9,833 research outputs found
Nondestructive selective probing of phononic excitations in a cold Bose gas using impurities
We introduce a detector that selectively probes the phononic excitations of a
cold Bose gas. The detector is composed of a single impurity atom confined by a
double-well potential, where the two lowest eigenstates of the impurity form an
effective probe qubit that is coupled to the phonons via density-density
interactions with the bosons. The system is analogous to a two-level atom
coupled to photons of the radiation field. We demonstrate that tracking the
evolution of the qubit populations allows probing both thermal and coherent
excitations in targeted phonon modes. The targeted modes are selected in both
energy and momentum by adjusting the impurity's potential. We show how to use
the detector to observe coherent density waves and to measure temperatures of
the Bose gas down to the nano-Kelvin regime. We analyze how our scheme could be
realized experimentally, including the possibility of using an array of
multiple impurities to achieve greater precision from a single experimental
run.Comment: 11+4 pages, 7 figure
Fragility of a class of highly entangled states of many quantum-bits
We consider a Quantum Computer with n quantum-bits (`qubits'), where each
qubit is coupled independently to an environment affecting the state in a
dephasing or depolarizing way. For mixed states we suggest a quantification for
the property of showing {\it quantum} uncertainty on the macroscopic level. We
illustrate in which sense a large parameter can be seen as an indicator for
large entanglement and give hypersurfaces enclosing the set of separable
states. Using methods of the classical theory of maximum likelihood estimation
we prove that this parameter is decreasing with 1/\sqrt{n} for all those states
which have been exposed to the environment.
Furthermore we consider a Quantum Computer with perfect 1-qubit gates and
2-qubit gates with depolarizing error and show that any state which can be
obtained from a separable initial state lies inbetween a family of pairs of
certain hypersurfaces parallel to those enclosing the separable ones.Comment: 9 Pages, RevTe
3D MHD Simulations of Planet Migration in Turbulent Stratified Disks
We performed 3D MHD simulations of planet migration in stratified disks using
the Godunov code PLUTO, where the disk is turbulent due to the
magnetorotational instability. We study the migration for planets with
different planet-star mass ratios . In agreement with previous
studies, for the low-mass planet cases ( and ),
migration is dominated by random fluctuations in the torque. For a Jupiter-mass
planet for , we find a reduction of
the magnetic stress inside the orbit of the planet and around the gap region.
After an initial stage where the torque on the planet is positive, it reverses
and we recover migration rates similar to those found in disks where the
turbulent viscosity is modelled by an viscosity. For the
intermediate-mass planets ( and ) we
find a new and so far unexpected behavior. In some cases they experience
sustained and systematic outwards migration for the entire duration of the
simulation. For this case, the horseshoe region is resolved and torques coming
from the corotation region can remain unsaturated due to the stresses in the
disk. These stresses are generated directly by the magnetic field. The
magnitude of the horseshoe drag can overcome the negative Lindblad contribution
when the local surface density profile is flat or increasing outwards, which we
see in certain locations in our simulations due to the presence of a zonal
flow. The intermediate-mass planet is migrating radially outwards in locations
where there is a positive gradient of a pressure bump (zonal flow).Comment: Accepted for publication in Ap
Effects of magnetic doping and temperature dependence on phonon dynamics in CaFe\_{1-x}Co\_{x}AsF compounds (x = 0, 0.06, 0.12)
We report detailed measurements of composition as well as temperature
dependence of the phonon density-of-states in a new series of FeAs compounds
with composition CaFe1\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12). The composition as
well as temperature dependence of phonon spectra for CaFe\_{1-x}Co\_{x}AsF (x =
0, 0.06, 0.12) compounds have been measured using time of flight IN4C and IN6
spectrometers at ILL, France. The comparison of phonon spectra at 300 K in
these compounds shows that acoustic phonon modes up to 12 meV harden in the
doped compounds in comparison to the parent CaFeAsF. While intermediate energy
phonon modes from 15 meV to 25 meV are also found to shift towards high
energies only in the 12 % Co doped CaFeAsF compound. The experimental results
for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) are quite different from our
previous phonon studies on parent and superconducting MFe2As2 (M=Ba, Ca, Sr)
where low-energy acoustic phonon modes do not react with doping, while the
phonon spectra in the intermediate range from 15 to 25 K are found to soften in
these compounds. We argue that stronger spin phonon interaction play an
important role for the emergence of superconductivity in these compounds. The
lattice dynamics of CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds is also
investigated using the ab-initio as well as shell model phonon calculations. We
show that the nature of the interaction between the Ca and the Fe-As layers in
CaFeAsF compounds is quite different compared with our previous studies on
CaFe2As2.Comment: 19 pages, 5 figure
Phonon spectra in CaFe2As2 and Ca0.6Na0.4Fe2As2: Measurement of the pressure and temperature dependence and comparison with ab-initio and shell model calculations
We report the pressure and temperature dependence of the phonon
density-of-states in superconducting Ca0.6Na0.4Fe2As2 (Tc=21 K) and the parent
