604 research outputs found
Amplified Sensitivity of Nitrogen-Vacancy Spins in Nanodiamonds using All-Optical Charge Readout
Nanodiamonds containing nitrogen-vacancy (NV) centers offer a versatile
platform for sensing applications spanning from nanomagnetism to in-vivo
monitoring of cellular processes. In many cases, however, weak optical signals
and poor contrast demand long acquisition times that prevent the measurement of
environmental dynamics. Here, we demonstrate the ability to perform fast,
high-contrast optical measurements of charge distributions in ensembles of NV
centers in nanodiamonds and use the technique to improve the spin readout
signal-to-noise ratio through spin-to-charge conversion. A study of 38
nanodiamonds, each hosting 10-15 NV centers with an average diameter of 40 nm,
uncovers complex, multiple-timescale dynamics due to radiative and
non-radiative ionization and recombination processes. Nonetheless, the
nanodiamonds universally exhibit charge-dependent photoluminescence contrasts
and the potential for enhanced spin readout using spin-to-charge conversion. We
use the technique to speed up a relaxometry measurement by a factor of
five.Comment: 13 pages, 14 figure
Optical Signatures of Quantum Emitters in Suspended Hexagonal Boron Nitride
Hexagonal boron nitride (h-BN) is a tantalizing material for solid-state
quantum engineering. Analogously to three-dimensional wide-bandgap
semiconductors like diamond, h-BN hosts isolated defects exhibiting visible
fluorescence, and the ability to position such quantum emitters within a
two-dimensional material promises breakthrough advances in quantum sensing,
photonics, and other quantum technologies. Critical to such applications,
however, is an understanding of the physics underlying h-BN's quantum emission.
We report the creation and characterization of visible single-photon sources in
suspended, single-crystal, h-BN films. The emitters are bright and stable over
timescales of several months in ambient conditions. With substrate interactions
eliminated, we study the spectral, temporal, and spatial characteristics of the
defects' optical emission, which offer several clues about their electronic and
chemical structure. Analysis of the defects' spectra reveals similarities in
vibronic coupling despite widely-varying fluorescence wavelengths, and a
statistical analysis of their polarized emission patterns indicates a
correlation between the optical dipole orientations of some defects and the
primitive crystallographic axes of the single-crystal h-BN film. These
measurements constrain possible defect models, and, moreover, suggest that
several classes of emitters can exist simultaneously in free-standing h-BN,
whether they be different defects, different charge states of the same defect,
or the result of strong local perturbations
Optimizing future dark energy surveys for model selection goals
We demonstrate a methodology for optimizing the ability of future dark energy
surveys to answer model selection questions, such as `Is acceleration due to a
cosmological constant or a dynamical dark energy model?'. Model selection
Figures of Merit are defined, exploiting the Bayes factor, and surveys
optimized over their design parameter space via a Monte Carlo method. As a
specific example we apply our methods to generic multi-fibre baryon acoustic
oscillation spectroscopic surveys, comparable to that proposed for SuMIRe PFS,
and present implementations based on the Savage-Dickey Density Ratio that are
both accurate and practical for use in optimization. It is shown that whilst
the optimal surveys using model selection agree with those found using the Dark
Energy Task Force (DETF) Figure of Merit, they provide better informed
flexibility of survey configuration and an absolute scale for performance; for
example, we find survey configurations with close to optimal model selection
performance despite their corresponding DETF Figure of Merit being at only 50%
of its maximum. This Bayes factor approach allows us to interpret the survey
configurations that will be good enough for the task at hand, vital especially
when wanting to add extra science goals and in dealing with time restrictions
or multiple probes within the same project.Comment: 12 pages, 16 figure
Testing for double inflation with WMAP
With the WMAP data we can now begin to test realistic models of inflation
involving multiple scalar fields. These naturally lead to correlated adiabatic
