416 research outputs found
Nonlinear interactions with an ultrahigh flux of broadband entangled photons
We experimentally demonstrate sum-frequency generation (SFG) with entangled
photon-pairs, generating as many as 40,000 SFG photons per second, visible even
to the naked eye. The nonclassical nature of the interaction is exhibited by a
linear intensity-dependence of the nonlinear process. The key element in our
scheme is the generation of an ultrahigh flux of entangled photons while
maintaining their nonclassical properties. This is made possible by generating
the down-converted photons as broadband as possible, orders of magnitude wider
than the pump. This approach is readily applicable for other nonlinear
interactions, and may be applicable for various quantum-measurement tasks.Comment: 4 pages, 2 figures, Accepted to Phys. Rev. Let
The observable effects of a photospheric component on GRB's and XRF's prompt emission spectrum
A thermal radiative component is likely to accompany the first stages of the
prompt emission of Gamma-ray bursts (GRB's) and X-ray flashes (XRF's). We
analyze the effect of such a component on the observable spectrum, assuming
that the observable effects are due to a dissipation process occurring below or
near the thermal photosphere. We consider both the internal shock model and a
'slow heating' model as possible dissipation mechanisms. For comparable energy
densities in the thermal and the leptonic component, the dominant emission
mechanism is Compton scattering. This leads to a nearly flat energy spectrum
(\nu F_\nu \propto \nu^0) above the thermal peak at ~10-100 keV and below
10-100 MeV, for a wide range of optical depths 0.03 <~ \tau_{\gamma e} <~ 100,
regardless of the details of the dissipation mechanism or the strength of the
magnetic field. At lower energies steep slopes are expected, while above 100
MeV the spectrum depends on the details of the dissipation process. For higher
values of the optical depth, a Wien peak is formed at 100 keV - 1 MeV, and no
higher energy component exists. For any value of \tau_{\gamma e}, the number of
pairs produced does not exceed the baryon related electrons by a factor larger
than a few. We conclude that dissipation near the thermal photosphere can
naturally explain both the steep slopes observed at low energies and a flat
spectrum above 10 keV, thus providing an alternative scenario to the optically
thin synchrotron - SSC model.Comment: Discussion added on the results of Baring & Braby (2004); Accepted
for publication in Ap.
Spectral components in the bright, long GRB 061007: properties of the photosphere and the nature of the outflow
We present a time-resolved spectral analysis of the bright, long GRB 061007
(z=1.261) using Swift BAT and Suzaku WAM data. We find that the prompt emission
of GRB 061007 can be equally well explained by a photospheric component
together with a power law as by a Band function, and we explore the
implications of the former model. The photospheric component, which we model
with a multicolour blackbody, dominates the emission and has a very stable
shape throughout the burst. This component provides a natural explanation for
the hardness-intensity correlation seen within the burst and also allows us to
estimate the bulk Lorentz factor and the radius of the photosphere. The
power-law component dominates the fit at high energies and has a nearly
constant slope of -1.5. We discuss the possibility that this component is of
the same origin as the high-energy power laws recently observed in some Fermi
LAT bursts.Comment: Accepted for publication in MNRA
Synchrotron emission in small scale magnetic field as possible explanation for prompt emission spectra of gamma-ray bursts
Synchrotron emission is believed to be a major radiation mechanism during
gamma-ray bursts' (GRBs) prompt emission phase. A significant drawback of this
assumption is that the theoretical predicted spectrum, calculated within the
framework of the ``internal shocks'' scenario using the standard assumption
that the magnetic field maintains a steady value throughout the shocked region,
leads to a slope F_\nu \propto \nu^{-1/2} below 100 keV, which is in
contradiction to the much harder spectra observed. This is due to the electrons
cooling time being much shorter than the dynamical time. In order to overcome
this problem, we propose here that the magnetic field created by the internal
shocks decays on a length scale much shorter than the comoving width of the
plasma. We show that under this assumption synchrotron radiation can reproduce
the observed prompt emission spectra of the majority of the bursts. We
calculate the required decay length of the magnetic field, and find it to be
\~10^4 - 10^5 cm (equivalent to 10^5 - 10^6 skin depths), much shorter than the
characteristic comoving width of the plasma, ~3*10^{9} cm. We implement our
model to the case of GRB050820A, where a break at <~ 4 keV was observed, and
show that this break can be explained by synchrotron self absorption. We
discuss the consequences of the small scale magnetic field scenario on current
models of magnetic field generation in shock waves.Comment: Extended explanation on alternative emission models, the radiative
efficiency and derivation of eq. 2. Minor typos and English corrections;
Accepted for publication in Ap.
(Semi-)Predictive Discretization During Model Selection
In this paper, we present an approach to discretizing multivariate continuous data while learning the structure of a graphical model. We derive the joint scoring function from the principle of predictive accuracy, which inherently ensures the optimal trade-off between goodness of fit and model complexity (including the number of discretization levels). Using the so-called finest grid implied by the data, our scoring function depends only on the number of data points in the various discretization levels. Not only can it be computed efficiently, but it is also independent of the metric used in the continuous space. Our experiments with gene expression data show that discretization plays a crucial role regarding the resulting network structure
Peak energy clustering and efficiency in compact objects
We study the properties of plasmas containing a low energy thermal photon
component at comoving temperature \theta \equiv kT'/m_e c^2 \sim 10^{-5} -
10^{-2} interacting with an energetic electron component, characteristic of,
e.g., the dissipation phase of relativistic outflows in gamma-ray bursts
(GRB's), X-ray flashes, and blazars. We show that, for scattering optical
depths larger than a few, balance between Compton and inverse-Compton
scattering leads to the accumulation of electrons at values of . For optical depths larger than ~ 100, this leads to a peak in the
comoving photon spectrum at 1-10 keV, very weakly dependent on the values of
the free parameters. In particular, these results are applicable to the
internal shock model of GRB, as well as to slow dissipation models, e.g. as
might be expected from reconnection, if the dissipation occurs at a
sub-photospheric radii. For GRB bulk Lorentz factors ~ 100, this results in
observed spectral peaks clustering in the 0.1-1 MeV range, with conversion
efficiencies of electron into photon energy in the BATSE range of ~ 30%.Comment: Extended explanations about the electron energy balance; Refine
figures; Accepted for publication in Ap.
