440 research outputs found
Fast sensing of double-dot charge arrangement and spin state with an rf sensor quantum dot
Single-shot measurement of the charge arrangement and spin state of a double
quantum dot are reported, with measurement times down to ~ 100 ns. Sensing uses
radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade
regime. The sensor quantum dot is up to 30 times more sensitive than a
comparable quantum point contact sensor, and yields three times greater signal
to noise in rf single-shot measurements. Numerical modeling is qualitatively
consistent with experiment and shows that the improved sensitivity of the
sensor quantum dot results from reduced screening and lifetime broadening.Comment: related papers at http://marcuslab.harvard.ed
About the dynamics and thermodynamics of trapped ions
This tutorial introduces the dynamics of charged particles in a
radiofrequency trap in a very general manner to point out the differences
between the dynamics in a quadrupole and in a multipole trap. When dense
samples are trapped, the dynamics is modified by the Coulomb repulsion between
ions. To take into account this repulsion, we propose to use a method,
originally developed for particles in Penning trap, that model the ion cloud as
a cold fluid. This method can not reproduce the organisation of cold clouds as
crystals but it allows one to scale the size of large samples with the trapping
parameters and the number of ions trapped, for different linear geometries of
trap.Comment: accepted for publication in the "Modern Applications of Trapped Ions"
special issu
The WINGS Survey: a progress report
A two-band (B and V) wide-field imaging survey of a complete, all-sky X-ray
selected sample of 78 clusters in the redshift range z=0.04-0.07 is presented.
The aim of this survey is to provide the astronomical community with a complete
set of homogeneous, CCD-based surface photometry and morphological data of
nearby cluster galaxies located within 1.5 Mpc from the cluster center. The
data collection has been completed in seven observing runs at the INT and
ESO-2.2m telescopes. For each cluster, photometric data of about 2500 galaxies
(down to V~23) and detailed morphological information of about 600 galaxies
(down to V~21) are obtained by using specially designed automatic tools.
As a natural follow up of the photometric survey, we also illustrate a long
term spectroscopic program we are carrying out with the WHT-WYFFOS and AAT-2dF
multifiber spectrographs. Star formation rates and histories, as well as
metallicity estimates will be derived for about 350 galaxies per cluster from
the line indices and equivalent widths measurements, allowing us to explore the
link between the spectral properties and the morphological evolution in high-
to low-density environments, and across a wide range in cluster X-ray
luminosities and optical properties.Comment: 12 pages, 10 eps figures, Proceedings of the SAIt Conference 200
The prevalence of MRI-defined spinal pathoanatomies and their association with Modic changes in individuals seeking care for low back pain
Modic changes are of increasing interest, however their age and gender prevalence are not well described. To date, the associations between Modic changes and other common vertebral pathologies have only been described in small samples (n < 100). Our aim was, in a large dataset of people with low back pain, to (1) describe the prevalence of a range of spinal pathoanatomies, and (2) examine the association between Modic changes and stages of intervertebral disc (IVD) pathology. Common pathologies were coded from the lumbar spine MRIs from 4,233 consecutive people imaged while attending a publicly-funded secondary care outpatient facility in Denmark. Prevalence data were calculated by pathology and by vertebral level. Prevalence was also calculated by age and gender categories for Modic changes. The association between stages of IVD pathology (degeneration, bulge, herniation) and Modic changes at L4/5 and L5/S1 was expressed using prevalence ratios (PR) and 95% confidence intervals. The prevalence of Modic changes and IVD pathology were greater in L4/5 and L5/S1, compared with the upper lumbar spine. There was no significant gender difference in prevalence of Modic changes (p = 0.11). The prevalence of IVD disc pathology occurring concurrently with Modic changes ranged from 11.5 to 17.5% (Type 1), 8.5 to 12.7% (Type 2) and 17.1 to 25.6% (Type 1 and/or 2) while the prevalence occurring in the absence of Modic changes ranged from 0.5 to 6.3% (Type 1), 0.3 to 4.9 (Type 2), 0.8 to 9.7% (Type 1 and/or 2). The associated PR for IVD pathology occurring concurrently with Modic changes ranged from 1.8 to 29.2 (p < 0.05). The highest PR (29.2) was between degeneration and Modic changes, indicating that it is rare for Modic changes to occur without disc degeneration.Spinal pathoanatomy was common in this population, particularly IVD pathologies, and a consistent trend of a relatively greater prevalence in the lower lumbar spine was identified. Modic changes were more likely to be present among individuals with IVD pathology than without, which may implicate mechanical factors as being one aetiological pathway for Modic changes, although other hypotheses may equally explain this association