and isocurvature (entropy) perturbations with a running spectral index. We
present the first full (9 parameter) likelihood analysis of double inflation
with WMAP data and find that despite the extra freedom, supersymmetric hybrid
potentials are strongly constrained with less than 7% correlated isocurvature
component allowed when standard priors are imposed on the cosomological
parameters. As a result we also find that Akaike & Bayesian model selection
criteria rather strongly prefer single-field inflation, just as equivalent
analysis prefers a cosmological constant over dynamical dark energy in the late
universe. It appears that simplicity is the best guide to our universe.Comment: 7 pages, 6 figure
Optimizing baryon acoustic oscillation surveys II: curvature, redshifts, and external datasets
We extend our study of the optimization of large baryon acoustic oscillation
(BAO) surveys to return the best constraints on the dark energy, building on
Paper I of this series (Parkinson et al. 2007). The survey galaxies are assumed
to be pre-selected active, star-forming galaxies observed by their line
emission with a constant number density across the redshift bin. Star-forming
galaxies have a redshift desert in the region 1.6 < z < 2, and so this redshift
range was excluded from the analysis. We use the Seo & Eisenstein (2007)
fitting formula for the accuracies of the BAO measurements, using only the
information for the oscillatory part of the power spectrum as distance and
expansion rate rulers. We go beyond our earlier analysis by examining the
effect of including curvature on the optimal survey configuration and updating
the expected `prior' constraints from Planck and SDSS. We once again find that
the optimal survey strategy involves minimizing the exposure time and
maximizing the survey area (within the instrumental constraints), and that all
time should be spent observing in the low-redshift range (z<1.6) rather than
beyond the redshift desert, z>2. We find that when assuming a flat universe the
optimal survey makes measurements in the redshift range 0.1 < z <0.7, but that
including curvature as a nuisance parameter requires us to push the maximum
redshift to 1.35, to remove the degeneracy between curvature and evolving dark
energy. The inclusion of expected other data sets (such as WiggleZ, BOSS and a
stage III SN-Ia survey) removes the necessity of measurements below redshift
0.9, and pushes the maximum redshift up to 1.5. We discuss considerations in
determining the best survey strategy in light of uncertainty in the true
underlying cosmological model.Comment: 15 pages, revised in response to referees remarks, accepted for
publication in MNRAS. 2nd paper in a series. Paper 1 is at
http://arxiv.org/abs/astro-ph/070204
Accuracy of the Cosmed K5 portable calorimeter
Purpose The purpose of this study was to assess the accuracy of the Cosmed K5 portable metabolic system dynamic mixing chamber (MC) and breath-by-breath (BxB) modes against the criterion Douglas bag (DB) method. Methods Fifteen participants (mean age±SD, 30.6±7.4 yrs) had their metabolic variables measured at rest and during cycling at 50, 100, 150, 200, and 250W. During each stage, participants were connected to the first respiratory gas collection method (randomized) for the first four minutes to reach steady state, followed by 3-min (or 5-min for DB) collection periods for the resting condition, and 2-min collection periods for all cycling intensities. Collection periods for the second and third methods were preceded by a washout of 1–3 min. Repeated measures ANOVAs were used to compare metabolic variables measured by each method, for seated rest and each cycling work rate. Results For ventilation (VE) and oxygen uptake (VO2), the K5 MC and BxB modes were within 2.1 l/min (VE) and 0.08 l/min (VO2) of the DB (p≥0.05). Compared to DB values, carbon dioxide production (VCO2) was significantly underestimated by the K5 BxB mode at work rates ≥150W by 0.12–0.31 l/min (p\u3c0.05). K5 MC and BxB respiratory exchange ratio values were significantly lower than DB at cycling work rates ≥100W by 0.03–0.08 (p\u3c0.05). Conclusion Compared to the DB method, the K5 MC and BxB modes are acceptable for measuring VE and VO2 across a wide range of cycling intensities. Both K5 modes provided comparable values to each other
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