compound CaFe2As2, using inelastic neutron scattering. We observe no
significant change in the phonon spectrum for Ca0.6Na0.4Fe2As2 at 295 K up to
pressures of 5 kbar. The phonon spectrum for CaFe2As2 shows softening of the
low-energy modes by about 1 meV when decreasing the temperature from 300 K to
180 K. There is no appreciable change in the phonon density of states across
the structural and anti-ferromagnetic phase transition at 172 K. These results,
combined with our earlier temperature dependent phonon density of states
measurements for Ca0.6Na0.4Fe2As2, indicate that the softening of low-energy
phonon modes in these compounds may be due to the interaction of phonons with
electron or short-range spin fluctuations in the normal state of the
superconducting compound as well as in the parent compound. The phonon spectra
are analyzed with ab-initio and empirical model calculations giving partial
densities of states and dispersion relations.Comment: 14 pages, 6 figure
Directional emission of light from a nano-optical Yagi-Uda antenna
The plasmon resonance of metal nanoparticles can enhance and direct light
from optical emitters in much the same way that radio frequency (RF) antennas
enhance and direct the emission from electrical circuits. In the RF regime, a
typical antenna design for high directivity is the Yagi-Uda antenna, which
basically consists of a one-dimensional array of antenna elements driven by a
single feed element. Here, we present the experimental demonstration of
directional light emission from a nano-optical Yagi-Uda antenna composed of an
array of appropriately tuned gold nanorods. Our results indicate that
nano-optical antenna arrays are a simple but efficient tool for the spatial
control of light emission.Comment: 4 pages, including 4 figure
Editorial: crime patterns in time and space: the dynamics of crime opportunities in urban areas
The routine activity approach and associated crime pattern theory emphasise how crime emerges from spatio-temporal routines. In order to understand this crime should be studied in both space and time. However, the bulk of research into crime patterns and related activities has investigated the spatial distributions of crime, neglecting the temporal dimension. Specifically, disaggregation of crime by place and by time, for example hour of day, day of week, month of year, season, or school day versus none school day, is extremely relevant to theory. Modern data make such spatio-temporal disaggregation increasingly feasible, as exemplified in this special issue. First, much larger data files allow disaggregation of crime data into temporal and spatial slices. Second, new forms of data are generated by modern technologies, allowing innovative and new forms of analyses. Crime pattern analyses and routine activity inquiries are now able to explore avenues not previously available. The unique collection of nine papers in this thematic issue specifically examine spatio-temporal patterns of crime to; demonstrate the value of this approach for advancing knowledge in the field; consider how this informs our theoretical understanding of the manifestations of crime in time and space; to consider the prevention implications of this; and to raise awareness of the need for further spatio-temporal research into crime event
Reflected Light from Sand Grains in the Terrestrial Zone of a Protoplanetary Disk
We show that grains have grown to ~mm size (sand sized) or larger in the
terrestrial zone (within ~3 AU) of the protoplanetary disk surrounding the 3
Myr old binary star KH 15D. We also argue that the reflected light in the
system reaches us by back scattering off the far side of the same ring whose
near side causes the obscuration.Comment: 22 pages, 5 figures. To be published in Nature, March 13, 2008.
Contains a Supplemen
Multiple scattering of polarized radiation by non-spherical grains: first results
We present the first numerical radiative transfer simulation of multiple
light scattering in dust configurations containing aligned non-spherical
(spheroidal) dust grains. Such models are especially important if one wants to
explain the circular polarization of light, observed in a variety of
astronomical objects. The radiative transfer problem is solved on the basis of
the Monte Carlo method. Test simulations, confirming the correct numerical
implementation of the scattering mechanism, are presented. As a first
application, we investigate the linear and circular polarization of light
coming from a spherical circumstellar shell. This shell contains perfectly
aligned prolate or oblate spheroidal grains. The most remarkable features of
the simulated linear polarization maps are so-called polarization null points
where the reversal of polarization occurs. They appear in the case when the
grain alignment axis is perpendicular to the line of sight. The maps of
circular polarization have a sector-like structure with maxima at the ends of
lines inclined to the grain alignment axis by \pm 45\degr.Comment: 13 pages, 14 figures, accepted by A&
The role of surface plasmons in the decay of image-potential states on silver surfaces
The combined effect of single-particle and collective surface excitations in
the decay of image-potential states on Ag surfaces is investigated, and the
origin of the long-standing discrepancy between experimental measurements and
previous theoretical predictions for the lifetime of these states is
elucidated. Although surface-plasmon excitation had been expected to reduce the
image-state lifetime, we demonstrate that the subtle combination of the spatial
variation of s-d polarization in Ag and the characteristic non-locality of
many-electron interactions near the surface yields surprisingly long
image-state lifetimes, in agreement with experiment.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
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