Ultracold dense gas of deeply bound heteronuclear molecules
Recently, the quest for an ultracold and dense ensemble of polar molecules
has attracted strong interest. Polar molecules have bright prospects for novel
quantum gases with long-range and anisotropic interactions, for quantum
information science, and for precision measurements. However, high-density
clouds of ultracold polar molecules have so far not been produced. Here, we
report a key step towards this goal. Starting from an ultracold dense gas of
heteronuclear 40K-87Rb Feshbach molecules with typical binding energies of a
few hundred kHz and a negligible dipole moment, we coherently transfer these
molecules into a vibrational level of the ground-state molecular potential
bound by >10 GHz. We thereby increase the binding energy and the expected
dipole moment of the 40K-87Rb molecules by more than four orders of magnitude
in a single transfer step. Starting with a single initial state prepared with
Feshbach association, we achieve a transfer efficiency of 84%. While dipolar
effects are not yet observable, the presented technique can be extended to
access much more deeply bound vibrational levels and ultimately those
exhibiting a significant dipole moment. The preparation of an ultracold quantum
gas of polar molecules might therefore come within experimental reach.Comment: 5 pages, 5 figure
Modeling the high-energy emission in GRB 110721A and implications on the early multiwavelength and polarimetric observations
GRB 110721A was detected by the Gamma-ray Burst Monitor and the Large Area
Telescope (LAT) onboard the Fermi satellite and the Gamma-ray Burst Polarimeter
onboard the IKAROS solar mission. Previous analysis done of this burst showed:
i) a linear polarization signal with position angle stable () and high degree of , ii) an extreme peak
energy of a record-breaking at 152 MeV, and iii) a subdominant prompt
thermal component observed right after the onset of this burst. In this paper,
the LAT data around the reported position of GRB 110721A are analysed with the
most recent software and then, the LAT light curve above 100 MeV was obtained.
The LAT light curve is modelled in terms of adiabatic early-afterglow external
shocks when the outflow propagates into a stellar wind. Additionally, we
discuss the possible origins and also study the implications of the
early-afterglow external shocks on the extreme peak energy observed at 152
MeV, the polarization observations and the subdominant prompt thermal
component.Comment: 9 pages and one figure. Accepted for publication in Ap
Quantum-inspired interferometry with chirped laser pulses
We introduce and implement an interferometric technique based on chirped
femtosecond laser pulses and nonlinear optics. The interference manifests as a
high-visibility (> 85%) phase-insensitive dip in the intensity of an optical
beam when the two interferometer arms are equal to within the coherence length
of the light. This signature is unique in classical interferometry, but is a
direct analogue to Hong-Ou-Mandel quantum interference. Our technique exhibits
all the metrological advantages of the quantum interferometer, but with signals
at least 10^7 times greater. In particular we demonstrate enhanced resolution,
robustness against loss, and automatic dispersion cancellation. Our
interferometer offers significant advantages over previous technologies, both
quantum and classical, in precision time delay measurements and biomedical
imaging.Comment: 6 pages, 4 figure
Impacts of selected Ecological Focus Area options in European farmed landscapes on climate regulation and pollination services: a systematic map protocol
Background: This systematic map protocol responds to an urgent policy need to evaluate key environmental benefits of new compulsory greening measures in the European Unionâs Common Agricultural Policy (CAP), with the aim of building a policy better linked to environmental performance. The systematic map will focus on Ecological Focus Areas (EFAs), in which larger arable farmers must dedicate 5% of their arable land to ecologically beneficial habitats, landscape features and land uses. The European Commissionâs Joint Research Centre has used a software tool called the âEFA calculatorâ to inform the European Commission about environmental benefits of EFA implementation. However, there are gaps in the EFA calculatorâs coverage of ecosystem services, especially âglobal climate regulationâ, and an opportunity to use systematic mapping methods to enhance its capture of evidence, in advance of forthcoming CAP reforms. We describe a method for assembling a database of relevant, peer-reviewed research conducted in all agricultural landscapes in Europe and neighbouring countries with similar biogeography, addressing the primary question: what are the impacts of selected EFA features in agricultural land on two policy-relevant ecosystem service outcomesâglobal climate regulation and pollination? The method is streamlined to allow results in good time for the current, time-limited opportunity to influence reforms of the CAP greening measures at European and Member State level. Methods: We will search four bibliographic databases in English, using a predefined and tested search string that focuses on a subset of EFA options and ecosystem service outcomes. The options and outcomes are selected as those with particular policy relevance and traction. Only articles in English will be included. We will screen search results at title, abstract and full text levels, recording the number of studies deemed non-relevant (with reasons at full text). A systematic map database that displays the meta-data (i.e. descriptive summary information about settings and methods) of relevant studies will be produced following full text assessment. The systematic map database will be published as a MS-Excel database. The nature and extent of the evidence base will be discussed, and the applicability of methods to convert the available evidence into EFA calculator scores will be assessed
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