4,974 research outputs found
Phase detection at the quantum limit with multi-photon Mach-Zehnder interferometry
We study a Mach-Zehnder interferometer fed by a coherent state in one input
port and vacuum in the other. We explore a Bayesian phase estimation strategy
to demonstrate that it is possible to achieve the standard quantum limit
independently from the true value of the phase shift and specific assumptions
on the noise of the interferometer. We have been able to implement the protocol
using parallel operation of two photon-number-resolving detectors and
multiphoton coincidence logic electronics at the output ports of a
weakly-illuminated Mach-Zehnder interferometer. This protocol is unbiased and
saturates the Cramer-Rao phase uncertainty bound and, therefore, is an optimal
phase estimation strategy.Comment: 4 pages, 5 figures replaced fig. 1 to correct graphics bu
Scale-Invariance and the Strong Coupling Problem
The effective theory of adiabatic fluctuations around arbitrary
Friedmann-Robertson-Walker backgrounds - both expanding and contracting -
allows for more than one way to obtain scale-invariant two-point correlations.
However, as we show in this paper, it is challenging to produce scale-invariant
fluctuations that are weakly coupled over the range of wavelengths accessible
to cosmological observations. In particular, requiring the background to be a
dynamical attractor, the curvature fluctuations are scale-invariant and weakly
coupled for at least 10 e-folds only if the background is close to de Sitter
space. In this case, the time-translation invariance of the background
guarantees time-independent n-point functions. For non-attractor solutions, any
predictions depend on assumptions about the evolution of the background even
when the perturbations are outside of the horizon. For the simplest such
scenario we identify the regions of the parameter space that avoid both
classical and quantum mechanical strong coupling problems. Finally, we present
extensions of our results to backgrounds in which higher-derivative terms play
a significant role.Comment: 17 pages + appendices, 3 figures; v2: typos fixe
Density Perturbations in the Ekpyrotic Scenario
We study the generation of density perturbations in the ekpyrotic scenario
for the early universe, including gravitational backreaction. We expose
interesting subtleties that apply to both inflationary and ekpyrotic models.
Our analysis includes a detailed proposal of how the perturbations generated in
a contracting phase may be matched across a `bounce' to those in an expanding
hot big bang phase. For the physical conditions relevant to the ekpyrotic
scenario, we re-obtain our earlier result of a nearly scale-invariant spectrum
of energy density perturbations. We find that the perturbation amplitude is
typically small, as desired to match observation.Comment: 36 pages, compressed and RevTex file, one postscript figure file.
Minor typographical and numerical errors corrected, discussion added. This
version to appear in Physical Review
The four fixed points of scale invariant single field cosmological models
We introduce a new set of flow parameters to describe the time dependence of
the equation of state and the speed of sound in single field cosmological
models. A scale invariant power spectrum is produced if these flow parameters
satisfy specific dynamical equations. We analyze the flow of these parameters
and find four types of fixed points that encompass all known single field
models. Moreover, near each fixed point we uncover new models where the scale
invariance of the power spectrum relies on having simultaneously time varying
speed of sound and equation of state. We describe several distinctive new
models and discuss constraints from strong coupling and superluminality.Comment: 24 pages, 6 figure
Beta Glucan: Health Benefits in Obesity and Metabolic Syndrome
Despite the lack of international agreement regarding the definition and classification of fiber, there is established evidence on the role of dietary fibers in obesity and metabolic syndrome. Beta glucan (β-glucan) is a soluble fiber readily available from oat and barley grains that has been gaining interest due to its multiple functional and bioactive properties. Its beneficial role in insulin resistance, dyslipidemia, hypertension, and obesity is being continuously documented. The fermentability of β-glucans and their ability to form highly viscous solutions in the human gut may constitute the basis of their health benefits. Consequently, the applicability of β-glucan as a food ingredient is being widely considered with the dual purposes of increasing the fiber content of food products and enhancing their health properties. Therefore, this paper explores the role of β-glucans in the prevention and treatment of characteristics of the metabolic syndrome, their underlying mechanisms of action, and their potential in food applications
The Coronavirus Pandemic: A Pitfall or a Fast Track for Validating Cell Therapy Products?
