injecting an initial coherent field into the optical cavity
one can significantly decrease the intensity fluctuations at the expense of
reducing the correlations, thus allowing for optical control over the quantum
statistical properties of matter waves.Comment: 4 page
Discrete Symmetry and Stability in Hamiltonian Dynamics
In this tutorial we address the existence and stability of periodic and
quasiperiodic orbits in N degree of freedom Hamiltonian systems and their
connection with discrete symmetries. Of primary importance in our study are the
nonlinear normal modes (NNMs), i.e periodic solutions which represent
continuations of the system's linear normal modes in the nonlinear regime. We
examine the existence of such solutions and discuss different methods for
constructing them and studying their stability under fixed and periodic
boundary conditions. In the periodic case, we employ group theoretical concepts
to identify a special type of NNMs called one-dimensional "bushes". We describe
how to use linear combinations such NNMs to construct s(>1)-dimensional bushes
of quasiperiodic orbits, for a wide variety of Hamiltonian systems and exploit
the symmetries of the linearized equations to simplify the study of their
destabilization. Applying this theory to the Fermi Pasta Ulam (FPU) chain, we
review a number of interesting results, which have appeared in the recent
literature. We then turn to an analytical and numerical construction of
quasiperiodic orbits, which does not depend on the symmetries or boundary
conditions. We demonstrate that the well-known "paradox" of FPU recurrences may
be explained in terms of the exponential localization of the energies Eq of
NNM's excited at the low part of the frequency spectrum, i.e. q=1,2,3,....
Thus, we show that the stability of these low-dimensional manifolds called
q-tori is related to the persistence or FPU recurrences at low energies.
Finally, we discuss a novel approach to the stability of orbits of conservative
systems, the GALIk, k=2,...,2N, by means of which one can determine accurately
and efficiently the destabilization of q-tori, leading to the breakdown of
recurrences and the equipartition of energy, at high values of the total energy
E.Comment: 50 pages, 13 figure
Improved Survival After Acute Graft-Versus-Host Disease Diagnosis in the Modern Era
Acute graft-versus-host disease remains a major threat to a successful outcome after allogeneic hematopoietic cell transplantation. While improvements in treatment and supportive care have occurred, it is unknown whether these advances have resulted in improved outcome specifically among those diagnosed with acute graft-versus-host disease. We examined outcome following diagnosis of grade II-IV acute graft-versus-host disease according to time period, and explored effects according to original graft-versus-host disease prophylaxis regimen and maximum overall grade of acute graft-versus-host disease. Between 1999 and 2012, 2,905 patients with acute myeloid leukemia (56%), acute lymphoblastic leukemia (30%) or myelodysplastic syndromes (14%) received a sibling (24%) or unrelated donor (76%) blood (66%) or marrow (34%) transplant and developed grade II-IV acute graft-versus-host disease (n=497 for 1999–2001, n=962 for 2002–2005, n=1,446 for 2006–2010). The median (range) follow-up was 144 (4–174), 97 (4–147) and 60 (8–99) months for 1999–2001, 2002–2005, and 2006–2010, respectively. Among the cohort with grade II-IV acute graft-versus-host disease, there was a decrease in the proportion of grade III-IV disease over time with 56%, 47%, and 37% for 1999–2001, 2002–2005, and 2006–2012, respectively (P\u3c0.001). Considering the total study population, univariate analysis demonstrated significant improvements in overall survival and treatment-related mortality over time, and deaths from organ failure and infection declined. On multivariate analysis, significant improvements in overall survival (P=0.003) and treatment-related mortality (P=0.008) were only noted among those originally treated with tacrolimus-based graft-versus-host disease prophylaxis, and these effects were most apparent among those with overall grade II acute graft-versus-host disease. In conclusion, survival has improved over time for tacrolimus-treated transplant recipients with acute graft-versus-host disease
Controlling for transactions bias in regional house price indices
Transactions bias arises when properties that trade are not a random sample of the total housing stock. Price indices are susceptible because they are typically based on transactions data. Existing approaches to this problem rely on Heckman-type correction methods, where a probit regression is used to capture the differences between properties that sell and those that do not sell in a given period. However, this approach can only be applied where there is reliable data on the whole housing stock. In many countries—the UK included—no such data exist and there is little prospect of correcting for transactions bias in any of the regularly updated mainstream house price indices. Thispaper suggests a possible alternative approach, using information at postcode sector level and Fractional Probit Regression to correct for transactions bias in hedonic price indices based on one and a half million house sales from 1996 to 2004, distributed across 1200 postcode sectors in the South East of England
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