Quantized conductance doubling and hard gap in a two-dimensional semiconductor-superconductor heterostructure
The prospect of coupling a two-dimensional (2D) semiconductor heterostructure
to a superconductor opens new research and technology opportunities, including
fundamental problems in mesoscopic superconductivity, scalable superconducting
electronics, and new topological states of matter. For instance, one route
toward realizing topological matter is by coupling a 2D electron gas (2DEG)
with strong spin-orbit interaction to an s-wave superconductor. Previous
efforts along these lines have been hindered by interface disorder and unstable
gating. Here, we report measurements on a gateable InGaAs/InAs 2DEG with
patterned epitaxial Al, yielding multilayer devices with atomically pristine
interfaces between semiconductor and superconductor. Using surface gates to
form a quantum point contact (QPC), we find a hard superconducting gap in the
tunneling regime, overcoming the soft-gap problem in 2D
superconductor-semiconductor hybrid systems. With the QPC in the open regime,
we observe a first conductance plateau at 4e^2/h, as expected theoretically for
a normal-QPC-superconductor structure. The realization of a hard-gap
semiconductor-superconductor system that is amenable to top-down processing
provides a means of fabricating scalable multicomponent hybrid systems for
applications in low-dissipation electronics and topological quantum
information.Comment: includes main text, supplementary information and code for
simulations. Published versio
Two-dimensional epitaxial superconductor-semiconductor heterostructures: A platform for topological superconducting networks
Progress in the emergent field of topological superconductivity relies on
synthesis of new material combinations, combining superconductivity, low
density, and spin-orbit coupling (SOC). For example, theory [1-4] indicates
that the interface between a one-dimensional (1D) semiconductor (Sm) with
strong SOC and a superconductor (S) hosts Majorana modes with nontrivial
topological properties [5-8]. Recently, epitaxial growth of Al on InAs
nanowires was shown to yield a high quality S-Sm system with uniformly
transparent interfaces [9] and a hard induced gap, indicted by strongly
suppressed sub gap tunneling conductance [10]. Here we report the realization
of a two-dimensional (2D) InAs/InGaAs heterostructure with epitaxial Al,
yielding a planar S-Sm system with structural and transport characteristics as
good as the epitaxial wires. The realization of 2D epitaxial S-Sm systems
represent a significant advance over wires, allowing extended networks via
top-down processing. Among numerous potential applications, this new material
system can serve as a platform for complex networks of topological
superconductors with gate-controlled Majorana zero modes [1-4]. We demonstrate
gateable Josephson junctions and a highly transparent 2D S-Sm interface based
on the product of excess current and normal state resistance
Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
We present a homogeneous photometric and spectroscopic analysis of 18 stars
along the evolutionary sequence of the metal-poor globular cluster NGC 6397
([Fe/H] = -2), from the main-sequence turnoff point to red giants below the
bump. The spectroscopic stellar parameters, in particular stellar-parameter
differences between groups of stars, are in good agreement with broad-band and
Stroemgren photometry calibrated on the infrared-flux method. The spectroscopic
abundance analysis reveals, for the first time, systematic trends of iron
abundance with evolutionary stage. Iron is found to be 31% less abundant in the
turnoff-point stars than in the red giants. An abundance difference in lithium
is seen between the turnoff-point and warm subgiant stars. The impact of
potential systematic errors on these abundance trends (stellar parameters, the
hydrostatic and LTE approximations) is quantitatively evaluated and found not
to alter our conclusions significantly. Trends for various elements (Li, Mg,
Ca, Ti and Fe) are compared with stellar-structure models including the effects
of atomic diffusion and radiative acceleration. Such models are found to
describe the observed element-specific trends well, if extra (turbulent) mixing
just below the convection zone is introduced. It is concluded that atomic
diffusion and turbulent mixing are largely responsible for the sub-primordial
stellar lithium abundances of warm halo stars. Other consequences of atomic
diffusion in old metal-poor stars are also discussed.Comment: 20 pages (emulateapj), 11 figures, accepted for publication in Ap
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