The global COVID-19 pandemic has prompted urgent need for potential therapies for severe respiratory consequences resulting from coronavirus infection. New therapeutic agents that will attenuate ongoing inflammation and at the same time promote regeneration of injured lung epithelial cells are urgently needed. Cell-based therapies, primarily involving mesenchymal stromal cells (MSCs) and their derivatives, are currently investigated worldwide for SARS-CoV-2-induced lung diseases. A significant number of academic centers and companies globally have already initiated such trials. However, at a time of unprecedented need, it is also foreseen that families and caregivers will seek all available options, including access to cell-based and other investigational products, even before proven safety and efficacy as well as regulatory approval. This should not be an excuse for opportunists to sell or advertise unproven therapies of any kind. "Compassionate use" should be conducted in the context of a clinical investigation framed by strict ethical and regulatory permissions, with the goal of obtaining mechanistic information wherever possible
3D visualisation tool for improving the resilience to urban and coastal flooding in Torbay, UK
This is the final version of the article. Available from Elsevier via the DOI in this record.Torbay, located in South West England, UK, is one of the Case Studies on the EU-funded project EU-CIRCLE, which is aimed at enhancing resilience of Critical Infrastructures (CI) to natural hazards. The region includes three urban centres (Torquay, Paignton and Brixham) and hosts more than 3 million tourists every year that contribute over £450 million to local economy. However, flooding, including coastal, fluvial and pluvial, has been a major threat to the area with more than 15 major incidents occurring since 1999. Rising sea levels, combined with increasing rainfall intensity, linked to climate change, are expected to exacerbate the problem. Better adaptation strategies are needed to safeguard CIs and services while improving resilience to climate hazards. EU-CIRCLE partners are engaged in a review of the existing capacity of flood defenses and the drainage systems in Torbay. To enhance the risk communication with the stakeholders, we adopted a high performance flood model to analyse the flood risk to Cis under a wide range of scenarios. The results are integrated into an innovative 3D visualization tool, showing the progress over time of any flood scenario in the region, via a fully interactive interface allowing stakeholders to better understand flood impacts to CIs.The work presented in this paper was partially funded by the ongoing EC H2020 EU-CIRCLE (GA 653824) and the
FP7 PEARL (Preparing for Extreme And Rare events in coastaL regions, GA 603663) projects
Cosmological Perturbations in a Big Crunch/Big Bang Space-time
A prescription is developed for matching general relativistic perturbations
across singularities of the type encountered in the ekpyrotic and cyclic
scenarios i.e. a collision between orbifold planes. We show that there exists a
gauge in which the evolution of perturbations is locally identical to that in a
model space-time (compactified Milne mod Z_2) where the matching of modes
across the singularity can be treated using a prescription previously
introduced by two of us. Using this approach, we show that long wavelength,
scale-invariant, growing-mode perturbations in the incoming state pass through
the collision and become scale-invariant growing-mode perturbations in the
expanding hot big bang phase.Comment: 47 pages, 4 figure
Long-wavelength approximation for string cosmology with barotropic perfect fluid
The field equations derived from the low energy string effective action with
a matter tensor describing a perfect fluid with a barotropic equation of state
are solved iteratively using the long-wavelength approximation, i.e. the field
equations are expanded by the number of spatial gradients. In the zero order, a
quasi-isotropic solution is presented and compared with the general solution of
the pure dilaton gravity. Possible cosmological models are analyzed from the
point of view of the pre-big bang scenario. The second order solutions are
found and their growing and decaying parts are studied.Comment: 19 pages, 1 figur
Generalizing Galileons
The Galileons are a set of terms within four-dimensional effective field
theories, obeying symmetries that can be derived from the dynamics of a
3+1-dimensional flat brane embedded in a 5-dimensional Minkowski Bulk. These
theories have some intriguing properties, including freedom from ghosts and a
non-renormalization theorem that hints at possible applications in both
particle physics and cosmology. In this brief review article, we will summarize
our attempts over the last year to extend the Galileon idea in two important
ways. We will discuss the effective field theory construction arising from
co-dimension greater than one flat branes embedded in a flat background - the
multiGalileons - and we will then describe symmetric covariant versions of the
Galileons, more suitable for general cosmological applications. While all these
Galileons can be thought of as interesting four-dimensional field theories in
their own rights, the work described here may also make it easier to embed them
into string theory, with its multiple extra dimensions and more general
gravitational backgrounds.Comment: 16 pages; invited brief review article for a special issue of
Classical and Quantum Gravity. Submitted to CQ
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